Acoustic Glossary

A-weighting: A frequency-response adjustment of a sound-level meter that makes its reading conform, very roughly, to human response.

Absorb: Sound waves are converted into energy such as heat that is then captured by insulation.


Absorption coefficient: The fraction of sound energy that is absorbed at any surface. It has a value between 0 and 1 and varies with the frequency and angle of incidence of the sound.

AC3: See Dolby Digital.

Acoustic impedance: Z=R+jX ; [ML-4T-1] ; mks acoustic ohm *(Pa • s/m3) ---of a surface, for a given frequency, the complex quotient obtained when the sound pressure averaged over the surface is divided by the volume velocity through the surface. The real and imaginary components are called, respectively, acoustic resistance and acoustic reactance.

Acoustic material: Any material considered in terms of its acoustical properties. Commonly and especially, a material designed to absorb sound.

Acoustics: The science of sound. It can also refer to the effect a given environment has on sound.

AES: Audio Engineering Society.


Airflow resistance, R: [ML-4T-1]; mks acoustic ohm *(Pa•s/m3) ---the quotient of the air pressure difference across a specimen divided by the volume velocity of airflow through the specimen. The pressure difference and the volume velocity may be either steady or alternating.

Airflow resistivity: *[ML-3T-1]; mks rayl/m *(Pa•s/m3) ---of a homogeneous material, the quotient of its specific airflow resistance divided by its thickness.


Ambience: The acoustic characteristics of a space with regard to reverberation. A room with a lot of reverb is said to be "live"; one without much reverb is said to be "dead".

Ambient noise: The composite of airborne sound from many sources near and far associated with a given environment. No particular sound is singled out for interest.


Analog: An electrical signal whose frequency and level vary continuously in direct relationship to the original electrical or acoustical signal.

Anechoic: Without echo.

Anechoic chamber: A room designed to suppress internal sound reflections. Used for acoustical measurements.

Arithmetic mean sound pressure level: Of several related sound pressure levels measured at different positions or sound pressure levels measured at different positions or different times, or both, in a specified frequency band, the sum of the sound pressure levels divided by the number of levels.


Artificial reverberation: Reverberation generated by electrical or acoustical means to simulate that of concert halls, etc., added to a signal to make it sound more lifelike.

ASA: Acoustical Society of America.



Audible frequency range: The range of sound frequencies normally heard by the human ear. The audible range spans from 20Hz to 20,000Hz

Audiophile: A person who is interested in sound reproduction.

Auditory area: The sensory area lying between the threshold of hearing and the threshold of feeling or pain.

Aural: Having to do with the auditory mechanism.

Average room absorption coefficient: Total room absorption in sabins or metric sabins, divided by total room surface area in consistent units of square feet or square meters.


Axial mode: The room resonances associated with each pair of parallel walls.

Background noise: Noise from all sources unrelated to a particular sound that is the object of interest. Background noise may include airborne, structureborne, and instrument noise.

Baffle: A moveable barrier used to achieve separation of signals from different sources. The surface or board upon which a loudspeaker is mounted.

Bandpass filter: A filter that attenuates signals both below and above the desired passband.

Bandwidth: The total frequency range of any system. Usually specified as something like: 20-20,000Hz plus or minus 3 dB.

Bass: The lower range of audible frequencies.

Beaming: The phenomenon of sound being emitted within a comparatively small solid angle. This characteristic becomes more acute as the frequency increases.


Binaural: A situation involving listening with two ears.

Block: Building a continuous barrier that traps or stops air movement from one side of a partition to the other, greatly reducing airborne sound transmission.

Boomy: Listening term, refers to an excessive bass response that has a peak(s) in it.


Bright: Listening term. Usually refers to too much upper frequency energy.

Broad band noise: Spectrum consisting of a large number of frequency components, none of which is individually dominant.

Byte: A term used in digital systems. One byte is equal to 8 bits of data. A bit is the elemental "high" or "low" state of a binary system.

Cavity: A space between wall studs or attic joists where insulation is typically installed.

Channel balance: In a stereo system, the level balances between left and right channels. Properly balanced, the image should be centered between the left-right speakers. In a home-theater system, refers to achieving correct balance between all the channels of the system.


Clipping: Refers to a type of distortion that occurs when an amplifier is driven into an overload condition. Usually the "clipped" waveform contains an excess of high-frequency energy. The sound becomes hard and edgy. Hard clipping is the most frequent cause of "burned out" tweeters. Even a low-powered amplifier or receiver driven into clipping can damage tweeters which would otherwise last virtually forever.

Coherence: Listening term. Refers to how well integrated the sound of the system is.

Coloration: Listening term. A visual analog. A "colored" sound characteristic adds something not in the original sound. The coloration may be euphonically pleasant, but it is not as accurate as the original signal.

Comb filter: A distortion produced by combining an electrical or acoustical signal with a delayed replica of itself. The result is constructive and destructive interference that results in peaks and nulls being introduced into the frequency response. When plotted to a linear frequency scale, the response resembles a comb, hence the name.

Compression: In audio, compression means to reduce the dynamic range of a signal. Compression may be intentional or one of the effects of a system that is driven to overload. It is also the portion of a sound wave in which molecules are pushed together, forming a region with higher-than-normal atmospheric pressure.

Critical band: In human hearing, only those frequency components within a narrow band, called the critical band, will mask a given tone. Critical bandwidth varies with frequency but is usually between 1/6 and 1/3 octaves.

Critical distance: The distance from a sound source at which direct sound and reverberant sound are at the same level.

Critical frequency: The frequency below which standing waves cause significant room modes.

Cross-talk: Unwanted breakthrough of one channel into another. Also refers to the distortion that occurs when some signal from a music source that you are not listening to leaks into the circuit of the source that you are listening to.

Crossover frequency: In a loudspeaker with multiple radiators, the crossover frequency is the 3dB point of the network dividing the signal energy.

Cutoff frequency: Of an anechoic wedge or set of wedges, the lowest frequency above which the normal incidence sound absorption coefficient is at least .990.

Cycles per second: The frequency of an electrical signal or sound wave. Measured in Hertz (Hz).

Damp: To cause a loss or dissipation of the oscillatory or vibrational energy of an electrical or mechanical system.

DB: The term used to identify ten times the common logarithm of the ratio of two like quantities proportional to power or energy. (See level, sound transmission loss.) Thus, one decibel corresponds to a power ratio of 100.1.

DB (A): A sound-level meter reading with an A-weighting network simulating the human-ear response at a loudness level of 40 phons.

DB (B): A sound-level meter reading with a B-weighting network simulating the human-ear response at a loudness level of 70 phons.



Decay rate: d, [T-1]; dB/s?for airborne sound, the rate of decrease of vibratory acceleration, velocity, or displacement level after the excitation has stopped.



Diffraction: A change in the direction of propagation of sound energy in the neighborhood of a boundary discontinuity, such as the edge of a reflective or absorptive surface.

Diffuse field: An environment in which the sound pressure level is the same at all locations and the flow of sound energy is equally probable in all directions.

Digital: A numerical representation of an analog signal. Pertaining to the application of digital techniques to common tasks.

Dipole: An open-back speaker that radiates sound equally front and rear. The front and rear waves are out of phase and cancellation will occur when the wavelengths are long enough to "wrap around". The answer is a large, wide baffle or to enclose the driver creating a monopole.

Directivity index (DI): The difference between sound pressure level in any given direction in the acoustic far field and the average sound pressure level in that field.

Distortion: Anything that alters the musical signal. There are many forms of distortion, some of which are more audible than others.

Divergence: The spreading of sound waves which, in a free field, causes sound pressure levels in the far field of a source to decrease with increasing distance from the source.

Dolby Digital: Is a five-channel system consisting of left, center, right and left rear, right rear channels. All processing is done in the digital domain . Unlike Dolby Prologic in which the rear effects channels are frequency limited to approx. 100-700Hz, Dolby Digital rear channels are specified to contain the full 20-20Khz frequency content. The AC3 standard also has a separate subwoofer channel for the lowest frequencies.

Dolby Prologic: Is a four-channel system consisting of left, center, right and rear channel, (the single rear channel is usually played through two speakers).

DSP: Digital Signal Processing. DSP can be used to create equalization, compression, etc. of a digital signal.

DTS: Digital Theater System. A multi-channel encoding/decoding system. Used in some movie theaters. Also now included in some home-theater processors. A competitor to Dolby Digital.

DVD: Digital Video Disc or Digital Versatile Disc. A relatively new standard that seeks to combine better-than-laser-disc quality video with better-than-CD quality audio in a disc the size of a CD. Requires special players. Seems to be a viable candidate to replace both Laser Discs and CDs, but the jury is still out.

Dynamic headroom: The ability of an audio device to respond to musical peaks. For example, an amplifier may only be capable of a sustained 100 watts, but may be able to achieve peaks of 200 watts for the fraction of a second required for an intense, quick sound. In this example the dynamic headroom would equal 3 dB.

Dynamic range: The range between the loudest and the softest sounds that are in a piece of music, or that can be reproduced by a piece of audio equipment without distortion (a ratio expressed in decibels). In speech, the range rarely exceeds 40 dB; in music, is greatest in orchestral works, where the range may be as much as 75 dB.

Dyne: The force that will accelerate a 1-gram mass at the rate of 1 cm/sec. The old standard reference level for sound pressure was .0002 dyne/sq. cm. The same level today is expressed as 20 micro pascals, or 20 uPa.

Echo: A delayed return of sound that is perceived by the ear as a discrete sound image.

Echograms: A record of the very early reverberatory decay of sound in a room.

EES: Early, early sound. Structure-borne sound may reach the microphone in a room before the air-borne sound because sound travels faster through denser materials.

EFC: Energy-frequency curve.

EFP: Eckoustic Functional Panel

EFTC: Energy-frequency-time curve.

Ensemble: Musicians must hear each other to function properly; in other words ensemble must prevail. Diffusing elements surrounding the stage area contribute greatly to ensemble.

Equal loudness contour: A contour representing a constant loudness for all audible frequencies. The contour having a sound pressure level of 40 dB at 1,000Hz is arbitrarily defined as the 40-phon contour.

Equalization: The process of adjusting the frequency response of a device or system to achieve a flat or other desired response.

Equalizer: A device for adjusting the frequency response of a device or system.

ETC: Energy-time curve.

Euphonic: Pleasing. As a descriptive audio term, usually refers to a coloration or inaccuracy that none-the-less may be sonically pleasing.

Extension: How extended a range of frequencies the device can reproduce accurately. Bass extension refers to how low a frequency tone will the system reproduce, high-frequency extension refers to how high in frequency will the system play.

Far field: That part of the sound field in which sound pressure decreases inversely with distance from the source. This corresponds to a reduction of approximately 6 dB in level for each doubling distance.

Feedback, acoustic: Unwanted interaction between the output and input of an acoustical system, e.g., between the loudspeaker and the microphone of a system.

FFP: Flat Functional Panel

FFT: Fast Fourier Transform. An iterative program that computes the Fourier Transform in a shorter time.

Fiber glass insulation: An energy-efficient glass fiber insulation used to ensure the best thermal and noise control performance available.

Fidelity: As applied to sound quality, the faithfulness to the original.

Field sound transmission class, FSTC: Sound transmission class calculated in accordance with Classification E 413 using values of field transmission loss.

Field transmission loss, FTL: Sound transmission loss measured in accordance with Annex A1 of Test Method E 336.

Filter, band pass: A filter that passes all frequencies between a low-frequency cutoff point or a high-frequency cutoff point.

Filter, high pass: A filter that passes all frequencies above a cutoff frequency.

Filter, low pass: A filter that passes all frequencies below a certain cutoff frequency.

Flame spread: A measure of the time it takes for flame to spread. Compared to red oak, whose Flame Spread Index (FSI) is 100 in accordance with ASTM E 84. The infill of IAC sound absorptive metal panels has an FSI of 20.

Flanking transmission: Transmission of sound from the source to a receiving location by a path other than that under consideration.

Fletcher-Munson Curve: Our sensitivity to sound depends on its frequency and volume. Human ears are most sensitive to sounds in the midrange. At lower volume levels humans are less sensitive to sounds away from the midrange, bass and treble sounds "seem" reduced in intensity at lower listening levels.

Flutter: A repetitive echo set up by parallel reflecting surfaces.

Fourier analysis: Application of the Fourier transform to a signal to determine its spectrum.

Free field: An environment in which a sound wave may propagate in all directions without obstructions or reflections. Anechoic rooms can produce such an environment under controlled conditions.

Frequency: The measure of the rapidity of alterations of a periodic signal, expressed in cycles per second or Hz.

Frequency response: The changes in the sensitivity of a circuit ,device, or room with frequency.

FTC: Frequency-time curve.

Fuel contributed: A measure of temperature rise in flame as a function of the time compared to red oak whose fuel contribution index (FCI) is 100 in accordance with ASTM E 84. The infill of IAC sound absorptive metal panels has an FSI of 15.

Fundamental: The lowest frequency of a note in a complex wave form or chord.

Fusion zone: All reflections arriving at the observer's ear within 20 to 40 msec of the direct sound are integrated, or fused together, with a resulting apparent increase in level and a pleasant change of character. This is the Haas effect.

Gain: To increase in level. The function of a volume control.

Grain: Listening term. A sonic analog of the grain seen in photos. A sort of "grittiness" added to the sound.

Grating, diffraction: An optical grating consists of minute, parallel lines used to break light down into its component colors. The principle is now used to achieve diffraction of acoustical waves.

Grating, reflection phase: An acoustical diffraction grating to produce diffusion of sound.

Haas effect: See fusion zone. Also called the precedence effect. Delayed sounds are integrated by the auditory apparatus if the fall on the ear within 20 to 40 msec of the direct sound. The level of the delayed components contributes to the apparent level of the sound, and it is accompanied by a pleasant change in character.

Hard room: A room in which the surfaces have very low values of sound absorption and are therefore highly reflective.

Harmonics: Also called overtones, these are vibrations at frequencies that are multiples of the fundamentals. Harmonics extend without limit beyond the audible range. They are characterized as even-order and odd-order harmonics. A second-order harmonic is two times the frequency of the fundamental; a third order is three times the fundamental; a fourth order is four times the fundamental; and so forth. Each even-order harmonic second, fourth, sixth, etc.-is one octave or multiples of one octave higher than the fundamental; these even-order overtones are therefore musically related to the fundamental. Odd-order harmonics, on the other hand third, fifth, seventh, and up-create a series of notes that are not related to any octave overtones and therefore may have an unpleasant sound. Audio systems that emphasize odd-order harmonics tend to have a harsh, hard quality.

Headroom: The ability of an amp to go beyond its rated power for short durations in order to reproduce musical peaks without distortion. This capability is often dependent on the power supply used in the design.

Hearing sensitivity: The human ear is less sensitive at low frequencies than in the midrange. Turn your volume knob down and notice how the bass seems to "disappear". To hear low bass requires an adequate SPL level. To hear 25Hz requires a much higher SPL level than to hear 250Hz.

Helmholtz resonator: A reactive, tuned, sound absorber. A bottle is such a resonator. They can employ a perforated cover or slats over a cavity.

Hertz: the unit of frequency, abbreviated Hz. The same as cycles per second.

High-pass filter: See filter, high pass.

Imaging: Listening term. A good stereo system can provide a stereo image that has width, depth and height. The best imaging systems will define a nearly holographic re-creation of the original sound.

Impact insulation class, IIC: A single-number rating derived from measured values of normalized impact sound pressure levels I accordance with Annex A1 of Test Method E 492. It provides an estimate of the impact sound insulating performance of a floor-ceiling assembly.

Impedance: The opposition to the flow of electric or acoustic energy measured in ohms.

Impulse: A very short, transient, electric or acoustic signal.

Impulse response: Sound pressure versus time measurement showing how a device or room responds to an impulse.

In phase: Two periodic waves reaching peaks and going through zero at the same instant are said to be "in phase."

Initial time-delay gap: The time gap between the arrival of the direct sound and the first sound reflected from the surfaces of the room.

Insertion loss, IL: Of a silencer or other sound-reducing element, in a specified frequency band, the decrease in sound power level, measured at the location of the receiver, when a sound insulator or a sound attenuator is inserted in the transmission path between the source and the receiver.

Intensity: Acoustic intensity is sound energy flux per unit area. The average rate of sound energy transmitted through a unit area normal to the direction of sound transmission.

Interference: The combining of two or more signals results in an interaction called interference. This may be constructive or destructive. Another use of the term is to refer to undesired signals.

Inverse-square law: Under far field/free field conditions, sound intensity varies inversely with the square of the distance from the source. In pure spherical divergence of sound from a point source in free space, the sound pressure level decreases 6 dB for each doubling of the distance.

Isolate: A dampening mechanism made a part of the assembly or system, which reduces structureborne vibrations from passing through the structure.

ITD: Initial time-delay gap.

JAES: Journal of the Audio Engineering Society.

JASA: Journal of the Acoustical Society of America.

Joist: Horizontal wood framing member set from wall to wall to support the boards of a floor or ceiling.

KHz: 1,000Hz.

Law of the first wave front: The first wave front falling on the ear determines the perceived direction of the sound.

LEDE: Live end dead end.

Level reduction, LR: In a specified frequency band, the decrease in sound pressure level, measured at the location of the receiver, when a barrier or other sound-reducing element is placed between the source and the receiver.

Level, L: Ten times the common logarithm of the ratio of a quantity proportional to power or energy to a reference quantity of the same kind. (See sound power level, sound pressure level.) the quantity so obtained is expressed in decibels.

Linear: A device or circuit with a linear characteristic means that a signal passing through it is not distorted.

Live end dead end: An acoustical treatment plan for rooms in which one end is highly absorbent and the other end reflective and diffusive.

Logarithm: An exponent of 10 in the common logarithms to the base 10. For example, 10 to the exponent 2=100; the log of 100=2.

Loudness: A subjective term for the sensation of the magnitude of sound. The subjective response to a sound level.

Loudspeaker: An electroacoustical transducer that changes electrical energy to acoustical energy.

Masking: The amount (or the process) by which the threshold of audibility for one sound is raised by the presence of another (masking) sound.

Mass law: An approximation that describes the Sound Transmission Loss (TL) of a limp, flexible barrier in terms of mass density and frequency. For each doubling of the weight or frequency of a partition, mass law predicts a 6 dB increase in TL.

Mean free path: For sound waves in an enclosure, it is the average distance traveled between successive reflections.

Metric sabin: [L2]---the unit of measure of sound absorption in the metre-kilogram-second system of units.

Microphone: An acoustical-electrical transducer by which sound waves in air may be converted to electrical signals.

Midrange: A speaker, (driver), used to reproduce the middle range of frequencies. A midrange is combined with a woofer for low frequencies and a tweeter for high frequencies to form a complete, full-range system.

Millisecond: One thousandth of a second, abbreviated ms or msec.

Modal resonance: See mode.

Mode: A room resonance. Axial modes are associated with pairs of parallel walls. Tangential modes involve four room surfaces and oblique modes all six surfaces. Their effect is greatest at low frequencies and for small rooms.

Monaural: See monophonic.

Monitor: Loudspeaker used in the control of a recording studio.

Monophonic: Single-channel sound.

Monopole: Any speaker that encloses the back-wave of the speaker device even though part of this back-wave may be released via a port or duct. The primary radiation at most frequencies will be from the driver front. If the driver is not enclosed it becomes a dipole.

Muddy: Listening term. A sound that is poorly defined, sloppy or vague. For example, a "muddy" bass is often boomy with all the notes tending to run together.

Muting: To greatly decrease the volume level. Many receivers and pre-amplifiers have a muting control which allows the volume level to be cut way down without changing the master volume control. Great for when the phone rings.

NAB: National Association of Broadcasters.

Near field: Locations close to the sound source between the source and the far field. The near field is typically characterized by large sound pressure level variations with small changes in measurement position from the source.

Noise: Interference of an electrical or acoustical nature. Random noise is a desirable signal used in acoustical measurements. Pink noise is random noise whose spectrum falls at 3 dB per octave: it is useful for use with sound analyzers with constant percentage bandwidths. Unwanted, bothersome, or distracting sound.

Noise criteria: Standard spectrum curves by which a given measured noise may be described by a single NC number.

Noise isolation class, NIC: A single-number rating calculated in accordance with Classification E 413 using measured values of noise reduction. It provides an estimate of the sound isolation between two enclosed spaces that are acoustically connected by one or more paths.

Noise reduction (NR): The difference in sound pressure level between any two points along the path of sound propagation. As an example, noise reduction is the term used to describe the difference in sound pressure levels between the inside and outside of an enclosure.

Noise reduction coefficient (NRC): The arithmetic average, to the nearest multiple of .05, of the sound absorption coefficients in the 1/3 octave bands centered at 250Hz, 500Hz, 1000Hz, and 2000Hz..

Normal incidence sound absorption: *; [dimensionless]---of a surface, at a specified frequency, the fraction of the perpendicularly incident sound power absorbed or otherwise not reflected.

Normal mode: A room resonance. See mode.

Normalized noise isolation class, NNIC: A single-number rating calculated in accordance with Classification E 413 using measured values of normalized noise reduction. (See normalized noise reduction.)

Normalized noise reduction, NNR: Between two rooms, in a specified frequency band, the value that the noise reduction in a given field test would have if the reverberation time in the receiving room were .5 s.

Null: A low or minimum point on a graph. A minimum pressure region in a room.

Oblique mode: See mode.

Octave: An octave is a doubling or halving of frequency. 20Hz-40Hz is often considered the bottom octave. Each octave you add on the bottom requires that your speakers move four times as much air!

Octave bands: Frequency ranges in which the upper limit of each band is twice the lower limit. Octave bands are identified by their geometric mean frequency, or center frequency.

One-third octave bands: Frequency ranges where each octave is divided into one-third octaves with the upper frequency limit being 2* (1.26) times the lower frequency. Identified by the geometric mean frequency of each band.

Overtone: A component of a complex tone having a frequency higher than the fundamental.

Particle velocity, u: [LT-1]; m/s---a fluctuating velocity superimposed by the presence of sound on the other velocities that the particles of the medium may have. In analogy with alternating voltage its magnitude can be expressed in several ways, such as instantaneous particle velocity or peak particle velocity. In air, the other velocities are those due to thermal agitation and wind currents.

Passive absorber: A sound absorber that dissipates sound energy as heat.

Peak sound pressure level: LPK[nd] ----ten times the common logarithm of the square of the ratio of the largest absolute value of the instantaneous sound pressure in a stated frequency band during a specified time interval to the reference sound pressure of 20 micro pascals.

PFC: Phase-frequency curve.

Phase: Phase is the measure of progression of a periodic wave. Phase identifies the position at any instant which a periodic wave occupies in its cycle. It can also be discribed as the time relationship between two signals.

Phase shift: The time or angular difference between two signals.

Phon: The unit of loudness level of a tone.


Pitch: A subjective term for the perceived frequency of a tone.

Plenum: An absorbent-lined cavity through which conditioned air is routed to reduce noise.

Polarity: The positive or negative direction of an electrical, acoustical, or magnetic force. Two identical signals in opposite polarity are 180 degrees apart at all frequencies. Polarity is not frequency dependent.

Pressure zone: As sound waves strike a solid surface, the particle velocity is zero at the surface and the pressure is high, thus creating a high-pressure layer near the surface.

Psychoacoustics: The study of the interaction of the auditory system and acoustics.

Pure tone: A tone with no harmonics. All energy is concentrated at a single frequency.


Rarefaction: The portion of a sound wave in which molecules are spread apart, forming a region with lower-than-normal atmospheric pressure. The opposite of compression.

RASTI: Rapid Speech Transmission Index expressed in a decimal range of 0.2 for "bad" to 1.00 for "Excellent"


RC (Room Criteria) curves: Undesirable rumble can result if NC curves are determined mainly by low frequency noise. Similarly, a hissing effect can result from NC level being controlled by higher frequency sounds. To achieve a better balance between low frequency and high frequency components, RC curves have been established for which the objective is to design spectra that meet an RC curve within + 2 dB at all frequencies. A spectrum that exceeds an RC curve by more than 5 dB at frequencies below 250Hz is likely to result in unacceptable rumble. Above 2000Hz, on the other hand, a spectrum more than 5 dB higher than the RC curve might have too much of a hissing quality.

Reactance: The opposition to the flow of electricity posed by capacitors and inductors.

Reactive absorber: A sound absorber, such as the Helmholtz resonator which involves the effects of mass and compliance as well as resistance.

Receiving room: In architectural acoustical measurements, the room in which the sound transmitted from the source room is measured.

Reflection: For large surfaces compared to the wavelength of impinging sound, sound is reflected much as light is reflected, with the angle of incidence equaling the angle of reflection.

Reflection-phase grating: A diffuser of sound energy using the principle of the diffraction grating.

Refraction: The bending of sound waves traveling through layered media with different sound velocities.

Resistance: The quality of electrical or acoustical circuits that results in dissipation of energy through heat.

Resonance: A natural periodicity, or the reinforcement associated with this periodicity.

Resonant frequency: Any system has a resonance at some particular frequency. At that frequency, even a slight amount of energy can cause the system to vibrate. A stretched piano string, when plucked, will vibrate for a while at a certain fundamental frequency. Plucked again, it will again vibrate at that same frequency. This is its natural or resonant frequency. While this is the basis of musical instruments, it is undesirable in music-reproducing instruments like audio equipment.

Response: See frequency response.

Reverberant sound field: The sound in an enclosed or partially enclosed space that has been reflected repeatedly or continuously from the boundaries.

Reverberation: The persistence of sound in an enclosed or partially enclosed space after the source of sound has stopped; by extension, in some contexts, the sound that so persists.

Reverberation room: A room so designed that the reverberant sound field closely approximates a diffuse sound field, both in the steady state when the sound source is on, and during the decay after the source of sound has stopped.

Reverberation time: The tailing off of a sound in an enclosure because of multiple reflections from the boundaries.

RFZ: Reflection-free zone.

Room mode: The normal modes of vibration of an enclosed space. See mode.

RT60: Reverberation time.

Sabin: [L2]---the unit of measure of sound absorption in the inch-pound system.

Sabine: The originator of the Sabine reverberation equation.

Self-extinguishing: A material which will not support combustion when external source of flame is removed.

Self-noise, n: Extraneous non-acoustical signals, generated or induced in a measurement system.

Signal-to-noise (SN) ratio: The range or distance between the noise floor (the noise level of the equipment itself) and the music signal.

Sine wave: A periodic wave related to simple harmonic motion.

Slap back: A discrete reflection from a nearby surface.

Smoke developed: Measure of smoke density developed by a material when compared with red oak, which has a smoke density index (SDI) of 100 in accordance with ASTM E 84. The infill of IAC sound absorptive metal panels has an SDI of 20.

Soft room: Room with highly sound absorptive surfaces.

Sone: The unit of measurement for subjective loudness.

Sound: Sound is vibrational disturbance, exciting hearing mechanisms, transmitted in a predictable manner determined by the medium through which it propagates. To be audible the disturbance must fall within the frequency range 20Hz to 20,000Hz.

Sound absorption: (1) The process of dissipating sound energy. (2) The property possessed by materials, objects and structures such as rooms of absorbing sound energy. (3) A: [L2]; metric sabin---in a specified frequency band, the measure of the magnitude of the absorptive property of a material, an object, or a structure such as a room.

Sound absorption coefficient, *: [dimensionless]; metric sabin/m* ---of a surface, in a specified frequency band, the measure of the absorptive property of a material as approximated by the method of Test Method C423. Ideally, the fraction of the randomly incident sound power absorbed or otherwise not reflected.

Sound attenuation: The reduction of the intensity of sound as it travels from the source to a receiving location. Sound absorption is often involved as, for instance, in a lined duct. Spherical spreading and scattering are other attenuation mechanisms.

Sound energy density, D: [ML-1T-2]; J/m---the quotient obtained when the sound energy in a region is divided by the volume of the region. The sound energy density at a point is the limit of that quotient as the volume that contains the point approaches zero.

Sound energy, E: [ML2T-2]; J-energy added to an elastic medium by the presence of sound, consisting of potential energy in the form of deviations from static pressure and of kinetic energy in the form of particle velocity.

Sound insulation: The capacity of a structure to prevent sound from reaching a receiving location. Sound energy is not necessarily absorbed; impedance mismatch, or reflection back toward the source, is often the principal mechanism.

Sound intensity, I: [MT-3]; W/m2 the quotient obtained when the average rate of energy flow in a specified direction and sense is divided by the area, perpendicular to that direction, through or toward which it flows. The intensity at a point is the limit of that quotient as the area that includes the point approaches zero.

Sound isolation: The degree of acoustical separation between two locations, especially adjacent rooms.

Sound level: Of airborne sound, a sound pressure level obtained using a signal to which a standard frequency-weighting has been applied.

Sound power level, Lp: Of airborne sound, ten times the common logarithm of the ratio of the sound power under consideration of the standard reference power of 1 pW. The quantity so obtained is expressed in decibels.

Sound power, W: [ML2T-3]; W---in a specified frequency band, the rate at which acoustic energy is radiated from a source. In general, the rate of flow of sound energy, whether from a source, through an area, or into an absorber.

Sound pressure level (SPL): Given in decibels (dB) is an expression of loudness or volume. A 10 dB increase in SPL represents a doubling in volume. Live orchestral music reaches brief peaks in the 105 dB range and live rock easily goes over 120 dB.

Sound pressure, p: [ML-1T-2]; Pa-a fluctuating pressure superimposed on the static pressure by the presence of sound. In analogy with alternating voltage its magnitude can be expressed in several ways, such as instantaneous sound pressure or peak sound pressure, but the unqualified term means root-mean-square sound pressure. In air, the static pressure is barometric pressure.

Sound receiver: One or more observation points at which sound is evaluated or measured. The effect of sound on an individual receiver is usually evaluated by measurements near the ear or close to the body.

Sound spectrograph: An instrument that displays the time, level, and frequency of a signal.

Sound transmission class, STC: A single-number rating, calculated in accordance with Classification E 413 using values of sound transmission loss. It provides an estimate of the performance of a partition in certain common sound insulation problems. A single number rating that indicates the sound transmission loss of a partition or ceiling system between adjacent closed rooms, abbreviated STC.

Sound transmission coefficient, r: [dimensionless]---of a partition, in a specified frequency band, the fraction of the airborne sound power incident on the partition that is transmitted by the partition and radiated on the other side.

Sound transmission loss, TL: Of a partition, in a specified frequency band, ten times the common logarithm of the ratio of the airborne sound power incident on the partition to the sound power transmitted by the partition and radiated on the other side. The quantity so obtained is expressed in decibels. The reduction in sound level when sound passes through a partition or ceiling system.

Sound waves: Sound waves can be thought of like the waves in water. Frequency determines the length of the waves; amplitude or volume determines the height of the waves. At 20Hz, the wavelength is 56 feet long! These long waves give bass its penetrating ability, (why you can hear car boomers blocks away).

Soundstage: A listening term that refers to the placement of a stereo image in a fashion that replicates the original performance. A realistic soundstage has proportional width, depth and height.

Source room: In architectural acoustical measurements, the room that contains the noise source or sources.

Speaker level: Taken from the speaker terminals. This signal has already been amplified.

Specific airflow resistance, r: [ML-2T-1]. Mks rayl (Pa-s/m)---the product of the airflow resistance of a specimen and its area. This is equivalent to the quotient of the air pressure difference across the specimen divided by the linear velocity, measured outside the specimen, of airflow through the specimen.

Spectral balance: Balance across the entire frequency spectrum of the audio range.

Spectrum: the distribution of the energy of a signal with frequency.

Spectrum analyzer: An instrument for measuring, and usually recording, the spectrum of a signal.

Specular reflections: Mirrorlike reflections of sound (angle of incidence equals angle of reflection) from a flat surface. Reflections that do not spread out.

Speech intelligibility: A measure of sound clarity that indicates the ease of understanding speech. It is a complex function of psychoacoustics, signal-to-noise ratio of the sound source, and direct-to-reverberant energy within the listening environment.

Speech transmission index, STI: A single number that indicates the effect of a transmission system on speech intelligibility.

Spherical divergence: Sound diverges spherically from a point source in free space.

Splaying: Walls are splayed when they are constructed somewhat "off square," i.e., a few degrees from the normal rectilinear form.

Standing wave: A resonance condition in an enclosed space in which sound waves traveling in one direction interact with those traveling in the opposite direction, resulting in a stable condition.

Steady-state: A condition devoid of transient effects.

Stereo: From the Greek meaning solid. The purpose of stereo is not to give you separate right and left channels, but to provide the illusion of a three-dimensional, holographic image between the speakers.

Structureborne noise: Generation and propagation of time-dependent motions and forces in solid materials which result in unwanted radiated sound.

Stud: An upright 2 x 4 or 2 x 6 wall framing member.

Subwoofer: A speaker designed exclusively for low-frequency reproduction. A true subwoofer should be able to at least reach into the bottom octave (20-40Hz). There are many "subwoofers" on the market that would be more accurately termed "woofers".

Superposition: Many sound waves may transverse the same point in space, the air molecules responding to the vector sum of the demands of the different waves.

T60: See RT60.

Tangential mode: A room mode produced by reflections off four of the six surfaces of the room.

TDS: Time-delay spectrometry.

TEF: Time, energy, frequency.

Threshold of feeling (pain): The sound pressure level that makes the ears tickle, located about 120 dB above the threshold of hearing.

Threshold of hearing: The lowest level sound that can be perceived by the human auditory system. This is close to the standard reference level of sound pressure, 20uPA.

THX: Refers to a series of specifications for surround sound systems. Professional THX is used in commercial movie theaters. Home THX specifications are not published and manufacturers must sign non-disclosure waivers before submitting their products for THX certification. Manufacturers that receive certification for their products must pay a royalty on units sold.

Timbral: Refers to the overall frequency balance of a system. In a perfect world, all systems would have complete tonal neutrality. With current technology, this ideal is approached but not met. Listening to many equally "good" speakers will reveal that some sound warmer than others, some sound brighter etc. In a surround sound system it is important that all speakers have a close timbral match for the highest degree of sonic realism.

Timbre: The quality of a sound that distinguishes it from other sounds of the same pitch and volume. The distinctive tone of an instrument or a singing voice.

Time-delay spectrometry: A sophisticated method for obtaining anechoic results in echoic spaces.

Tonal:, adj.: In reference to audible sound, capable of exciting an auditory sensation having pitch.

Tone: A tone results in an auditory sensation of pitch.

Tone burst: A short signal used in acoustical measurements to make possible differentiating desired signals from spurious reflections.

Total harmonic distortion (THD): Refers to a device adding harmonics that were not in the original signal. For example: a device that is fed a 20Hz sine wave that is also putting out 40Hz, 80Hz, etc. Not usually a factor in most modern electronics, but still a significant design problem in loudspeakers.

Transient response: The ability of a component to respond quickly and accurately to transients. Transient response affects reproduction of the attack and decay characteristics of a sound.

Transients: Instantaneous changes in dynamics, producing steep wave fronts.

Transparency: Listening term. An analog that can be best "pictured" in photography. The more "transparent" the sound, the clearer the auditory picture.

Treble: The higher frequencies of the audible spectrum.

Tuning frequency: The helmholtz resonant frequency of a box. Also refers to the resonant frequency of other types of systems.

Vibration isolation: A reduction, attained by the use of a resilient coupling, in the capacity of a system to vibrate in response to mechanical excitation.

Volume: Colloquial equivalent of sound level.

Warmth: A listening term. The opposite of cool or cold. In terms of frequency, generally considered the range from approx. 150Hz-400Hz. A system with the "proper" warmth will sound natural within this range.

Watt: The unit of electrical or acoustical power.

Wattage: Is the unit of power used to rate the output of audio amplifiers. For a wattage number to have meaning the distortion level and impedance must also be specified.

Wave: A regular variation of an electrical signal or acoustical pressure.

Wavelength: The distance the sound wave travels to complete one cycle. The distance between one peak or crest of a sine wave and the next corresponding peak or crest. The wavelenth of any frequency may be found by dividing the speed of sound by the frequency. (speed of sound at sea level is 331.4 meters/second or 1087.42 feet/second).

Weighting: Adjustment of sound-level meter response to achieve a desired measurement.

White noise (ANS): Noise with a continuous frequency spectrum and with equal power per unit bandwidth. For example, equal power in any band of 100-Hz width.


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