specification that can clearly identify frequencies which shift with
vehicle operating conditions, primarily vehicle speed, when the
frequency content of the desired signal and any background noise is
unknown.'' Since no practicable test methodology consistent with the
requirements of an FMVSS has been developed to date to objectively
determine frequency shifting, the agency is not including a requirement
for frequency shifting in the final rule.
    Nevertheless, the agency encourages manufacturers to include
frequency shifting in their development of alert sounds as this
shifting does provide aural information to pedestrians about whether
they are at risk or not and about the distance, speed, and acceleration
of approaching vehicles. These are useful cues for pedestrian
navigation.
    In the future, should a practicable, objective method to quantify
frequency shifting of vehicle alert sounds be developed, NHTSA may
reconsider its decision to exclude a frequency shifting requirement
from the safety standard.
Relative Volume Change
    Because it is not feasible to include requirements for frequency
shifting in the final rule for the reasons discussed above, the agency
has decided to include in the final rule a requirement for vehicle-
emitted sound level or ``volume'' rather than in frequency to increase
as the vehicle increases speed. The agency has decided to include this
volume change requirement as a means for pedestrians to utilize the
sounds emitted by a vehicle to determine if a vehicle is accelerating
or decelerating. The agency understands that the concept of ``relative
volume change'' is not a direct replacement for frequency shifting, but
we believe it is a reasonable alternative. While frequency shifting
would be a more certain method for determining vehicle acceleration and
deceleration, volume change will provide useful audible information to
pedestrians about the operating state of nearby vehicles. We believe
that the volume change specifications will partially compensate for the
absence of pitch shifting requirements.
    To better understand the concept, as a vehicle approaches a
pedestrian at a constant speed, the pedestrian would hear the vehicle
alert sound increase in volume, identifying that the vehicle is
approaching but maybe not accelerating or decelerating. However, if the
vehicle is approaching a pedestrian and accelerating (or decelerating),
the alert sound will increase (or decrease) in volume more rapidly as
the vehicle approaches while transitioning between 0 km/h and 10 km/h,
between 10 km/h and 20 km/h, and between 20 km/h and 30 km/h. A rapid
ramp up in volume as the vehicle approaches will be indicative of a
vehicle accelerating, and a rapid reduction in volume as the vehicle
approaches will be indicative of a vehicle decelerating.
    The minimum detection thresholds which are contained in this final
rule increase with speed. Consequently, vehicles that meet the minimum
requirements, without exceeding them, will have an innate volume
increase commensurate with the increase in speed. The minimum
specifications incorporate a volume change of approximately 6 dB
between stationary and 10 km/h, approximately 6 dB between 10 km/h and
20 km/h, and approximately 5 dB between 20 km/h and 30 km/h. However,
manufacturers could design alert signals that have only a single sound
level, such as one that meets the highest sound level requirements
(those required at 30 km/h) across all speeds (thus exceeding the
minimum levels at stationary, 10 km/h

and 20 km/h). In this case, the alert would have no built-in volume
change with increasing or decreasing speed, and the potential
pedestrian cue to increasing or decreasing vehicle speed would not
exist. The ``relative volume change'' requirement specified in this
final rule will ensure a minimum sound level increase and decrease as a
vehicle reaches each successive higher or lower speed operating
condition.
    In discussing the minimum acoustic requirements for the eight one-
third octave bands in the NPRM, NHTSA said the minimum requirements in
each one-third octave band increased as the vehicle increased in speed
to give pedestrians more time to detect faster moving vehicles and to
allow the pedestrian to determine whether the vehicle was accelerating
or decelerating. While the minimum acoustic requirements in the NPRM
increased for each test speed, the NPRM did not include maximum sound
requirements for each test speed. This meant that a vehicle could
comply with the requirements of the NPRM by meeting the minimum
acoustic requirements for the highest test speed for all test speeds
without any variation in the sound produced by the vehicle. In other
words, a vehicle alert system could be designed such that it would emit
the loudest required sound level in all test conditions from stationary
up to 30 km/h. Under this scenario, a pedestrian would have limited
ability to detect changes in vehicle speed without pitch shifting
because the sound produced by the vehicle would not change as the
vehicle changed speed. To eliminate this possibility, NHTSA has
included the volume change requirements in the final rule to ensure
that the alert sound varies produced as vehicle changes speed.
    Since an alert signal's acoustic components can change from one
operating condition to the next, changes in the overall SPL level will
not necessarily correspond to changes in the level of individual one-
third octave bands. Also, the overall sound pressure level is
influenced by bands that are outside of the range of one-third octaves
covered by NHTSA's specifications (i.e., those greater than 5000 Hz and
less than 315 Hz). Therefore, in order to evaluate changes in perceived
volume level, we will consider only the one-third octave bands that
account for sound energy contained in the range from 315 Hz to 5000 Hz.
Normalized one-third octave band values are derived by subtracting the
minimum one-third octave values specified for the stationary operating
condition from each of the one-third octave band alert measurements.
This normalization process allows measurements of different one-third
octave bands to be compared by accounting for the differences in the
minimum levels specified for each band. The logarithmic sum of the
thirteen normalized one-third octave band levels is then determined
(i.e., the ``band sum'').
[GRAPHIC] [TIFF OMITTED] TR14DE16.010

    Finally, the relative volume change is calculated as the difference
in these band sum values between consecutive operating speed
conditions.
    Evaluating the increase in band sum values from one speed to the
next then provides a metric for ``relative volume change.'' This
approach allows for the tracking of volume as a function of speed, as
the volume is characterized by the sound pressure levels above the
minimum levels required at the baseline stationary operating condition.
It also allows for the rejection of one-third octave bands outside of
the range of interest (315 Hz to 5000 Hz). Another key characteristic
of this approach is that frequency is not tracked, which provides
design flexibility because different one-third octave bands can be
prominent at different speeds.
    The relative volume change procedure will utilize the same vehicle
measurement data collected for the determination of compliance with the
minimum detection standards. That is, the volume change determination
uses the average values for the thirteen one-third octave bands of the
first four valid, ambient-corrected runs, from the louder side of the
vehicle (left or right), for each operating condition (Stationary, 10
km/h, 20 km/h, and 30 km/h). By comparing the calculated band sum at a
given operating speed with the band sum value for the next lower speed
condition, a relative volume change can be computed.
    An example calculation is provided in Figure 9.

[GRAPHIC] [TIFF OMITTED] TR14DE16.011

    Figure 9 illustrates the four-step procedure used to calculate the
relative volume change for sample data for the 10 km/h to 20 km/h
conditions as follows:
    Step 1: Calculate the average measured one-third octave band level
for each of the 13 one-third octave bands (315 Hz to 5000 Hz) using the
four valid test runs identified for each of the test operating
scenarios (stationary, 10 km/h (11+/- 1km/h), 20 km/h (21+/- 1km/h),
and 30 km/h (31+/- 1km/h)).
    Step 2: Calculate the normalized values for each of the 13 one-
third octave bands for each of the operating scenarios, relative to the
minimum SPL requirements specified for the stationary operating
scenario. The normalized values are calculated by subtracting the
minimum SPL values specified for the stationary operating condition
from each of the one-third octave band averages calculated for each
operating scenario (stationary, 10 km/h (11+/- 1km/h), 20 km/h (21+/-
1km/h), and 30 km/h (31+/- 1km/h)).
    Step 3: Calculate the BAND SUM for each critical operating scenario
(stationary, 10 km/h (11+/- 1km/h), 20 km/h (21+/- 1km/h), and 30 km/h
(31+/- 1km/h)) as follows:
[GRAPHIC] [TIFF OMITTED] TR14DE16.012

Where:

i represents each of the 13 one-third octave bands.
Normalized Band Leveli is the calculated normalized value for each
of the 13 one-third octave bands.

    Step 4: Calculate the relative volume change between each operating
scenario (stationary to 10 km/h; 10 km/h to 20 km/h; 20 km/h to 30 km/
h) by subtracting the BAND SUM of the lower speed test case from the
BAND SUM of the next higher speed test case.
    The performance specifications for the relative volume change
requirement were derived based upon the minimum detection standards for
each operating condition. The minimum detection standards increase with
speed such that, if a vehicle just meets the minimum standards at each
operating condition, its relative volume change would be approximately
6 dB between stationary and 10 km/h, approximately 6 dB between 10 km/h
and 20 km/h, and approximately 5 dB between 20 km/h and 30 km/h. It is
the agency's desire to ensure that vehicles equipped with compliant
alert sounds are only as loud as they need to be for detection by

pedestrians, and not excessively louder. To meet the relative volume
change requirements, a manufacturer could simply increase the sound
levels well beyond the minimum standards to achieve the required
separation at each speed interval. However, we believe that
manufacturers will also want to reduce alert sounds to the greatest
extent possible while meeting the minimum standards in order to
maximize customer satisfaction and minimize environmental noise. To
accomplish the goal of minimizing excessive noise, the relative volume
change values should not exceed the already established differences of
6 dB, 6 dB, and 5 dB built into the minimum operating condition
specifications. The relative volume change specifications that NHTSA
has decided to require are provided in Table 17.

          Table 17--Minimum Relative Volume Change Requirements
------------------------------------------------------------------------
                                                              Minimum
                                                             relative
              Critical operating scenarios                volume change,
                                                                dB
------------------------------------------------------------------------
Between:
  Stationary and 10 km/h................................               3
  10 km/h and 20 km/h...................................               3
  20 km/h and 30 km/h...................................               3
------------------------------------------------------------------------

    These performance levels were established using the following
criteria. First, as explained above, to minimize alert sound levels,
the maximum volume change between operating scenarios would be 6 dB, 6
dB, and 5 dB, respectively. So, as a starting point, the relative
volume change requirements should not exceed these values. Second, a
manufacturer might choose to design an alert signal that exceeds the
minimum values at a given speed and just meets the minimum values at
the next higher speed. Such a design would have a decreased relative
volume change, i.e., less than 5 dB or 6 dB, between operating
conditions. Third, as discussed in the NPRM, the sound level change
that can be discerned by an untrained observer is approximately 3 dB,
so the relative volume change between each successive operating
scenario should be at least 3 dB in order to be useful. Considering all
these criteria, we want to target relative volume changes within the
range of 3 dB to 6 dB. Within this range, we have decided to specify 3
dB as the minimum volume change requirement for the transitions between
successive operating conditions. This means that the manufacturer can
incorporate a 3 dB volume change or any level above 3 dB to meet the
specified requirements. The minimum requirement of 3 dB between each
operating condition ensures the volume change will be discernable while
providing manufacturers with the greatest flexibility in the design of
their alert systems.
    It is NHTSA's expectation that the volume change requirement will
provide pedestrians with the audible cues needed to discern vehicle
acceleration and deceleration. However, we reiterate that frequency
shifting still is a useful characteristic of a vehicle alert system,
and we encourage system designers to incorporate frequency shifting
even though this final rule does not include specific requirements for
it.
    Lastly, in regards to the commenters who requested that the
proposed test procedure for frequency shifting be modified to allow for
indoor testing and/or testing at the component level, those comments
are no longer applicable since the agency has decided to exclude a
frequency shifting test. In regard to comments about indoor and
component testing in general, we have addressed that issue in Section
III.K of today's final rule, where we have stated that NHTSA will
conduct compliance testing on complete vehicles on outdoor test tracks.

H. Sameness

    The NPRM criterion for sameness was that the alert sound of two
example vehicles must have a sound pressure level within 3 dB(A) in
every one-third octave band between 315 Hz and 5000 Hz. That
requirement would limit the amount of variation in one-third octave
bands over a range of frequencies when measured on a stationary
vehicle. We proposed that requirement as an objective way to determine
if the alert sounds produced by two different vehicles of the same make
and model are the same.
    In the NPRM, the agency interpreted the PSEA language on sameness
as applying ``only to sound added to a vehicle for the purposes of
complying with the NHTSA regulation'' [NPRM, p. 2804]. The proposed
sameness criteria were not intended to apply to sounds generated by a
vehicle's tires or body parts or by the mechanical operations of the
vehicle.
    In the NPRM, NHTSA stated that we interpret a vehicle ``model'' as
a specific grouping of similar vehicles within a vehicle line. The
Federal Motor Vehicle Theft Prevention Standard,\140\ defines vehicle
line as ``a name which a manufacturer applies to a group of vehicles of
the same make that have the same body or chassis, or otherwise are
similar in construction or design.'' If a manufacturer calls a group of
vehicles by the same general name as it applies to another group, but
adds a further description to that name (e.g., Ford Fusion Hybrid, or
Toyota Prius Three), the further description indicates a unique model
within that line.
---------------------------------------------------------------------------

    \140\ 49 CFR part 541.
---------------------------------------------------------------------------

    Also, the NPRM conveyed that the requirement for vehicles of the
same make and model to have the same sound or set of sounds does not
apply across model years. For example, a model year 2020 Prius Two
could have a different sound than a 2019 Prius Two (same model but
different model years). A 2019 Prius Two could have a different sound
than a 2019 Prius Four (same model year but different models). All
Prius Two's from the 2019 model year would be required to emit the same
sound or set of sounds (same model and model year).
    The PSEA includes language that requires ``the same sound or set of
sounds for all vehicles of the same make and model.'' We interpreted
this to mean that a manufacturer may choose to equip a vehicle to have
different sounds for different operating modes such as forward,
reverse, and stationary [NPRM, p. 2804]. Each sound would have to meet
the corresponding performance requirements in each operating mode. We
did not interpret this language in the PSEA to mean that a vehicle can
have more than one alert sound for a given operating mode, such as a
suite of sounds that a driver can select from according to personal
preference.
    In general, commenters from industry stated that speaker tolerances
make it impossible to make all vehicles of the same year/make/model
produce the same sound in accordance with the NPRM criterion, i.e., to
have the same sound level, within 3.0 dB, in each of the
thirteen specified one-third octave bands. Also, industry commenters
favor an indoor, component-level test for sameness, rather than an
outdoor test conducted on an ISO pad.
    Advocacy groups that provided comments on the proposed sameness
requirement generally supported it, or supported some performance-based
assessment of sameness, but did not suggest specific technical criteria
for such a performance test.
    Alliance/Global stated on behalf of their member companies that the
classification of sounds by an objective metric that would determine
sameness first needs to have ``sameness'' defined. The NPRM proposal
for a three decibel limit in each one-third octave band is not
sufficient for the measurement uncertainty, let alone production
variation, according to Alliance/Global.

Alliance/Global recommended that sameness be measured at a component
level under indoor laboratory conditions. They stated that their only
practical course of action to assure sameness between two vehicles is
to compare the input signals to the speakers (the output from the
signal generator or the programmed digital sound file). Alliance/Global
stated that measuring sameness through microphone recordings of
operating vehicles is not possible as a practical matter. Furthermore,
due to the variation in production speakers, it also is not reasonable
to require them to emit the same sound within the proposed three
decibel specification. They acknowledged that the requirement cannot be
deleted altogether because it is included in the PSEA. Alliance/Global
also agreed with OICA that NHTSA should allow manufacturers the option
of demonstrating compliance with the sameness requirement through
comparisons of elements such as the software sound file, input to the
speakers, etc.
    OICA stated that the proposed sameness criterion needs revision,
pointing out that industry has already shown that even 6 dB may not be
a sufficient tolerance between vehicles of the same make and model.
OICA stated that the measurement uncertainty is the most significant
factor, and that the proposed allowance of 3 dB is not commensurate
with the measurement uncertainty. OICA suggested that NHTSA should
carefully consider how sameness is defined as that will drive the
necessary measurement procedures. OICA noted that sound-generating
devices that use the same software will inherently have the same sound,
even when the sound is altered slightly through various factors such as
installation into a vehicle. Using the same software also means that
vehicles will produce the same sound even when the hardware is changed
somewhat, according to OICA. OICA also noted that NHTSA could resolve
issues with measurement of Sameness by specifying a requirement that
applies to the software sound file. Citing the PSEA language, ``The
Secretary shall allow manufacturers to provide each vehicle with one or
more sounds that comply with the motor vehicle safety standard at the
time of manufacture,'' OICA stated that vehicle manufacturers should be
allowed to offer vehicles to customers with more than one alert sound
and to equip vehicles with multiple alert sounds for the driver to
select from during vehicle operation, as long as each of the sounds
fulfils the minimum requirements defined in the safety standard. OICA
suggested that the language of Section S5.3 should state that two
vehicles of the same make, model, and model year must ``emit the same
sound within a set of sounds,'' and that their overall sound level
should be required to be within 6 dB(A).
    Denso stated that this requirement is not feasible for a number of
reasons. For one, there is inherent variability in vehicle sound
characteristics and in speaker and amplifier characteristics and
performance. When combining this variability, it is very difficult to
limit the sound difference to within 3 dB(A) between two vehicles, even
for vehicles having nominally identical sound systems, according to
Denso. Denso stated that sound pressure levels will decrease by
approximately one decibel when the ambient temperature increases from 0
to 40 degrees Celsius. Therefore, Denso suggested it is very difficult
to measure the sound level within a tolerance of 1.5 dB
with good repeatability in outdoor conditions. In addition, since the
perception of sound depends on ambient conditions (wind direction, wind
speed, temperature, atmospheric pressure, etc.) and surrounding noise,
Denso stated that ICE vehicles of the same model have up to a 3 dB and
greater sound level difference. For these reasons, Denso requested that
NHTSA not adopt a requirement for sameness.
    The SAE stated that, although 3 dB may be an acceptable tolerance
on overall SPL, it is not sufficient for one-third octave bands. SAE
also stated that restricting one-third octave band variation does not
guarantee sameness in any reasonable sense related to this regulation.
Sounds can be filtered to meet the same one-third octave requirements,
yet still could be perceived as substantially different by pedestrians.
SAE provided an example of two sound files having the same overall SPL
and very similar average spectral distribution, but different time
signals. Despite their similarities, the two sound files were from
recordings of completely different sounds. SAE stated that this
demonstrates how sounds can appear to be similar based on a selected
measurement criterion when in fact they might be very different in how
they sound to listeners.
    Honda stated the criterion for sameness in the NPRM is too
stringent and cannot be complied with due to the variability of sound-
producing devices. An attachment to Honda's comment graphically
represented the variability in repeated testing of the same vehicles.
[We note there was very little explanation of the data in Honda's
comment; the graphic showed that one-third octave band measurements in
repeated tests of the same vehicle appeared to vary by up to about 7
dB; but the results were quite different for the various one-third
octaves and for the different test vehicles Honda tested, with
variability in some instances being close to zero.] Honda suggested
that NHTSA should specify an overall sound level and require that there
be two peak frequencies that fall within specified frequency ranges.
    Advocates for Highway and Auto Safety stated that, to ensure that
different vehicles of the same make/model have the same sound, the
agency must establish a test procedure for comparing different vehicles
of the same make and model to ensure compliance and production
uniformity along with meeting the FMVSS sound requirements.
    Accessible Designs for the Blind stated that sameness should be
tested at all speeds from idle up to the crossover point speed. ADB
stated it does not believe that testing at idle only is appropriate for
establishing the standard. ADB stated that changing a vehicle's tires
or body design is likely to affect the vehicle's sound profile and
therefore it is essential that the single sound specified be well
documented as detectable and localizable under common traffic and
ambient sound conditions by visually-impaired pedestrians who are at
least 60 years of age. There will be differences in the perceived sound
even if it is generated using the same wav file. The nature of the
loudspeaker and where and how it is mounted will also result in
differences. Perceived sound will, of course, also vary by road
surface. ADB rejected the notion that a variety of sounds will be
consistently and accurately recognized by pedestrians as coming from
vehicles. Any added sound should be the same for all EVs and HVs in
order to be maximally recognized and quickly interpreted as being a
vehicular sound, according to ADB. ADB stated that having more than one
sound is likely to decrease any safety benefit added sound might
provide for visually-impaired pedestrians.
    In a February 2014 letter to NHTSA co-signed by the Alliance,
Global, the NFB, and the ACB, the co-signers jointly submitted their
mutually agreed-upon position about aspects of the PSEA's sameness
requirement. They stated that vehicles with the same overall sound
pressure level, within a reasonable engineering and manufacturing
tolerance, should be considered as having the same sound.

    The joint letter said that vehicles of different model years should
not be considered to be the same make and model. In other words, only
vehicles of the same make, model, and model year should be required to
emit the same sound.
    The joint commenters also expressed their agreement about two other
aspects of the PSEA Sameness requirement: First, OEMs should have
flexibility to provide EV/HVs with some number of driver-selectable
sounds instead of just a single sound; and second, OEMs should be
allowed to install updated sounds once per model year to address any
dissatisfaction that might arise on the part of vehicle owners with the
alert sounds their HV/EVs are originally manufactured with. The latter
would be separate from updates that OEMs might need to make to remedy a
noncompliance or for conducting a recall, as provided for in the PSEA.
The joint commenters believe the language of the PSEA, which uses the
terms ``one or more sounds'' and also ``sound or set of sounds'' allows
for driver-selectable sounds and voluntary updating of sounds.
    We note that NHTSA did not receive comments specifically in
response to our request for comment on the extent to which changing a
vehicle's tires or body design would affect the vehicle's sound profile
for the purposes of determining whether two example vehicles have the
same sound.
Agency Response to Comments
    In light of the comments the agency received on the NPRM sameness
requirement, we have reconsidered the proposed requirement and have
decided that it is not appropriate for the final rule. We agree with at
least one shortcoming that was pointed out by several commenters: Even
if two vehicles' alert sounds are within three dB(A) in each specified
one-third octave band, the alerts would not necessarily sound the same
because sounds that have identical one-third octave sound pressure
levels can vary considerably in terms of how they are perceived by a
listener. In fact, it is possible for completely different types of
sounds to have similar one-third octave band levels, even across a wide
range of frequency bands.
    We now believe that the NPRM metric based on A-weighted one-third
octave band sound pressure levels would be suitable only to identify
``defective'' sounds, i.e., to identify when two sounds that are
intended by design to sound the same are not the same, for example if a
particular test vehicle had a damaged speaker. The main reason for this
is that the NPRM method has relatively low resolution and would not
distinguish between tonal signals and noise signals, which are
different by definition but can have the same one-third octave band
spectra. Consequently, even if two vehicles of the same make and model
were to comply with the NPRM criterion, there would be little assurance
that they in fact produce identical alert sounds.
    We also acknowledge the concern expressed in comments that speakers
used in alert systems have some inherent manufacturing variation.
However, NHTSA has not conducted tests to verify the level of speaker
variation claimed by commenters.
    Regarding the Alliance/Global suggestion that overall sound
pressure levels produced by two vehicles should be used to determine
whether they are the same, we do not believe that method would provide
a meaningful comparison. That approach would merely characterize how
loudly two vehicles' alert sounds are perceived. That approach would
not evaluate other acoustic characteristics that make sounds alike such
as phase or spectral shape, and it normally would not distinguish
between sounds that are obviously different to listeners. For example,
music, construction noise, and thunder all can have the same overall A-
weighted sound pressure level.
Other Sameness Metrics Considered by NHTSA
    Subsequent to concluding that a requirement based on one-third
octave levels is not appropriate for the final rule, the agency
considered various alternatives for objectively determining that alert
sounds among vehicles of the same make and model are the same.
    To address issues with the NPRM approach, we considered two
additional types of acoustic metrics to evaluate the similarity of the
alert sounds on vehicles of the same make and model: Power Spectrum
Analysis and Frequency Response Functions (FRF). These are both
acoustic metrics that could be used to analyze the actual output of the
alert system speaker to quantify the difference between two sound
signals. Both of these metrics characterize amplitude and frequency.
The FRF is sensitive to phase as well. Both metrics have higher
resolution than one-third octave bands.
    Power spectrum analysis generally has resolution sufficient for
signals that do not change over time. However, temporal differences
such as time reversal (e.g., playing of a signal in reverse) and
amplitude modulations which change the perceived character of a sound
may not show up as significant differences in the power spectrum of two
signals. For this metric to be useful for evaluating sameness, it
probably would be necessary to evaluate the statistical correlation
(R\2\ value) of the power spectra of two sound signals and to specify a
degree of correlation that must be achieved in order for the two sounds
to be considered the same. For a variety of reasons including a lack of
any established procedure using this method and also repeatability
concerns, we do not know if it is feasible to develop a compliance
requirement based on this method.
    Frequency Response Functions would provide a better comparison. For
some alert sounds, the FRF could be used to show that certain periodic
variations are highly correlated between two signals. However, other
signal variations may not be correlated. Additionally, an evaluation of
the FRF would require a standardized method to synchronize the phase
between the two signals, and the agency currently does not have any
such method.
    Overall, we have concluded that comparisons using Power Spectrum
Analysis or Frequency Response Functions might provide a higher degree
of confidence than the NPRM method that two unknown signals are the
same, but developing a requirement and test procedure based on these
metrics for a compliance test application may involve considerable
additional agency research and testing.
    Furthermore, in order for either of these metrics to be useful in a
compliance test, the measurement variability of the data collected for
a sameness evaluation would have to be extremely low, such that even
small differences in measurements of two example vehicles could be
attributed to actual differences in their alert sounds. As discussed in
the Repeatability/Reproducibility section (Section III.K) of this
preamble, we have determined that the variability of pedestrian alert
sound measurements is on the order of several decibels when measured on
a vehicle in operation (although stationary tests like those used for
Sameness tend to be somewhat less variable.) Although the level of
variability of the NHTSA measurement procedure promulgated in today's
final rule is sufficiently low for stationary, reverse, and pass-by
tests, we believe it is inadequate for a sameness evaluation using
power spectra and FRFs. For these metrics to be useful for sameness, we
would need to obtain a clean signal prior to its exposure to external
influences like speaker tolerances and ambient noise fluctuations.

    Another option would be to evaluate the alert signal at the point
where it is transmitted to the alert system speaker, i.e., at the
speaker input. While speaker input would have very high repeatability,
this approach would require that the speaker inputs must be physically
accessible, which the agency has found is not always the case. For
example, speakers might be integrated into a sealed module that
incorporates the control electronics, making access difficult without
destructive measures.
    Another option is to evaluate the signal at the point where it is
generated internally in the alert system. On typical alert systems,
this would amount to evaluating the actual digital source of the alert
sound, such as a wav file, or an equivalent digital element of the
alert system from which the signal originates. NHTSA may not have the
means to extract a digital file for a compliance evaluation of a test
vehicle and would need the assistance of the vehicle manufacturer. At
that point, a more practical option might be for NHTSA to simply
request that information from the vehicle manufacturer. However, even
if an OEM were to provide NHTSA with a digital source file from two
vehicles of the same make and model, it is uncertain whether the agency
could verify that they are identical.
    Because alternative acoustic metrics have these issues, we believe
they are not viable for a regulatory application, and we have decided
not to adopt acoustic metrics for the sameness requirement in the final
rule. Instead, as detailed later in this section, we have concluded
that the final rule requirement for sameness should be based on
certification by vehicle manufacturers that vehicles of the same make
and model are designed to have identical alert sounds. That is, they
must certify that vehicles of the same make, model, and model year are
the same with respect to their alert system hardware and software
components, the source of the alert sound (such as a digital file) and
vehicle inputs used to vary the sound, as well as all other elements of
the alert system.
Other Sameness Issues--Selectable Sounds and Mid-Year Updates
    In the proposed regulatory text in the NPRM, paragraph S8 was
included to prevent alert sound modifications, except in case of a
vehicle recall. That section of the regulatory text also prohibited
systems from being designed to allow access by anyone other than the
OEM or a service provider, so that individuals would not be able to
tamper with or replace the alert sound in their vehicles.
    The joint comment of the Alliance, Global, the NFB, and the ACB
addressed both the issue of ``selectable'' sounds and the issue of
alert sounds being updated or improved after vehicles are delivered to
customers. Regarding the first issue, the joint commenters stated that
they believe the PSEA allows vehicles to be equipped with more than one
sound for a given operating condition. This comment would mean, for
example, that a particular vehicle make/model might have an alert sound
X, an alert sound Y, and an alert sound Z for when the vehicle is in
forward motion at a given speed, and the driver could select X, Y, or Z
based on personal preference and could switch among those choices at
any time. Regarding the second issue, the joint commenters stated the
PSEA allows a manufacturer or dealer to provide vehicle owners with
opportunities at any time during a model year to update the alert sound
or sounds with which their vehicle came equipped from the factory. They
contended that this allowance exists under the PSEA even in cases where
the original sound is not defective or out of compliance with the
safety standard, and that updates may be provided for aesthetic
purposes rather than for remedy of a recalled alert system (the latter
being expressly provided for in the PSEA.)
    Given our understanding of the PSEA, we are not including
provisions requested by these commenters that would allow for driver-
selectable pedestrian alert sounds and mid-year updates of pedestrian
alert sounds. As such, the provision in paragraph S8 of the NPRM
regulatory text, which specifically prohibits alert sound modifications
except for recall purposes and also prohibits systems designed so as to
allow manipulation or modification of the alert sound by anyone other
than the OEM or a service provider, is adopted in this final rule
without modification. We believe that this approach is necessary to
satisfy the requirements contained in the PSEA language and that
allowing a means for owners to select or modify alert sounds, or to
allow vehicle manufacturers, dealers, or other vehicle service entities
to replace or update alert sounds outside the auspices of a recall
action, would be in conflict with the language of the PSEA.
Furthermore, by not allowing driver-selectable sounds, the final rule
adheres more closely to the PSEA requirement that vehicles of a given
make and model must have the same alert sound.
Compliance Evaluation of Sameness
    After fully considering the NPRM comments on sameness and other
acoustic metrics, we have concluded that the compliance requirement for
sameness in this final rule should not be based on acoustic performance
measurements, including the one proposed in the NPRM. The difficulties
and unknowns with comparing direct measurements of acoustic metrics, as
well as the potential need for more agency research in this area if we
decided to use any of the metrics discussed above, leads us to conclude
that, currently, the most effective and expedient way for NHTSA to
evaluate sameness is to explicitly require that specific design aspects
of vehicle alert systems must be the same, particularly the software
and hardware that comprise the systems.
    Although this approach would not be based on acoustic measurement,
it would provide assurance that the design of alert systems on vehicles
of a given make and model are consistent from one vehicle to the next
because the vehicle manufacturer would be certifying not just that the
sounds are the same but that the hardware and software components that
are used to generate the alert sound are the same from vehicle to
vehicle.
    This approach is consistent with the comments NHTSA received in
response to the NPRM. In response to NHTSA's request for comment in the
NPRM regarding its proposed method of measuring whether the sound
produced by two vehicles was the same, the Alliance/Global joint
comment stated that the only way to verify sameness was to measure the
digital signal output of the sound generator or to examine the digital
sound file itself. Alliance/Global further referenced statements by
OICA supporting a method of determining sameness based on the
examination of the software and hardware making up the sound generation
system. Alliance/Global stated in their comments that ``OICA notes that
current sound generating devices that use the same software will
inherently have the same sound, even when the sound is altered slightly
through various factors, such as installation into a vehicle. The
Alliance and Global agree with OICA that NHTSA should allow
manufacturers the option of demonstrating compliance with the sameness
requirement through comparisons such as: The software sound file, input
to the speakers, etc.'' After reviewing the comments and its own data,
NHTSA agrees that the best method for satisfying the requirement in the
PSEA to require vehicles of the same

make and model to make the same sound is to examine the hardware and
software of the subject vehicles and to require that hardware and
software to be the same.
    As stated previously, we believe that the Vehicle Safety Act and
PSEA requirement can be satisfied by this methodology. Aside from being
a requirement in the PSEA, requiring vehicles of the same make and
model to emit the same sound limits the universe of sounds produced by
EVs and HVs that pedestrians, both blind and sighted, must be able to
identify as vehicle sounds. This is important because pedestrians must
be able to recognize the sound produced by an EV or an HV as a vehicle-
emitted sound for this rule to reduce crashes between pedestrians and
EVs and HVs.
    If we can establish that vehicles of the same make and model are
alike with respect to the hardware and software they utilize for their
alert systems, that information will be sufficient to establish their
sameness because the sounds they generate would be effectively the
same. That is, if two vehicles are designed the same in regard to
having the same software and hardware to generate alert sounds, then
any overall differences in the sound produced would not be perceptible
in a meaningful way to pedestrians. Thus, this approach achieves the
intent of the PSEA sameness requirement.
    Consistent with the NPRM, we are applying the sameness criterion
only to sounds added to vehicles for the purpose of complying with this
final rule. In that way, tire noise, wind noise, and any other noise
associated with vehicle motion and that is not generated by the
pedestrian alert system is not subject to the sameness requirement.
    We note that NHTSA has taken a similar approach in other FMVSS
where we have relied on manufacturer's assurance and documentation that
a system is designed to comply with the safety standard. For example,
when NHTSA created the safety standard for Electronic Stability
Control, FMVSS No. 126, S5.6 ``ESC System Technical Documentation,''
was included for compliance of ESC systems with an understeer
requirement. In NHTSA's development of FMVSS No. 126, the agency was
unable to devise an understeer test that was both accurate and
repeatable. The agency instead took the approach of identifying certain
system design characteristics and verifying them by requesting
information from the OEM. Standard No. 126 lists items such as a system
diagram, a written explanation of the system operational
characteristics, a logic diagram, and a discussion of processor inputs
and calculations relating to vehicle understeer as examples of evidence
that may be used to validate the manufacturer's certification.
    In the case of pedestrian alert systems, we are taking that
approach. In our development of today's final rule on FMVSS No. 141, we
have not successfully devised a meaningful, accurate and repeatable
test for sameness. The reasons for this are discussed previously in
this section. Instead, we are including a requirement that critical
aspects of the alert system design must be the same from vehicle to
vehicle.
    We also believe that this approach is consistent with the Vehicle
Safety Act. While Congress intended that NHTSA issue performance
standards when it passed the Vehicle Safety Act, courts interpreting
the Vehicle Safety Act have recognized that in some instances it is
necessary for NHTSA to issue a design restrictive standard in order to
achieve a desired performance or to ensure safety.\141\ In Chrysler v.
Department of Transportation, the Sixth Circuit upheld a FMVSS issued
pursuant to the Vehicle Safety Act restricting the design of headlamps.
The court held that the design restriction on headlamps in the standard
was consistent with the Vehicle Safety Act because it fulfilled the
important safety purpose of ensuring that replacement headlamps were
readily available to consumers. We believe that the provisions in this
final rule requiring that certain aspects of the vehicle alert sound
system be the same in all vehicles of the same make and model, in
addition to fulfilling a requirement in the PSEA, fulfils the safety
purpose of helping pedestrians to recognize sounds produced by EVs and
HVs as vehicle emitted sounds.
---------------------------------------------------------------------------

    \141\ See Washington v. Dep't of Transp., 84 F.3d 1222, (10th
Cir. 1996); Chrysler Corp. v. Dep't of Transp., 515 F.2d 1052, 1058
(6th Cir. 1975).
---------------------------------------------------------------------------

    To implement this approach for the sameness requirement, we are
modifying the proposed regulatory text in paragraph S5.5 (was NPRM
paragraph S5.3) to state that any two vehicles of the same make, model,
and model year shall generate their pedestrian alert sound using the
same external sound generation system including the software and
hardware that are part of the system. Furthermore, we are adding a
definition of Pedestrian Alert System within the regulatory text of
S5.5 which lists the common components of pedestrian alert systems. In
this way, by certifying that a pedestrian alert system meets S5.5, the
manufacturer is explicitly certifying that the following specific
hardware and software components of the system are the same from
vehicle to vehicle: The alert system hardware components including
speakers, speaker modules, and control modules, as evidenced by
specific details such as part numbers and technical illustrations; the
location, orientation, and mounting of the hardware components within
the vehicle; the digital sound file or other digitally encoded source;
the software and/or firmware and algorithms which generate the
pedestrian alert sound and/or which process the digital source file to
generate a pedestrian alert sound; vehicle inputs including vehicle
speed and gear selector position utilized by the alert system; any
other design features necessary for vehicles of the same make, model,
and model year to have the same pedestrian alert sound at each given
operating condition specified in this safety standard.
    To verify the OEM's certification of an alert system in the
agency's annual compliance evaluations, NHTSA's Office of Vehicle
Safety Compliance may request that the manufacturer make available to
the agency specific design documentation relating to the alert system
used on same make, model, and model year vehicles. The documentation
that a manufacturer could provide to demonstrate that the sound
produced by two vehicles of the same make and model is the same may
include documents such as: A description of the source of the alert
sound, such as the digital sound file; a copy of the digital file (if
applicable); any algorithms for processing/manipulating the digital
file to generate an alert sound; vehicle inputs such as speed signal
that are needed to process and generate the alert sound; and details
such as part numbers showing that vehicles of the same make, model, and
model year are consistently equipped with identical alert system
components.

I. Customer Acceptance

    In the NPRM we discussed presentations provided by vehicle
manufacturers regarding consumer acceptance of adding sound to vehicles
to provide pedestrian detection. Nissan submitted a presentation
stating that over 60 percent of Nissan Leaf owners surveyed found that
added noise was acceptable if the overall sound pressure level of the
sound was 55 dB-A or quieter for the forward moving condition.
    The NPRM also discussed the ways in which NHTSA crafted the
proposal to account for concerns about the community noise impacts of
the

proposal so that sounds complying with the requirements of the final
rule would not unnecessarily contribute to noise pollution. In
consideration of community noise impacts the NPRM omitted the mid-range
frequencies from the proposed acoustic requirements as these are the
frequencies that contribute the most to increasing the overall sound
pressure level of sound.
    NHTSA also conducted a draft Environmental Assessment (EA) to
analyze the environmental effects of the proposed rule. The analysis in
the EA most relevant to analyzing the impact of the rule on consumer
acceptance is the single car pass-by analysis. This analysis is
designed to show what a person standing near the road way would hear
when a EV or HV emitting sound complying with the NPRM passed by. In an
urban ambient with an overall sound pressure level of 55 dB(A) a
listener standing near the roadway would not be able to perceive the
difference between a EV/HV that did not produce added sound and an EV/
HV that complied with the requirements of the NPRM.\142\ In a non-urban
ambient with an overall sound pressure level of 35 dB(A) the difference
between the single-vehicle pass-by for EVs/HVs meeting the minimum
sound requirements in the NPRM and those without the added sound would
be 3.1 to 6.3 dB, depending on speed, and 10.1 dB at stationary. In the
non-urban ambient a single vehicle pass by of an EV/HV meeting the
minimum sound requirements of the NPRM would produce less sound than an
average ICE vehicle although this difference would only be noticeable
at stationary.
---------------------------------------------------------------------------

    \142\ NHTSA, Minimum Sound Requirements for Hybrid and Electric
Vehicles; Draft Environmental Assessment (2013), at 39-40.
---------------------------------------------------------------------------

    We received several comments in response to the NPRM that certain
aspects of the proposal would be annoying to passengers or drivers or
would not be accepted by consumers. We also received several comments
from members of the general public stating that the whole concept of
adding any sound to hybrid and electric vehicles would be annoying and
would lead to decreased sales of EVs and HVs.
    Alliance/Global stated in their joint comment that the loudness and
frequency composition of sounds meeting the proposed requirements would
be unpleasant to vehicle occupants. Specifically sounds with minimum
content in eight one-third octave bands would be too loud to be
accepted by consumers.
    Alliance/Global further stated that because the proposed
requirements did not contain requirements for mid-range one-third
octave bands from 500 Hz to 2000 Hz, resulting sound would have a
shrill unpleasant character. Alliance/Global stated that, based on past
experience with shrill sounds, their members fear that costumers may be
unwilling to purchase EVs and HVs if they are equipped with sounds
meeting the proposed requirements.
    GM stated that the proposed sound levels and operating conditions
are in excess of the safety needs of pedestrians and further explained
that this would likely result in customer annoyance leading to
customers disabling the alert sound and also affecting vehicle
purchases. Chrysler and Honda also expressed concerns about
marketability and customer acceptance.
    Toyota also stated that sounds meeting the requirements of the NPRM
would be too loud and would discourage consumers from purchasing EVs
and HVs. Toyota commented that it had examined customer acceptance of
sounds meeting the NPRM specifications. Toyota used a prototype speaker
and included 56 Prius owners (ages 20 to 55 years old). Participants
were asked to drive an alert-equipped vehicle on a specific route and
then rate the sound. The operating conditions experienced during the
study included slow acceleration; 40 km/h pass-by; slow deceleration;
and 16 km/h pass-by. Toyota reported that 68 percent of the drivers
were somewhat dissatisfied or very dissatisfied with their overall
experience with the sound emitted by the test vehicle. Toyota asked the
participants how the sound might affect their future vehicle purchases,
and 54 percent of the drivers indicated a somewhat negative or very
negative impact, while 46 percent indicated no impact or a somewhat
positive impact. Toyota also mentioned that a sound meeting the
proposed requirements in the NPRM resulted in an increase in the
interior noise relative to the same vehicle with the alert system
turned off.
    WBU commented that allowing the sound to be emitted over fewer one-
third octave bands may alleviate manufacturers concerns about consumer
acceptance of alert systems.
    Several commenters also stated that requiring a sound while the
vehicle is stationary would lead to lower consumer acceptance of EVs
and HVs. Nissan submitted with its comment the result of a customer
survey that indicated that over 60 percent of costumers would accept an
idle sound with an overall sound pressure level of 49 dB-A or less.
    NHTSA also received comments from OICA stating that the
requirements in the NPRM requiring that the sound produced by EVs and
HVs contain tones would make sounds complying with the NPRM annoying to
vehicle occupants. Mercedes expressed concern that including
requirements for low one-third octave frequency bands down to 315 Hz
and broadband content down to 160 Hz may affect consumer acceptance of
sounds meeting the requirements of the NPRM because sounds with content
in this area of the spectrum are difficult to isolate from the vehicle
cabin.
Agency Response to Comments
    As discussed in Section III.E of this notice, the agency made
several changes to the acoustic requirements of the NPRM in this final
rule. In response to comments from manufacturers, the final rule allows
compliance with its acoustic requirements by placing minimum content in
the mid-range one-third octave bands from 500 Hz to 2000 Hz. We believe
that this change will increase manufacturer's flexibility to create
sounds that are pleasing to motorists and pedestrians. NHTSA does not
believe that the overall sound pressure level of sounds meeting the
requirements of this final rule will discourage consumers from
purchasing EVs or HVs or effect consumers acceptance of the
requirements in the final rule. The overall sound pressure level of
sounds meeting the requirements of the final rule for the 10 km/h pass
by are between 53-56 dB(A). According to Nissan's presentation, 60
percent of consumers would accept added sound to their vehicle if the
overall sound pressure level of the sound was 55 dB(A) or quieter for
the forward moving condition. NHTSA believes that the Nissan study
indicates that consumers will accept sounds meeting the requirements of
the final rule.
    While the minimum sound requirements in the final rule increase
above 55 dB(a) for the 20 km/h and 30 km/h pass-by tests, sound emitted
from other sources on the vehicle, such as the tires, increases as the
vehicle increases speed as well. NHTSA believes that the increased
sound from these other sources will limit the extent to which drivers
notice, and are negatively affected by, the sound produced in
compliance with this final rule at 20 km/h and 30 km/h.
    NHTSA finds that it is difficult to draw conclusions about consumer
acceptance of sounds meeting requirements of the final rule from the
survey submitted by Toyota. The Toyota survey does not breakout the
views of the participants in the survey by operating speed like the
survey

conducted by Nissan. One of the conditions included by Toyota was a 40
km/h pass-by for which the agency did not propose requirements in NPRM.
Furthermore, the Toyota study did not state the overall sound pressure
level of the sound to which the participants were exposed during the
test. We believe that reducing the number of required one-third octave
bands to either four or two and allowing manufacturers to comply with
the requirements of the final rule by placing minimum content in the
mid-range one-third octave bands from 500 Hz to 2000 Hz will allow
manufacturers more flexibility to create pleasing sounds.
    The final EA replicates the findings of the draft EA indicating
that sounds emitted by EVs/HVs in compliance with this final rule will
be noticeably louder than EVs/HVs without added noise but will produce
less sound than the average ICE vehicle. For this reason we do not
believe that the requirements in the final rule will lead to sounds
that will be so loud as to be annoying to drivers and pedestrians or to
effect consumers' desire to buy these vehicles. Furthermore, according
to the analysis of national annual noise caused by this final rule in
the Final EA, EVs and HVs subject to the final rule would only be
required to emit sound in compliance with this rule during 2.3 percent
of all travel hours in urban areas.\143\ Therefore, the amount of time
during which drivers and pedestrians would be exposed to sounds
produced in compliance with the final rule is limited which also limits
the possibility for annoyance to drivers and pedestrians.
---------------------------------------------------------------------------

    \143\ NHTSA, Minimum Sound Requirements for Hybrid and Electric
Vehicles; Final Environmental Assessment (2016), at p. 56. (docket
NHTSA-2011-0100).
---------------------------------------------------------------------------

    This is not the case for LSVs, however. These vehicles have top
speeds of greater than 20 mph and less than 25 mph and, because final
rule would require sound at speeds of up to 18.6 mph, sound is likely
to be nearly constant for these vehicles. In addition, these vehicles
are often open, lacking windows and, sometimes doors. For this reason,
occupants of these vehicles are likely to hear the required sounds more
so than occupants of other vehicles. However, we did not receive any
comments indicating that consumer acceptance of sounds required by this
final rule would be a greater issue for owners of LSVs than other
vehicles to which this rule applies.\144\
---------------------------------------------------------------------------

    \144\ Note that the category of Low Speed Vehicles is defined in
NHTSA regulations as vehicles whose top speed is more than 20 mph
and not more than 25 mph. Electric vehicles with top speed of 20 mph
or less, like many electric golf carts for example, are not
considered LSVs and, in fact, are not regulated as motor vehicles,
and thus are not subject to this final rule.
---------------------------------------------------------------------------

    The agency addressed comments regarding consumer acceptance of a
sound at stationary in Section III.I of this notice. We note briefly
here that we do not believe that the requirements in the final rule for
EVs and HVs to emit a sound at stationary will substantially affect
consumer acceptance of the requirements in the final rule. As indicated
by the survey conducted by Nissan, 60 percent of consumers accepted a
sound at stationary with an overall sound pressure level similar to the
levels required by the final rule.
    We note that the final rule does not contain the requirements for
broadband sound, low frequency content, and tones proposed in the NPRM.
In satisfying the mandate in the PSEA to establish minimum sound
requirements for EVs and HVs, NHTSA has taken several steps to minimize
the impacts of the requirements on drivers and pedestrians while also
ensuring that these vehicles are detectable to pedestrians when
operating at low speed. This includes reducing the number of required
bands and removing requirements for tones and low frequency content.
Given these changes from the NPRM to the final rule, NHTSA believes
manufacturers will be able to design pedestrian alert sounds that will
be accepted by drivers and pedestrians.

J. Test Conditions

Ambient Temperature Range for Testing
    In the NPRM, we proposed that, for sound measurement testing, the
ambient temperature be in the range 5 to 40 [deg]C. This proposal is
consistent with SAE J2889-1. However, SAE J 2889-1 contains a note
stating that testing of some vehicles may not be possible in warmer
weather conditions (above 20 [deg]C) since such things as battery
cooling fans (if there is one) will always be running. Since the NPRM
proposed that measurements that contain sounds emitted by any component
of a vehicle's battery thermal management system be considered not
valid, the NPRM stated that SAE J2889-1 note will also apply to FMVSS
No. 141 sound measurement testing. Therefore, in the NPRM preamble,
NHTSA requested comments on narrowing the permitted temperature range
to 5 to 20 [deg]C to improve test repeatability and to remove issues
with battery cooling fans running.
    We received comments from Alliance/Global and Honda regarding the
ambient temperature during testing. Both commenters were opposed to
narrowing the permitted temperature range to 5 to 20 [deg]C to improve
test repeatability and to remove issues with battery cooling fans
running. Honda also recommended that the ambient weather conditions be
measured at the specified microphone height in FMVSS No. 141 S6.4 with
a tolerance of 0.02 meters instead of the specified
microphone height with a tolerance of 0.0254 meters that
was proposed in the NPRM.
Agency Response to Comments
    After the NPRM was issued, NHTSA analyzed the sound measurement
repeatability data that it collected in 2012 for a Ford Fusion to
determine if there were systematic effects of the atmospheric
conditions, particularly temperature, on measured sound pressure level
for the vehicle's 10 km/h pass-by. This data consisted of 96 individual
measurements taken over a six-month period from April to September of
2012. For each individual measurement the following data was recorded:
     Overall Sound Pressure Level (dBA)
     Temperature ([deg]C)
     Wind Speed (m/s)
     Wind Direction (degrees from North)
     Atmospheric Pressure (Pa)
     Relative Humidity (%)
    Analysis of variance for each variable's effect on overall sound
pressure level showed no statistically significant variation (at the
[alpha] = 0.05 level) for any variable over the range of the data.
Linear modeling of all terms also showed no statistically significant
effect on overall sound pressure level for any variable.\145\
---------------------------------------------------------------------------

    \145\ For a complete analysis see, Garrott, W.R., Hoover, R.L.,
Evans, L.R., Gerdus, E., and Harris, J.R., ``2012 Quieter Vehicle
Testing Report: Measured Sound Levels for Electric, Hybrid Electric,
and Low Speed Vehicles'' Washington, DC, DOT/NHTSA, November 2016.
---------------------------------------------------------------------------

    Since ambient temperature has no statistically significant effect
on measured sound data, NHTSA agrees with the commenters that we should
not restrict ambient temperatures to between 5 [deg]C and 20 [deg]C
(however, we note that the tendency of thermal management system
cooling fans to activate at higher temperatures may effectively limit
testing to this temperature range). Doing so could limit compliance
testing opportunities while not providing any test accuracy or
repeatability benefit. We would expect a vehicle's thermal management
system to operate more frequently in tests during warmer ambient
conditions. As discussed in Section III.K, the agency has clarified
when a test can be deemed invalid, including instances when cooling
fans engage intermittently

during testing. Therefore, the final rule will permit sound
measurements to be made when the ambient temperature is in the range
from 5 [deg]C and 40 [deg]C.
    Honda's other recommendation was that the ambient weather
conditions be measured at the specified microphone height in FMVSS No.
141, paragraph S6.1, with a tolerance of 0.02 meters. NHTSA
agrees that the 0.02 meters tolerance instead of the
proposed height tolerance of 0.0254 meters that was
proposed in the NPRM is more consistent with SAE J2889-1.
    The NPRM used the microphone positions of S7.1 of SAE J2889-1 and
also used the microphone height tolerance of 0.02 meters.
It seems logically consistent to use the same height tolerance of
0.02 meters for the meteorological instrumentation. Making
this change is not expected to have any impact on the stringency of the
compliance test. It will merely make testing slightly easier to
perform. Therefore, the final rule will have a meteorological
measurement height tolerance of 0.02 meters (2.0 centimeters).
Tire Inflation Pressure
    In the NPRM, NHTSA proposed that, prior to sound measurement
testing, the vehicle's tires be inflated to the recommended tire
inflation pressure listed on the vehicle's tire placard.
    EMA recommended that NHTSA adopt the tire inflation pressure
requirements for medium and heavy trucks in FMVSS No. 121, Air Brake
Systems. NHTSA's proposal deviates from the test procedure in FMVSS No.
121 which states that tires will be inflated as specified by the
vehicle manufacturer for its GVWR.
    EMA cited two factors in support of its suggestion to harmonize the
test procedures in this final rule with those contained in FMVSS No.
121 for tire fitment and inflation pressure. First, EMA pointed out
that a conflict between FMVSS No. 121 and FMVSS No. 141 would add a
burden to manufacturers without any safety benefit by imposing a unique
tire inflation pressure specification for the new FMVSS. Second, EMA
stated that ``the tire inflation pressures on a heavy-duty vehicle's
certification label or tire information label may lead to inaccurate
tire inflations.'' EMA stated that a heavy-duty vehicle's certification
label or tire inflation pressure label contain the recommended cold
inflation pressures for the tires identified on those labels; however,
it is possible that the vehicle may be equipped with a tire not listed
on those two labels.
Agency Response to Comments
    The agency has considered EMA's comments and agrees that the
correct inflation pressure should be used for all applicable vehicles.
For passenger cars, multipurpose passenger vehicles, light trucks, and
buses (with GVWR of 4,536 kg or less) the requirement as proposed in
the NPRM is appropriate. For low-speed vehicles, the required
certification label generally includes tire size and inflation pressure
information. All low-speed vehicles tested to date by the agency's
Compliance division have shown the requisite tire inflation pressure
information on the certification label.
    To address EMA's comments and ensure that all vehicles subject to
the new safety standard are addressed in the language relating to
recommended inflation pressure, paragraph S6.6(e) of the regulatory
text has been revised.
Tire Conditioning
    In the NPRM, NHTSA proposed that, prior to sound measurement
testing, the vehicle's tires be conditioned by driving it around a
circle 30 meters (100 feet) in diameter at a speed that produces a
lateral acceleration of approximately 0.5 to 0.6 g for three clockwise
laps, followed by three counterclockwise laps. This tire conditioning
procedure was derived from ISO 362, ``Road Noise for Passenger Vehicle
Tires.''
    Honda and OICA recommended that NHTSA not require tire conditioning
prior to testing unless NHTSA can show differences in measured acoustic
data attributable to conditioning. OICA recommended changing the tire
conditioning language to state that before sound measurements are
started, the tires shall be brought to their normal operating
conditions.
Agency Response to Comments
    NHTSA does not have measured acoustic data showing differences that
are attributable to tire conditioning. However, NHTSA's goal for tire
conditioning matches the OICA recommendation that, before sound
measurements are started, the tires be brought to their normal
operating conditions. NHTSA also thinks that sound measurement testing
with brand new tires may produce non-representative sounds due to mold
vents and mold lubricant. The goal of tire conditioning is to remove
sound anomalies caused by these effects. We believe that achieving this
goal will require minimal effort during testing. Therefore, NHTSA will
retain tire conditioning in the final rule for passenger cars,
multipurpose passenger vehicles, light trucks, and buses with a GVWR of
4,536 kilograms or less, and low-speed vehicles. The final rule only
specifies how NHTSA (not manufacturers) will perform compliance testing
and, as with other NHTSA safety standards, manufacturers may elect not
to adopt specific portions of a test procedure if they are convinced
that doing so will not affect how their test results compare to the
results from NHTSA compliance testing.
Self-Locking Doors
    In the NPRM, NHTSA proposed that the test vehicle's doors are shut
and locked for all measurements of vehicle pedestrian alert sounds.
    NHTSA received comments on this topic from OICA and Alliance/
Global. Commenters requested that NHTSA clarify the vehicle condition
section of the final rule test procedure for self-locking doors by
adding a sentence saying that in the case of self-lockable vehicles,
the doors shall be locked before starting measurement.
Agency Response to Comments
    NHTSA does not think that it is necessary to add clarification
about vehicles with self-locking doors to the regulatory text. The
applicable proposed regulatory text, as contained in the NPRM, is
S6.6(b): ``The vehicle's doors are shut and locked and windows are
shut.'' This seems quite clear. This text requires that all doors,
whether self-locking or not, be locked prior to testing. This text is
used in this final rule in re-numbered paragraph S6.6(a).
Accessory Equipment
    In the NPRM, NHTSA proposed that, for sound measurement testing,
all accessory equipment (air conditioner, wipers, heat, HVAC fan,
audio/video systems, etc.) be turned off. We also stated that
propulsion battery cooling fans and pumps and other components of the
vehicle's propulsion battery thermal management system are not
considered accessory equipment.
    NHTSA received comments on this topic from OICA and Alliance/
Global. Commenters requested that NHTSA state that accessory equipment
that cannot be shut off need not be shut off. The commenters suggested
that the compliance test procedure prohibit the use of any results
which include sound from any vehicle systems other than those which
would be constantly engaged under the specified performance conditions.
Agency Response to Comments
    NHTSA's goal during compliance testing is to measure the sound

produced by the vehicle when it is in its quietest state after sale to
the general public. It is not to test the vehicle in some artificially
quiet state that will never be attained by the driving public. These
comments are in accord with NHTSA's goal for compliance testing. The
point made by commenters, that accessory equipment that cannot be shut
off need not be shut off, is sensible, is in the spirit of what NHTSA
is trying to accomplish, and clarifies a point not addressed
previously. Therefore, in the final rule we are adding the phrase
``that can be shut down'' to the proposed regulatory text of section
S6.6(c) in the NPRM that dealt with accessory equipment. The re-worded
requirement is in Section S6.6(b) of the final rule regulatory text.
Vehicle Test Weight
    In the NPRM, we proposed that, for sound measurement testing, the
vehicle test weight will be the curb weight (as defined in 571.3) plus
125 kilograms. Equipment, driver, and ballast should be evenly
distributed between the left and right side of the vehicle. The vehicle
test weight should not exceed the GVWR or Gross Axle Weight Ratings
(GAWRs) of the vehicle.
    Commenters addressed three issues related to vehicle test weight:
the need for the final rule to specify vehicle test weight, the need
for a vehicle test weight tolerance, and what the specified vehicle
test weight should be.
    Both Alliance/Global and OICA commented that vehicle test weight
has no effect on measured vehicle sounds. Honda commented that, since
FMVSS No. 141 testing is being conducted at relatively low vehicle
speeds (a maximum of 30 km/h), small changes in vehicle test weight
would have a minimal effect on measured vehicle sounds. Alliance/Global
and OICA both commented that, if the final rule does specify vehicle
test weight, then, for practical reasons, a vehicle test weight
tolerance should be specified. Alliance/Global and Honda both
recommended using the vehicle test weight specified in SAE J2889-1
(manufacturer-defined unloaded weight + one person + measurement
instruments).
Agency Response to Comments
    NHTSA believes that a vehicle test weight specification is
necessary. While we have not conducted research in this area, we
believe it is reasonable to anticipate that if a large load (relative
to the curb weight of the vehicle) is placed in a vehicle (say 1,000
pounds in a passenger car's trunk or 30,000 pounds on a heavy truck),
there would likely be some change in the sound produced by the vehicle
during testing. Therefore, we believe it is necessary to specify
vehicle test weight in the final rule.
    In specifying vehicle test weight in other rules, NHTSA has not
provided a weight tolerance. Organizations performing a test should
make reasonable efforts to comply with the test specifications exactly
as written. Therefore, we are choosing not to do so here and FMVSS No.
141 will not contain a vehicle test weight tolerance.
    Since NHTSA agrees with the commenters that the sound produced by a
vehicle at the relatively low test speeds being used for FMVSS No. 141
testing is not sensitive to minor changes in vehicle loading, minor
deviations in vehicle test weight from the exact values specified in
the rule should not have any effect.
    As to what the vehicle test weight specified in final rule should
be, NHTSA wants to measure sounds produced by lightly loaded vehicles.
We believe that, all else being equal, the tires of a heavily loaded
vehicle will produce a louder sound than will the tires of that same
vehicle when it is lightly loaded.
    NHTSA has identified three possible alternatives for vehicle test
weight in FMVSS No. 141. These are:
    1. Retain the NPRM vehicle test weight specification. This does not
seem to have any particular advantages and has multiple disadvantages.
Some of the disadvantages are that this test vehicle weight
specification does not match that contained in SAE J2889-1; this
vehicle test weight specification is not used by other FMVSS; and this
vehicle test weight specification imposes weight limits on NHTSA test
drivers. To elaborate on the last point, since the proposed NPRM
regulatory text would require the weight above vehicle curb weight to
be evenly balanced from side-to-side, the test driver for NPRM-based
compliance tests cannot weigh more than 62.5 kg (138 pounds). Since a
50th-percentile adult male weighs 76 kg (168 pounds), the use of this
vehicle test weight specification could create difficulties in finding
drivers to perform compliance testing.
    2. Specify the SAE J2889-1 vehicle test weight specification for
NHTSA tests. This was the method recommended by commenters. It would
harmonize with SAE J2889-1, and it has the advantage that NHTSA could
use any test drivers. It has two disadvantages. First, it would mean
that the weight of the test vehicle will vary with the weight of the
test driver (i.e., the test weight is not a precisely specified number
of pounds above the manufacturer-defined unloaded weight). This may not
matter since we believe that the external sounds generated by a vehicle
are relatively insensitive to vehicle weight. Second, this vehicle test
weight specification is inconsistent with any other FMVSS. A given
NHTSA test vehicle often is tested by NHTSA and by manufacturers to
determine compliance with multiple 100-series FMVSS at one time, with
compliance testing for one standard being performed right after that
for another. Adopting the SAE J2889-1 vehicle test weight specification
would require a test vehicle undergoing such a sequence of compliance
tests to be reloaded before and after FMVSS No. 141 testing slightly
increasing the costs of performing such testing.
    3. Specify a vehicle test weight that is specified by other NHTSA
FMVSS. These test weights are different depending on vehicle class and
brake system type. For pedestrian alert sound testing, a fairly lightly
loaded weight would be used, not the heavier loading specified in some
FMVSS. The vehicle test weight specifications used by other FMVSS are
as follows:
     FMVSS No. 105 is applicable to vehicles with hydraulic or
electric service brake systems and a GVWR greater than 3,500 kg (7,716
pounds). FMVSS No. 105 defines Lightly Loaded Vehicle Weight (LLVW),
for vehicles with a GVWR of 10,000 pounds or less, as equal to unloaded
vehicle weight plus 400 pounds including driver and instrumentation.
FMVSS No. 121 is applicable to vehicles with air brake systems. FMVSS
No. 121 tests at a weight equal to unloaded vehicle weight plus 500
pounds including driver and instrumentation plus not more than an
additional 1,000 pounds for a roll bar structure on the vehicle (if
needed).
     FMVSS No. 135 is applicable to vehicles with a GVWR of
3,500 kg (7,716 pounds) or less. FMVSS No. 135 defines Lightly Loaded
Vehicle Weight (LLVW) as equal to unloaded vehicle weight plus 180 kg
(396 pounds) including driver and instrumentation.
     FMVSS No. 500 is applicable to low speed vehicles. FMVSS
No. 500 defines the test weight as equal to unloaded vehicle weight
plus 78 kg (170 pounds) including driver and instrumentation.
    NHTSA does not believe that any one of these alternatives is better
for safety than any other. As was previously stated, NHTSA thinks that
the sound produced by a vehicle at the relatively low test speeds being
used for FMVSS No. 141 testing is not sensitive to minor changes in
vehicle loading. Therefore, NHTSA's goal in selecting a test vehicle
weight specification is to choose one that will minimize the economic
burden of performing compliance testing. We

think that this alternative is best achieved through the selection of
the third alternative listed above changing to the vehicle test weights
specified by other NHTSA FMVSS. Vehicle test weights will therefore be
specified by vehicle type and GVWR in the final rule.
Battery Charge During Testing
    In the NPRM, NHTSA proposed that, for sound measurement testing,
the vehicle's electric propulsion batteries, if any, be fully charged.
    NHTSA received comments on this topic from Advocates, Alliance/
Global, Honda, Navistar, and OICA. Advocates requested that NHTSA
either establish a battery charging procedure or require that the
vehicle be charged in accordance with the manufacturer's stated
charging procedure as outlined in vehicle documentation to ensure that
the ICE or other vehicle non-essential systems do not start during
sound testing procedures. Alliance/Global and OICA recommended using
the language from the charging procedure in SAE J2889-1. OICA stated
that many hybrids cannot be charged by external charge devices and that
by driving the vehicle a 100-percent charge level will nearly never be
reached. Honda pointed out that controlling the battery condition of a
hybrid vehicle to attain a specific level of charge can be difficult.
Honda recommended testing with the propulsion battery at a normal (as
is) condition and deleting this requirement as being unnecessary.
Navistar recommended that batteries be charged to the manufacturer's
recommended full state of charge.
Agency Response to Comments
    NHTSA agrees with Advocates that the battery needs to be
sufficiently charged during sound measurement testing so that the ICE
or other vehicle non-essential systems do not automatically activate.
Provided that this condition is met, the battery's state of charge
during sound measurement testing should have no impact on the safety of
the vehicle. NHTSA also agrees with commenters that precisely
controlling the battery condition of a hybrid vehicle to attain a
specific level of charge can be difficult. However, getting the
battery's state of charge during testing high enough that the ICE or
other vehicle non-essential systems do not automatically activate
should be feasible.
    Following review of the comments, NHTSA has decided to accept the
OICA and Alliance/Global recommendations and use the SAE J2889-1
language for the battery charge specifications in paragraph <IP_ADDRESS>.
This will accomplish our two objectives of (1) having a battery's state
of charge during testing be high enough that the ICE or other vehicle
non-essential systems do not automatically activate, and (2) specifying
a practicable, achievable, battery state of charge for testing.
Battery Thermal Management Systems
    In the NPRM, NHTSA proposed that measurements that included sounds
emitted by any component of a vehicle's propulsion battery thermal
management system are not considered valid. In addition, when testing a
hybrid vehicle with an ICE that runs intermittently, measurements that
contain sounds emitted by the ICE would not be considered valid
measurements.
    NHTSA received comments on this topic from OICA and Alliance/
Global. Commenters pointed out that the battery's thermal management
system might always be running when the vehicle is performing the test
scenarios. Therefore, they requested that NHTSA state that a battery
thermal management system that would normally be operating during the
specified test conditions need not be shut down. The commenters
suggested that the compliance test procedure prohibit the use of any
results which include sound from any vehicle systems other than those
which would be constantly engaged under the specified performance
conditions.
Agency Response to Comments
    NHTSA's goal during compliance testing is to measure the sound
produced by the vehicle when it is in its quietest state after sale to
the general public. It is not to test the vehicle in some artificially
quiet state that will never be attained by members of the driving
public. These comments are in accord with NHTSA's goal for compliance
testing. The commenters' statement, that a battery thermal management
system that would normally be operating during the specified test
conditions need not be shut down, is sensible and is consistent with
what NHTSA is trying to accomplish. Clarifying this will address an
important test factor that was not covered in the proposed version of
the regulatory text. This factor is addressed in S7.1.2 and S7.3.2 of
the regulatory text in this final rule. We have modified both of these
subsections by adding appropriate wording to include systems which
would be constantly engaged under the specified test performance
conditions (backing, stationary, forward motion at specified speeds).

K. Test Procedure

Indoor Testing

    In the NPRM, the agency tentatively concluded that outdoor
acoustics testing was preferable to indoor testing in hemi-anechoic
chambers. The agency explained that outdoor testing was more
representative of real-world vehicle-to-pedestrian interactions, and
that outdoor tests, especially pass-by tests, transmit to the
pedestrian not just vehicle-generated sounds (e.g., engine-powertrain
and pedestrian alert system), but also sounds from the vehicle body's
interaction with the atmosphere (wind noise) and road test surface
(tire noise). These complete sound profiles are transmitted to the
pedestrian over the ``outdoor ambient'' noise. Outdoor sounds also
contain a Doppler shift when the vehicle is moving relative to the
pedestrian.
    Conversely, the NPRM also explained, when a vehicle is tested on an
indoor dynamometer in a hemi-anechoic chamber, the body of the vehicle
is static and does not produce aerodynamic noise. The agency said that
it was unclear how representative the tire noise generated during
rotation on the curved dynamometer test rollers is of actual tire-road
noise. As explained, the vehicle approach and passing of the
microphones could be simulated by phasing a row of microphones next to
the vehicle, and interior tire noise could be digitally replaced with
exterior tire noise recordings, however, the agency has not determined
the fidelity of such methods.\146\ The agency voiced its concern about
both the availability of repeatable specifications for all aspects of
indoor testing and the availability of hemi-anechoic chambers in which
to conduct compliance testing.
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    \146\ see https://www.bksv.com/en/products/PULSE-analysis-software/acoustic-application-software/pass-by-noise-testing/indoor-testing-7793 (weblink last accessed 2November2016).
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    The NPRM mentioned the agency's belief that specifications for
outdoor testing have a more detailed history of objective and
repeatable performance than specifications for indoor testing. The
agency noted that a substantial amount of development and refinement
has gone into the test procedures and facilities used for outdoor
vehicle noise testing.
    The NPRM explained that SAE J2889-1 contains specifications on the
cut-off frequency of the indoor hemi-anechoic test facility and
requirements. However, the agency stated that it was not aware of
specifications for dynamometer drum surface textures, materials,
diameters, road loads coefficients (i.e., to produce

appropriate engine RPMs), etc. to allow comparable results between
different indoor dynamometers.
    Lastly, the NPRM explained that there are some advantages to
testing indoors. Testing in an indoor hemi-anechoic chamber would not
be influenced by weather conditions or high ambient noise levels that
can affect outdoor testing. Indoor testing could be more predictable
and time efficient than outdoor pass-by testing because testing time
would not be limited by weather and noise conditions at the test site.
The agency sought comment on the availability of hemi-anechoic
facilities that could accommodate indoor pass-by testing and the
desirability of including a test procedure for indoor pass-by testing
in this standard.
    Auto manufacturers and groups that represent them, along with SAE,
stated in their comments that the agency should allow indoor testing in
the compliance test procedure. According to Alliance/Global, OEMs would
prefer and support the use of indoor measurement facilities meeting
specifications contained in SAE J2889-1and ISO 16254. Alliance/Global
\147\ explained that in consideration of the practicability and
repeatability of the required tests, they believe that the test
conditions specified in the final rule should allow both the outdoor
testing and indoor hemi-anechoic testing which are specified in SAE
J2889-1. The Alliance/Global mentioned that some of its members have
indoor hemi-anechoic chambers for pass-by testing and some do not, but
all can gain access to them.
---------------------------------------------------------------------------

    \147\ NHTSA-2011-0148-0251.
---------------------------------------------------------------------------

    Honda stated it is necessary to include indoor test procedures in
the final rule and requested the agency allow use of an anechoic
chamber as an option for system testing. Honda stated that this option
will be more practical for automakers and can yield more consistent and
repeatable results without compromising the quality of the sound
measurements. Honda explained that indoor chamber tests are necessary
not only for pass-by tests, but for stationary vehicle tests using an
artificial speed signal and component-based pitch shifting tests.
    OICA stated that indoor test facilities meeting the specifications
in SAE J-2889-1 are an acceptable alternative to outdoor testing.
According to OICA, hemi-anechoic test facilities are widely available
for testing and should be allowed but not required. OICA mentioned that
some specifications for the facilities will be needed but did not
elaborate further.
    SAE explained that to achieve the goals of practical, repeatable,
and reproducible test results, the use of indoor and component level
test facilities are necessary. Furthermore, SAE stated that for
measuring the acoustic one-third octaves at any speed greater than
zero, the use of indoor facilities will be necessary to reduce
measurement uncertainty.
Agency Response to Comments
    In this final rule, the agency is specifying performance
requirements for vehicle-emitted sounds that are detectable and
recognizable to a pedestrian as a motor vehicle in operation. All
components of the vehicles' sound profile that convey the signature of
a motor vehicle in operation (including aerodynamic and tire noise) up
to the crossover speed are important facets of the vehicle's sound
performance. Upon consideration of the above comments, and as explained
further below, the agency has decided to only specify requirements for
outdoor testing as proposed in the NPRM. Vehicle manufacturers may
choose to test their vehicles indoors but the final rule has not added
that option to the regulatory text.
    As previously mentioned, the agency believes that outdoor testing
is more representative of real-world vehicle-to-pedestrian
interactions, and that outdoor tests, especially pass-by tests,
reproduce not just vehicle sounds that are internally generated (e.g.,
engine-powertrain and pedestrian alert system), but also sounds from
the vehicle body's interaction with the atmosphere (wind noise) and
road test surface (tire noise). When a vehicle is tested on an indoor
dynamometer in a hemi-anechoic chamber, the body of the vehicle is
static and does not produce aerodynamic noise. Additionally, the agency
does not know how representative the tire noise generated during
rotation on the curved dynamometer test rollers is of actual tire-road
noise.
    To date, the agency has had limited experience and access to
testing for and measuring acoustic sound levels on dynamometers in
hemi-anechoic test chambers. As we stated in the NPRM, the test setup
and test execution procedures for outdoor testing have long been
established.\148\ As mentioned previously, a substantial amount of
development and refinement has gone into the test procedures and
facilities used for outdoor vehicle noise testing. Establishment of
corresponding indoor procedures to be used in hemi-anechoic chambers on
dynamometers requires further development and validation. SAE J2889-1
contains specifications for indoor testing but does not appear to
provide the specifications for dynamometer drum surface textures,
materials, diameters, road loads coefficients (i.e., to produce
appropriate engine RPMs), etc. to allow comparable results between
different indoor dynamometers and outdoor ISO 10844 noise pads.
---------------------------------------------------------------------------

    \148\ 78 FR 2836 (Jan. 14, 2013).
---------------------------------------------------------------------------

    The agency continues to be concerned that hemi-anechoic chambers
that have four-wheel dynamometer drive capabilities are not widely
available for commercial testing. The agency was able to locate a large
number of outdoor 10844 noise pads in the United States, most of which
were available for paid use by outside parties. As mentioned in the
NPRM, one vehicle manufacturer stated that it has nine noise pads
throughout its global operations and we believe the standardized
outdoor noise pads have widespread commercial availability.
    While indoor testing is appealing because it eliminates inclement
weather and seasonal downtimes, which may provide more flexibility for
manufacturers, we believe this is outweighed by the fact that outdoor
testing will provide a more representative real-world condition
including realistic interaction of the vehicle and vehicle alert system
with the outdoor environment. The NHTSA acoustic measurement procedures
incorporate strategies such as the rejection of test runs having
extraneous background noise to ensure that interaction with the outdoor
environment does not affect test results.
    Several of the commenters explained that we should allow indoor
testing as specified in SAE J2889-1. In addition to conducting indoor
testing in a hemi-anechoic chamber using a dynamometer to simulate
vehicle motion, it is possible to conduct pass-by testing in an indoor
hemi-anechoic chamber, provided sufficient space is available to allow
testing of all test conditions. SAE J2889-1 seems to allow for both
methods of indoor testing. Full vehicle indoor pass-by testing in a
hemi-anechoic chamber without a dynamometer (i.e., an indoor track)
would capture elements of the vehicle sound profile (including
aerodynamic and tire noise) that contribute to the detectability of the
vehicle's sound signature until the vehicle reaches the crossover
speed. Therefore, indoor pass-by testing in a hemi-anechoic chamber is
able to record all aspects of the vehicle's sound profile while still
achieving the convenience and efficiency advantages of indoor

testing. In this case, an indoor pass-by procedure, without a
dynamometer, would be the same as the outdoor pass-by procedure
contained in Section <IP_ADDRESS> of SAE J2889-1 DEC 2014 except that the
50-meter radius free of reflecting objects around the test track would
not apply. The provision in SAE J2889-1 DEC 2014 that the hemi-anechoic
chamber used for indoor pass-by testing comply with ISO 3745 or ISO
26101 would ensure that reflection from the test enclosure would not
interfere with the vehicle's sound measurement.
    The Alliance/Global \149\ mentioned that some OEMs have indoor
facilities large enough to execute full vehicle pass-by tests at
required test speeds but did not provide corresponding details. The
agency is not aware of the availability of hemi-anechoic chambers that
are large enough to accommodate indoor pass-by tests and continues to
believe that the existence of such facilities is limited, which would
be an issue if NHTSA favored this approach as an option and wanted to
conduct its own compliance testing in such an environment.
---------------------------------------------------------------------------

    \149\ NHTSA-2011-0148-0251.
---------------------------------------------------------------------------

    SAE stated that when measuring the acoustic one-third octaves at
any speed in excess of zero, the use of indoor facilities is necessary
to reduce measurement uncertainty. SAE also explained that to achieve
the goals of practical, repeatable, and reproducible test results, the
use of indoor and component level test facilities are necessary. NHTSA
has issued a technical report presenting an analysis of its indoor test
data for hybrid and electric vehicles.\150\ This report includes the
analysis of acoustic measurements in hemi-anechoic chambers equipped
with chassis dynamometers. The analysis includes data for electric,
hybrid, and internal combustion engine vehicles and examines ambient
noise, repeatability and reproducibility of vehicle acoustic signals
(measurements). The analysis includes a limited comparison of indoor
and outdoor test data provided by Transport Canada and NHTSA in
conjunction with Transportation Research Center (TRC).
---------------------------------------------------------------------------

    \150\ See Hastings et al. ``Analysis of Indoor Test Data for
Hybrid and Electric Vehicles.'' (2015) U.S. Dept. of Transportation,
Washington, DC.
---------------------------------------------------------------------------

    Test results between two indoor test sites (General Motors Milford
Proving Grounds (MPG) and International Automotive Components (IAC))
and one outdoor test site (TRC) were compared. Repeatability, as
measured by standard errors for each indoor site was good. The
estimated mean value was found to be within 0.5 to 0.75 dB of the true
mean with 95% confidence depending on the one-third octave band being
analyzed. Reproducibility of estimated means between the two indoor
tests sites was about 2 dB on average; however, individual measurements
had significant variation resulting in a 95% confidence interval range
of +/-2.5 dB to +/-6.7 dB depending on the one-third octave band.
    In addition to comparing the two indoor test facilities to one
another, both facilities were also compared with outdoor measurements
made at TRC. Measurement reproducibility between each indoor test
facility and TRC was evaluated by comparing the average values of each
vehicle at each one-third octave band for each speed at the respective
sites. Results indicate that the indoor facilities tend to have higher
acoustic sound levels, especially at 20 and 30 km/h. Because the
differences are smaller at 10 km/h, it is not likely that the
differences in acoustic reflections from the indoor floor and the
outdoor pavement are causing the difference. Rather, it is likely that
the tire/dynamometer interaction is producing the higher sound pressure
levels. We believe that these results show that it may be necessary to
conduct further studies about the tire/dynamometer interaction before
any level of confidence can be established with the procedures
utilizing a dynamometer. Because our research shows that the tire/
dynamometer interaction could influence the repeatability of the test
and because there are no specifications for dynamometer drums or other
aspects of indoor testing that would increase repeatability, we believe
that the procedures for indoor testing are not currently sufficient to
be used by the agency for compliance testing.
    Considering confidence intervals of estimated mean values for
individual vehicle/speed/frequency pairs, the standard deviation
between TRC and MPG was as high as 5 dB and the standard deviation
between TRC and IAC was as high as 4.7 dB. Thus 95% confidence
intervals would be as large as +/-9.8 and +/-9.2 dB respectively. It is
important to keep in mind that these confidence intervals included not
only site-to-site differences, tire/dynamometer differences, and
differences as a result of using different vehicles and in some cases
different model years, therefore, these confidence intervals can be
considered a worst case. It is expected that confidence intervals for
the same vehicles would be smaller.
    In response to the SAE comment, we note the limited data available
seem to demonstrate that there is measurement variability inherent in
the procedures utilized indoors and outdoors. For the one-third octave
bands, higher levels of variability were noted between several indoor
facilities and between indoor and outdoor facilities. The variability
noted may be associated with different dynamometers used and the fact
that the comparison vehicles were not in all cases the exact same
vehicles. The agency believes that further research and specification
refinements are required to establish and properly validate indoor
testing utilizing dynamometers. Further discussion on test
repeatability and reproducibility is provided in Section III.K of this
document.
    In conclusion, after considering recent agency research and the
comments received on the NPRM, the agency continues to believe outdoor
testing on an ISO test pad is preferable to indoor testing in hemi-
anechoic chambers with dynamometers. Section S7 of the final rule
specifies the test procedures for outdoor testing.
    We again note that vehicle manufacturers' testing can deviate from
the procedures in an FMVSS, which communicate the method the agency
will use to determine whether a vehicle complies with the requirements
of that standard. Vehicle manufacturers may choose to test their
vehicles indoors for the purpose of demonstrating compliance with the
standard, but the final rule has not added that option to the
regulatory text. The agency believes that further developments,
refinements and validation are required before the indoor hemi-anechoic
chambers equipped with chassis dynamometers can be specified by the
agency. If further developments, data and information become available
in the future the agency may decide at that time to revisit the
possibility of adding the indoor testing option.
Test Surface for Compliance Testing
    In the NPRM, NHTSA proposed that the test surface used during
compliance testing meet the requirements of ISO 10844:2011.
    NHTSA received comments on this topic from OICA, Alliance/Global,
and EMA. OICA and Alliance/Global recommended that NHTSA allow
compliance testing on a test surface meeting the requirements of either
ISO 10844:2011 or ISO 10844:1994. They supported this recommendation by
stating that they believe that surfaces meeting the requirements of ISO
10844:1994 and ISO 10844:2011 are technically equivalent.

Agency Response to Comments
    NHTSA agrees with OICA and Alliance/Global that surfaces meeting
the requirements of ISO 10844:1994 and ISO 10844:2011 seem to be
technically equivalent. Our understanding is that the major impetus for
the 2011 update of the ISO 10844 standard was to incorporate laser
profilometry technology that has recently become available which allows
more precise measurements of the porosity of the surface. NHTSA's
understanding is that the majority of surfaces that are within the 1994
standard should pass the 2011 standard without change. We know that
this was the case for the Transportation Research Center, Inc.'s
(TRC's) ISO sound pad that has been used for much of NHTSA's testing.
Prior to NHTSA's testing, TRC's ISO sound pad was certified under ISO
10844:1994. At NHTSA's request, TRC recertified their sound pad under
ISO 10844:2011; this required certification testing but no structural
changes to the sound pad.
    Thus a 1994 certified sound pad is likely to generate a sound
profile equivalent to that generated on a 2011 certified surface.
During the NHTSA's 2011 testing, a Ford Fusion vehicle was tested on
both ISO 10844-1994 and ISO 10844-2011 surfaces and no significant
difference in sound profile levels were found.
    For light vehicle sound measurement, NHTSA has had no difficulties
in finding sound pads certified to ISO 10844-2011 for its testing.
    NHTSA prefers to harmonize FMVSS No. 141 with SAE J2889-1 absent
rationale for departing from that standard. The updated version of SAE
J2889-1 that was released in December 2014 specifies performing outdoor
sound testing on a surface that meets the requirements of ISO
10844:1994, ISO 10844:2011, or ISO 10844:2014. Since NHTSA believes
these three surfaces to be technically equivalent, we are expanding the
list of test surfaces specified for FMVSS No. 141 compliance testing to
include those certified to any of the above three versions of ISO
10844.
    Based on the preceding discussion, all types of vehicles to which
this rule applies will be tested on surfaces that meet either ISO
10844:1994, ISO 10844:2011, or ISO 10844:2014 specifications.
Vehicle Start-Up/Activation
    The NPRM proposed in Section S5.1.1 that a vehicle must emit sound
meeting the specifications for the stationary-but-active operating
condition ``within 500 milliseconds of activation of the vehicle's
starting system.'' The NPRM test procedure to measure compliance with
the proposed stationary-but-active condition included a separate
microphone two meters in front of the vehicle on the vehicle
centerline.\151\ We stated in the NPRM that this other microphone is
needed in addition to the two specified in SAE J2889-1 to measure the
sound that a pedestrian standing directly in front of a vehicle would
hear. We wanted to ensure that there was no drop off in sound level
from the side of the vehicle where the measurement is taken to the
front of the vehicle, where the sound would be beneficial in warning
pedestrians standing in front of the vehicle of its presence.
---------------------------------------------------------------------------

    \151\ The vehicle centerline is referred to as the CC' line in
the test setup diagram in J2889-1.
---------------------------------------------------------------------------

    There were a number of comments on the proposed stationary-but-
active requirement, focusing on two aspects of the regulatory language:
(1) The start-up delay of 500 milliseconds for the alert to begin, and
(2) the meaning of ``activation of the vehicle's starting system'' for
HVs and EVs.
    We note here that these two issues are directly related to the
sound-at-stationary requirement which is discussed in Section III.C,
``Critical Operating Scenarios,'' in today's final rule. Many of the
NPRM comments addressed start-up delay and definition of `activation'
to the extent that they opposed any requirement for an alert sound in
the ``Stationary-but-Active'' operating condition. Because comments on
the ``Stationary-but-Active'' operating condition were summarized in
that previous section of this final rule, and we wish to avoid
duplication, we are not repeating all of those comments here. Rather,
we focus here on aspects of the Stationary-but-Active comments that
directly relate to Start-up, the definition of Activation, and the
associated measurement procedure.
    Commenters, mainly OEMs, said that 500 milliseconds is too rapid to
emit sound in a controlled fashion, and that it is technically
unfeasible to achieve the one-third octave band levels in that short an
interval.
    Advocates stated that NHTSA should provide data to support the
requirement that the alert sound must initiate and meet the acoustic
specifications within 500 milliseconds of activation to justify that
this is an appropriate amount of time to warn pedestrians. Advocates
also suggested the agency should investigate the delay times of typical
vehicles, i.e., the delay between when a vehicle is started and when it
is able to begin moving. NHTSA's analysis to support the 500
milliseconds requirement also should consider whether a lower sound
level is appropriate for the parked condition.
    Honda stated that NHTSA should clarify the definition and the
measurement procedure of ``after the vehicle's starting system is
engaged'' in the NPRM. If the definition of ``activation is the instant
when the driver operates the vehicle's starting system, then it may be
possible to engage the alert sound within 500 milliseconds. However, it
may be difficult to consistently achieve the specified one-third octave
levels in each of the eight bands as specified by NHTSA in the proposed
rule.
    Mitsubishi stated that the alert sound should start when a vehicle
is shifted out of Park, and the 500 milliseconds interval should start
at that point. Mitsubishi stated that it would be technically
impracticable to meet the 500 milliseconds requirement from the moment
a driver first activates the propulsion system. Mitsubishi also pointed
out the need for NHTSA to define ``activation of the vehicle's starting
system.''
    Denso commented that 500 milliseconds is not enough time to
initiate the alert sound, and that only individual vehicle
manufacturers can determine how much of a delay is necessary for a
given vehicle. Denso also said that the safety risk to pedestrians can
be avoided if the alert sound is emitted beginning at the moment that a
vehicle commences motion. In that regard, Denso suggested introducing
minimum SPL requirements for a vehicle commencing-motion sound in place
of the minimum SPL requirements for a vehicle at ``start-up and
stationary but activated.''
    WMU stated that 500 milliseconds should provide enough time from a
safety standpoint because, in most cases, a driver does not initiate
movement for several seconds after first starting up a vehicle. This
would give any nearby pedestrian several seconds of acoustic warning.
    We also received comments from Alliance/Global stating that, for
testing in the stationary condition, we should amend the test procedure
to eliminate the additional measurement at a point two meters in front
of the vehicle on the vehicle centerline since that would have applied
only to the stationary test which they were in favor of excluding from
the final rule.
    A number of commenters challenged the proposed requirement on the
basis that 500 milliseconds is too short an interval for an alert
system to become active upon vehicle start-up because

vehicle manufacturers cannot ensure that an alert system is fully
engaged and operating at the required sound level in such a short
amount of time. Commenters stated that one reason for this is speaker
transients, i.e., once sound production begins it takes a while for it
to stabilize. Therefore, while a vehicle's alert system may be capable
of emitting some level of sound within 500 milliseconds, it may not
achieve the specified sound pressure levels in each one-third octave
band until a considerably longer time has elapsed after start-up.
    Commenters also questioned how NHTSA intends to measure the lag
time between starting system activation and the initiation of the alert
sound. OEMs and industry groups commented that the NPRM did not define
what ``activation of a vehicle's starting system'' means exactly.
Without an exact definition, any attempt to measure the lag time would
be subject to arbitrary selection of a starting point which could
result in inconsistent measurements.
Agency Response to Comments
    As a consequence of our decision discussed in Section III.C of this
final rule to require sound at stationary only when a vehicle's gear
selector is not in ``Park,'' and also due to the fact that vehicles are
designed so that they must be in ``Park'' in order to be started, the
proposed requirement for an alert to initiate within 500 milliseconds
of vehicle activation is no longer applicable. Therefore, that proposed
requirement is not included in this final rule.
    In addition, our decision on sound-at-stationary obviates the need
for NHTSA to define the term ``activation of the vehicle's starting
system'' as it appeared in the proposed S5.1.1 regulatory text. Because
alert system engagement will not depend on when a vehicle is started,
no definition of ``activation'' is necessary.
    We note that this decision does not mean that vehicles would have
to be in motion before they are required to emit an alert sound.
Vehicles that are not moving must emit an alert sound unless they are
in a condition typical of a vehicle that may remain parked for some
time. Vehicles that are stationary still would have to emit sound if
they are, for example, waiting at a red traffic light (assuming the
drivers do not shift to Park, in the case of automatic transmission
vehicle, or apply the parking brake in the case of manual transmission
vehicles). This means that vehicles that are in Park with an activated
ignition and which are not in traffic, and which therefore are unable
to drive off until they are put into gear, would not have to emit
sound. For example, vehicles that are parked but idling so that
occupants can use the heat or air-conditioning would not have to emit
sound. We recognize that this will distinguish EVs/HVs from ICE
vehicles since the latter emit sound whenever their engines are
running, even in Park (although this may not be the case for ICE
vehicles with stop-start capability.) On the other hand, an ICE vehicle
could be parked with its ignition in the `ON' position but with its
engine not running.
    We have decided to maintain the use of the additional front-center
microphone for determining compliance with the stationary-but-active
requirement. We believe this is important to ensure that pedestrians
standing or passing in front of EVs and HVs are able to detect them. If
the agency did not ensure that sounds produced by EVs and HVs met the
minimum sound requirements in today's final rule two meters in front of
the vehicle it would be possible that a pedestrian standing in front of
an EV or HV would not be able to hear it within the vehicle's safe
detection distance.
Vehicle Speed During Compliance Testing
    In the NPRM, NHTSA proposed that the instrumentation used to
measure vehicle speed during compliance testing be capable of
continuous speed measurement over the entire zone from the `AA' Line to
the `BB' Line with an accuracy of 1.0 km/h.
    NHTSA's proposal also set a speed tolerance for valid test runs.
For a test run to be valid, the vehicle speed must be within 1.0 km/h of the target speed for that run as the vehicle travels
through the measurement zone from the AA' Line to the PP' Line.
    NHTSA received comments on the instrumentation used to measure
vehicle speed during compliance testing from Honda and Alliance/Global.
Commenters requested that NHTSA allow independent,\152\ as well as
continuous, speed measurement during compliance testing. Honda
requested that the accuracy specification for speed measurement
equipment match that contained in SAE J2889-1 ( 0.5 km/h
for continuous speed measurement devices or  0.2 km/h for
independent speed measurement instrumentation). Alliance/Global also
requested that the accuracy specification for independent speed
measurement equipment match that contained in SAE J2889-1.
---------------------------------------------------------------------------

    \152\ SAE J2889-1 defines independent speed measurement as being
when two or more separate devices are used to measure the vehicle's
speed as it crosses the AA', BB', and PP' Lines. In comparison,
continuous speed measurement uses one device to measure the
vehicle's speed as it travels through the entire zone from the AA''
Line to the BB' Line.
---------------------------------------------------------------------------

    NHTSA received comments on the speed tolerance for valid test runs
while the vehicle is traveling forward from Alliance/Global. They
recommended changing the speed tolerance to -0.0/+2.0 km/h. Their
justification for recommending this is to correct the inconsistency
between the standard's performance requirement and compliance test
procedure while still maintaining an overall tolerance of 2.0 km/h.
Agency Response to Comments
    NHTSA wants to harmonize FMVSS No. 141 with SAE J2889-1 when
feasible and consistent with the agency's focus on safety. For the
instrumentation used to measure vehicle speed during compliance
testing, we see no reason not to harmonize with SAE J2889-1.
    Allowing independent speed measurement will not affect compliance
test severity (or the safety benefits provided by this standard)
because the 10 meters between the AA' Line and the PP' Line is not
enough distance to permit the vehicle to vary more than minimally from
the target speed.
    In the most recent versions of SAE J2889-1, the accuracy
specification for the continuous speed measurement instrumentation
(0.5 km/h) is tighter than the earlier SAE J2889 (Sept
2011) version and the NHTSA's proposal of 1.0 km/h. The SAE
J2889-1 continuous speed measurement accuracy specification is known to
be both feasible and practical since NHTSA's commercially-purchased
sound measurement equipment package includes speed measurement
instrumentation with an accuracy specification of 0.1 km/h.
The SAE J2889-1 independent speed measurement accuracy specification
(0.2 km/h) is tighter than the SAE J2889-1 continuous speed
measurement accuracy specification. While NHTSA does not have first-
hand knowledge of independent speed measurement, we believe that the
SAE J2889-1 accuracy specification should be both feasible and
practical. Therefore, NHTSA accepts Honda's recommendation and will
make the FMVSS No. 141 speed measurement instrumentation accuracy
specification identical to that contained in the most recent version of
SAE J2889-1.
    Alliance/Global made a good point regarding the speed tolerance for
valid test runs while the vehicle is traveling

forward. NHTSA's proposal required the vehicle to emit sounds having a
specified level that varied with the speed of the vehicle. The required
level varied in a stepwise manner with the steps occurring at multiples
of 10 km/h, i.e., at 10, 20, and 30 km/h. In other words, NHTSA
proposed that the vehicle emit sound with one sound pressure level at,
for example, 9.9 km/h and with a different sound pressure level at 10.0
km/h. NHTSA also proposed that compliance testing be performed at
multiples of 10 km/h, i.e., at 10, 20, and 30 km/h. The problem is
that, when testing at, for example, 10 km/h, due to the 1.0
km/h speed tolerance, valid tests could be performed at any speed from
9.0 through 11.0 km/h, inclusive. Therefore, a test performed at 9.9
km/h would be a valid test as would a test performed at 10 km/h.
However, as previously discussed, these two tests would have different
required sound pressure levels.
    The Alliance/Global suggestion would avoid this problem by changing
the speed tolerance to -0/+2 km/h. This would mean that a valid 10 km/h
test would have to have a speed in the range from 10.0 to 12.0 km/h,
inclusive. Alternatively, the proposed 10 km/h pass-by compliance test
would become an 11 km/h pass-by test with a 1.0 km/h speed
tolerance.
    The Alliance/Global suggestion is a departure from SAE J2889-1
(which has a 10 km/h pass-by test with a 1.0 km/h speed
tolerance). However, this idea allows NHTSA to vary the required level
of the sounds emitted by the vehicle in a stepwise manner with the
steps occurring at multiples of 10 km/h, i.e., at 10, 20, and 30 km/h.
Adopting this suggestion will have only a very minor effect on the
severity of FMVSS No. 141 compliance tests making them a little easier
to pass since each test will now, on the average, be performed at a 1.0
km/h faster speed. Therefore, tires, aerodynamics, etc., will
contribute slightly more sound thereby reducing the sound that needs to
be generated by the vehicle's external sound generation system.
However, the differences in sounds due to this 1.0 km/h speed up are
expected to be minor.
    Considering all of the preceding discussion, NHTSA has decided to
adopt the Alliance/Global suggestion and change the compliance test
speed tolerance to -0/+2 km/h. NHTSA will make this revised tolerance
applicable to all three moving vehicle compliance tests, including the
10, 20, and 30km/h pass-by tests.
Repeatability/Reproducibility
    NHTSA is addressing measurement variability in the final rule as a
result of comments that were received on the NPRM, coupled with
additional testing and analysis conducted by the agency which indicate
that measurement repeatability and reproducibility (the latter across
test facilities), may impact compliance testing results if not properly
accounted for. The NPRM discussed how the agency would attempt to
minimize test variability. However, adequate treatment was not given to
the potential effect measurement tolerance may have on compliance
testing.
    A critical component of every Federal motor vehicle safety standard
is a compliance test procedure that is objective, repeatable and
reproducible. The test procedure must be objective such that differing
parties, including OEMs and test laboratories will interpret and
execute the procedures the same way. The test procedure must be
repeatable and reproducible such that the results obtained are the same
results from test-to-test at the same test facility and across
different test facilities.
    In the NPRM, the agency discussed its approach for minimizing test
variability. The test procedure specified in the NPRM requires that all
tests be conducted on a track with a surface that meets the
requirements of ISO 10844:2011 which specifies, among other things, a
very particular type of pavement to be used so as to minimize the
contribution of tire noise to the sound measured. As mentioned in the
NPRM, using a specified test track surface would minimize test
variability.
    The NPRM also contained provisions for specific environmental
conditions (temperature and wind specifications), vehicle conditions
(tire set-up and conditioning, door and window opening adjustments,
vehicle accessory settings and vehicle loading), and track/
instrumentation layout restrictions. These provisions are also
important for minimizing test variability. The NPRM explained that the
instruments used to make the acoustical measurements required under our
proposal must meet the requirements of paragraph 5.1 of SAE J2889-1.
This SAE paragraph describes procedures for calibration of the
acoustical equipment. Use of such instruments and calibration
procedures will ensure that test measurements can be duplicated
repeatedly on the same vehicle at one facility, or at different test
facilities.
    In the NPRM, the agency addressed the issue of intermittent vehicle
sound caused by the vehicle's battery cooling fan by requiring that any
vehicle sound measurements taken while the cooling fan is operating be
discarded. At the time, the agency believed that this helped address
repeatability issues caused by battery cooling fans. The NPRM required
that for all operating conditions, four consecutive valid measurements
be within 2 dB(A). As explained, this repetition and decibel level
restriction would ensure repeatability of vehicle sounds without the
presence of unwanted ambient spikes, other non-vehicle sounds, or
intermittent sounds the vehicle may happen to make that are not
associated with its normal operating sound.
    The agency received individual comments from Honda, Alliance/
Global, Toyota, SAE, Nissan, and Denso. These comments generally fell
into two categories: The expected variance in recorded measurements in
terms of size and sources of variability; and the consequences of
manufacturers taking steps to address repeatability in compliance
testing.
    Honda offered two comments regarding measurement variability. The
first dealt with outdoor testing stating ``The Notice of Proposed
Rulemaking (NPRM) requires testing of the one-third octave requirement
at an outdoor site, but we are concerned that this poses practical
concerns due to the low repeatability of test results which will be
influenced by the presence of background noise.'' Honda also explained
that it believes the ``like vehicle requirements'' are too stringent,
and practically cannot be met due to the variability of sound producing
devices. Honda provided an attachment with plots that indicate the
differences in four tests by the same vehicle is more than 3dB.
    Alliance/Global stated, ``The loudness in NHTSA's proposal is
created by summing required broadband content in eight one-third octave
bands when the sound in each band is already loud enough for detection
purposes. The resultant sum is a sound that is, at a minimum, 6 dB
louder than necessary. When a compliance margin (for repeatability and
reproducibility) and production variation is added on, this proposed
alert sound becomes 9-12 dB louder than necessary. The decibel sound
scale is logarithmic, so this represents a doubling in the perceived
sound levels.''
    Alliance/Global further said that they were concerned that the run-
to-run variability is greater than the levels proposed in the NPRM.
They stated, ``Given the uncertainties noted by SAE for the measurement
of one-third octaves proposed in the NPRM, we

suggest that the tolerance should be increased to 9 dB. This applies to
all measures of performance for compliance purposes.''
    SAE discussed measurement uncertainties in its comments. SAE said
that for the measurements of overall Sound Pressure Levels (SPL) the
identified site-to-site variation at 80% confidence interval is 1.4 dB. SAE said that the uncertainty for the measurements of
one-third octave results ``has not yet been determined,'' but will be
larger than the uncertainty for the overall SPL. According to SAE, for
indoor measurements, the site-to-site variation of one-third octave
levels at 95% confidence interval is expected to be in excess of 2 dB. For outdoor measurements, the site-to-site variation at 95%
confidence interval is expected to be in excess of 6.0 dB.
According to SAE, these estimated uncertainties should be considered
when specifying tolerances for regulatory compliance. SAE also
mentioned that any variation in sound output due to vehicle component
production variability will be in addition to the measurements
variation noted.
    Denso commented on the variability of the speaker unit itself,
stating ``There is inherent variability in vehicle sound
characteristics and in speaker and amplifier characteristics and
performance. When combining this variability, it is very difficult to
limit the sound difference within 3 dB(A) between the two vehicles,
even for vehicles having nominally identical sound systems.'' Denso
also went on to comment that for a 40 degree rise in temperature (0
[deg]C to 40 [deg]C) the overall sound level would decrease by 1 dB.
Nissan, similar to Denso, suggested in its comments that sound levels
must be increased by the variation of speakers.
    In general, comments received stated that the variability present
in the vehicles sound measurement is higher than the agency accounted
for in the NPRM, and that variability could be substantial even when
using the measurement procedures set forth in SAE J2889-1. There was
also concern expressed by the commenters that if manufacturers increase
vehicle alert sound pressure levels above the minimum standards to
ensure a reasonable compliance margin, the vehicle alert sound may
become excessively loud.
Agency Response to Comments
    Upon review and further consideration of the comments received it
appears that the provisions for addressing variability included in the
NPRM and discussed above are not sufficient to properly address all the
test variability inherent in measuring vehicle acoustic alert sounds.
To further address the issue of variability, the agency has decided to
reduce the minimum standards required in this final rule by 4 dB in