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For other uses, see Headphones (disambiguation).
Headphones on a stand

Headphones (or "head-phones" in the early days of telephony and radio) are a pair of small loudspeakers that are designed to be held in place close to a user's ears. They are also known as earspeakers, earphones[1] or, colloquially, cans.[2] The alternate in-ear versions are known as earbuds or earphones.[1] In the context of telecommunication, a headset is a combination of headphone and microphone. Headphones either have wires for connection to a signal source such as an audio amplifier, radio, CD player, portable media player, mobile phone, video game consoles, electronic musical instrument, or have a wireless device, which is used to pick up signal without using a cable.

The different types of headphones have different sound reproduction characteristics. Closed-back headphones, for example, are good at reproducing bass frequencies. Headphones that use cables typically have either a 1/4 inch (6.35mm) jack or an 1/8 inch (3.5mm) jack for plugging the headphones into the sound source.


Brandes radio headphones, circa 1920.


Some very sensitive headphones, such as those manufactured by Brandes around 1919, were commonly used for early radio work. These early headphones used moving iron drivers, with either single ended or balanced armatures. The requirement for high sensitivity meant that no dampening was used, thus the sound quality was crude. These early models lacked padding, and often produced excessive clamping forces on the wearer's head. Their impedance varied; headphones used in telegraph and telephone work had an impedance of 75 ohms. Those used with early wireless radio had to be more sensitive and were made with more turns of finer wire. Impedance of 1000 to 2000 ohms was common, which suited both crystal sets and triode receivers.

In early powered radios, the headphone was part of the vacuum tube's plate circuit and carried dangerous voltages. It was normally connected directly to the positive high voltage battery terminal, and the other battery terminal was securely grounded. The use of bare electrical connections meant that users could be shocked if they touched the bare headphone connections while adjusting an uncomfortable headset.

In 1943, John C. Koss, an audiophile and jazz musician from Milwaukee, produced the first stereo headphones. Previously, headphones were used only by telephone and radio operators, and individuals in similar industries.


Headphones may be used with stationary CD and DVD players, home theater, personal computers, or portable devices (e.g. digital audio player/mp3 player, mobile phone, etc.). Cordless headphones are not connected to their source by a wire. Instead, they receive a radio or infrared signal encoded using a radio or infrared transmission link, such as FM, Bluetooth or Wi-Fi. These are powered receiver systems, of which the headphone is only a component. Cordless headphones are used with events such as a Silent disco or Silent Gig.

Sennheiser HD555 headphones, used in audio production environments (2007)

In the professional audio sector, headphones are used in live situations by disc jockeys with a DJ mixer, and sound engineers for monitoring signal sources. In radio studios, DJs use a pair of headphones when talking to the microphone while the speakers are turned off, to eliminate acoustic feedback while monitoring their own voice. In studio recordings, musicians and singers use headphones to play along to a backing track. In military applications, audio signals of many varieties are monitored using headphones.

Wired headphones are attached to an audio source by a cable. The most common connectors are 6.35 mm (¼″) and 3.5 mm phone connectors. The larger 6.35 mm connector is more common on fixed location home or professional equipment. Sony introduced the smaller, and now widely used, 3.5 mm "minijack" stereo connector in 1979, adapting the older monophonic 3.5 mm connector for use with its Walkman portable stereo tape player. The 3.5 mm connector remains the most widely used connector for portable application today. Adapters are available for converting between 6.35 mm and 3.5 mm devices.

Electrical characteristics[edit]

Electrical characteristics of dynamic loudspeakers may be readily applied to headphones, because most headphones are small dynamic loudspeakers.


Headphones are available with low or high impedance (typically measured at 1 kHz). Low-impedance headphones are in the range 16 to 32 ohms and high-impedance headphones are about 100-600 ohms. As the impedance of a pair of headphones increases, more voltage (at a given current) is required to drive it, and the loudness of the headphones for a given voltage decreases. In recent years, impedance of newer headphones has generally decreased to accommodate lower voltages available on battery powered CMOS-based portable electronics. This has resulted in headphones that can be more efficiently driven by battery powered electronics. Consequently, newer amplifiers are based on designs with relatively low output impedance.

The impedance of headphones is of concern because of the output limitations of amplifiers. A modern pair of headphones is driven by an amplifier, with lower impedance headphones presenting a larger load. Amplifiers are not ideal; they also have some output impedance that limits the amount of power they can provide. In order to ensure an even frequency response, adequate damping factor, and undistorted sound, an amplifier should have an output impedance less than 1/8 that of the headphones it is driving (and ideally, as low as possible). If output impedance is large compared to the impedance of the headphones, significantly higher distortion will be present.[5] Therefore, lower impedance headphones will tend to be louder and more efficient, but will also demand a more capable amplifier. Higher impedance headphones will be more tolerant of amplifier limitations, but will produce less volume for a given output level.

Historically, many headphones had relatively high impedance, often over 500 ohms in order to operate well with high impedance tube amplifiers. In contrast, modern transistor amplifiers can have very low output impedance, enabling lower impedance headphones. Unfortunately, this means that older audio amplifiers or stereos often produce poor quality output on some modern, low impedance headphones. In this case, an external headphone amplifier may be beneficial.


Sensitivity is a measure of how effectively an earpiece converts an incoming electrical signal into an audible sound. It thus indicates how loud the headphones will be for a given electrical drive level. It can be measured in decibels of sound pressure level per milliwatt (dB (SPL)/mW) or decibels of sound pressure level per volt (dB (SPL) / V).[6] Unfortunately, both definitions are widely used, often interchangeably. As the output voltage (but not power) of a headphone amplifier is essentially constant for most common headphones, dB/mW is often more useful if converted into dB/V using Ohm's Law:

dB (SPL)/V = dB (SPL)/mW - 10*\log10{\frac{Impedance}{1000}}

Alternatively, online calculators can be used.[7] Once the sensitivity per volt is known, the maximum volume for a pair of headphones can be easily calculated from the maximum amplifier output voltage. For example, for a headphone with a sensitivity of 100 dB (SPL)/V, an amplifier with an output of 1 root-mean-square (RMS) voltage will produce a maximum volume of 100 dB.

Pairing high sensitivity headphones with power amplifiers can produce dangerously high volumes and damage headphones. The maximum sound pressure level is a matter of preference, with some sources recommending no higher than 110 to 120 dB.[8] In contrast, the American Occupational Safety and Health Administration recommends an average SPL of no more than 85 dB(A) to avoid long-term hearing loss, while the European Union standard EN 50332-1:2013 recommends that volumes above 85 dB(A) include a warning, with an absolute maximum volume (defined using 40–4000 Hz noise) of no more than 100 dB to avoid accidental hearing damage.[9] Using this standard, headphones with sensitivities of 90, 100 and 110 dB (SPL)/V should be driven by an amplifier capable of no more than 3.162, 1.0 and 0.3162 RMS volts at maximum volume setting, respectively to reduce the risk of hearing damage.

The sensitivity of headphones is usually between about 80 and 125 dB/mW and usually measured at 1 kHz.[10]


Headphone size can affect the balance between fidelity and portability. Generally, headphone form factors can be divided into four separate categories: circumaural, supra-aural, earbud, and in-ear.


Circumaural headphones have large pads that surround the outer ear.



A pair of supra-aural headphones.


Open or closed back[edit]

Both circumaural and supra-aural headphones can be further differentiated by the type of earcups:

Open-back headphones have the back of the earcups open. This leaks more sound out of the headphone and also lets more ambient sounds into the headphone, but gives a more natural or speaker-like sound and more spacious "soundstage" - the perception of distance from the source.

Closed-back (or sealed) styles have the back of the earcups closed. They usually block some of the ambient noise, but have a smaller soundstage, giving the wearer a perception that the sound is coming from within their head. Closed-back headphones tend to be able to produce stronger low frequencies than open-back headphones.

Semi-open headphones, have a design that can be considered as a compromise between open-back headphones and closed-back headphones. This may imply that the result combines all the positive properties of both designs. Some believe the term "semi-open" is purely there for marketing purposes. While there is no exact definition for the term semi-open headphone, there are designs that can be considered as such. Where the open-back approach has hardly any measure to block sound at the outer side of the diaphragm, and the closed-back approach, really has a closed chamber at the outer side of the diaphragm, a semi-open headphone can have a chamber to block sound partially while leaving some sound through, via openings or vents.

Ear-fitting headphones[edit]


Earbuds/earphones sit in the outer ear

Earphones (popularly called "earbuds" in recent years) are very small headphones that are fitted directly in the outer ear, facing but not inserted in the ear canal. Earphones are portable and convenient, but many people consider them to be uncomfortable and prone to falling out.[12] They provide hardly any acoustic isolation and leave room for ambient noise to seep in; users may turn up the volume dangerously high to compensate, at the risk of causing hearing loss.[12][13] On the other hand, they let the user be better aware of their surroundings. Since the early days of the transistor radio, earphones have commonly been bundled with personal music devices. They are sold at times with foam pads for comfort.

In-ear headphones[edit]

Main article: In-ear monitor
In-ear monitors extend into the ear canal, providing isolation from outside noise.

In-ear headphones, also known as in-ear monitors (IEMs) or canalphones,[14] are small headphones with similar portability to earbuds which are inserted in the ear canal itself. IEMs are higher quality in-ear headphones and are used by audio engineers and musicians as well as audiophiles.

Because in-ear headphones engage the ear canal, they can be less prone to falling out and they block out much environmental noise. Lack of sound from the environment can be a problem when sound is a necessary cue for safety or other reasons, as when walking, driving, or riding near or in vehicular traffic.



Main article: Headset (audio)
A typical example of a headset used for voice chats.

A headset is a headphone combined with a microphone. Headsets provide the equivalent functionality of a telephone handset with hands-free operation. Among applications for headsets, besides telephone use, are aviation, theatre or television studio intercom systems, and console or PC gaming. Headsets are made with either a single-earpiece (mono) or a double-earpiece (mono to both ears or stereo). The microphone arm of headsets is either an external microphone type where the microphone is held in front of the user's mouth, or a voicetube type where the microphone is housed in the earpiece and speech reaches it by means of a hollow tube. Some headsets come in a choice of either behind-the-neck or no-headband design instead of the traditional over-the-head band.

Telephone headsets[edit]

Sony Ericsson Cordless bluetooth headset


For older models of telephones, the headset microphone impedance is different from that of the original handset, requiring a telephone amplifier for the telephone headset. A telephone amplifier provides basic pin-alignment similar to a telephone headset adaptor, but it also offers sound amplification for the microphone as well as the loudspeakers. Most models of telephone amplifiers offer volume control for loudspeaker as well as microphone, mute function and switching between headset and handset. Telephone amplifiers are powered by batteries or AC adaptors.

Ambient noise reduction[edit]

Among different types of headphones, in-ears are good at noise reduction


Passive noise isolation is essentially using the body of the earphone, either over or in the ear, as a passive earplug that simply blocks out sound. The headphone types that provide most attenuation are in-ear canal headphones and closed-back headphones, both circumaural and supra aural. Open-back and earbud headphones provide some passive noise isolation, but much less than the others. Typical closed-back headphones block 8 to 12 dB, and in-ears anywhere from 10 to 15 dB. Some models have been specifically designed for drummers, with the aim to be able to monitor the recorded sound while shutting out the sound coming directly from the drums at the same time as much as possible. Such headphones claim to reduce ambient noise by around 25 dB.


Transducer technology[edit]

Various types of transducer are used to convert electrical signals to sound in headphones.


A typical moving-coil headphone transducer

The moving coil driver, more commonly referred to as a "dynamic" driver is the most common type used in headphones. The operating principle consists of a stationary magnetic element affixed to the frame of the headphone which sets up a static magnetic field. The magnetic element in headphones is typically composed of ferrite or neodymium. The diaphragm, typically fabricated from lightweight, high stiffness to mass ratio cellulose, polymer, carbon material, or the like, is attached to a coil of wire (voice coil) which is immersed in the static magnetic field of the stationary magnet. The diaphragm is actuated by the attached voice coil, when the varying current of an audio signal is passed through the coil. The alternating magnetic field produced by the current through the coil reacts against the static magnetic field in turn, causing the coil and attached diaphragm to move the air, thus producing sound. Modern moving-coil headphone drivers are derived from microphone capsule technology.


Electrostatic loudspeaker diagram



Electrostatic headphones require a voltage source generating 100 V to over 1 kV, and are on the user's head. They do not need to deliver significant electric current, which limits the electrical hazard to the wearer in case of fault.


An electret driver functions along the same electromechanical means as an electrostatic driver. However the electret driver has a permanent charge built into it, where electrostatics have the charge applied to the driver by an external generator. Electret and electrostatic headphones are relatively uncommon. Electrets are also typically cheaper and lower in technical capability and fidelity than electrostatics.


Orthodynamic (also known as Planar Magnetic) headphones use similar technology to electrostatic headphones, with some fundamental differences. They operate similarly to Planar Magnetic Loudspeakers.

An orthodynamic driver consists of a relatively large membrane suspended between two sets of permanent, oppositely aligned, magnets.

The membrane is then electrically charged in order to induce movement in the membrane and produce sound.

Orthodynamic headphones have a number of advantages and disadvantages. Since the entire membrane is equally charged, and suspended within a uniform magnetic field, changes in the electrical charge produce uniform movement across the entire membrane surface. This reduces the possibility of distortion at different frequencies across different parts of the membrane, as may occur in dynamic drivers. This configuration also usually benefits from a good quality of bass response, due to the large surface area of the membrane producing the pressure waves required for low frequency sound.

Overall, orthodynamic headphones are considered to have very high-fidelity sound quality.

However, the large permanent magnet configurations cause orthodynamic headphones to often be considerably heavier than their dynamic or electrostatic counterparts. Whilst they do not require as much power as electrostatic headphones in order to give acceptable levels of volume and audio quality, they do require significantly more than most dynamic driver headphones. As such, most orthodynamic headphones require substantial amplification.

Balanced armature[edit]

Balanced armature transducer with armature balanced and exerting no force on diaphragm

A balanced armature is a sound transducer design primarily intended to increase the electrical efficiency of the element by eliminating the stress on the diaphragm characteristic of many other magnetic transducer systems. As shown schematically in the first diagram, it consists of a moving magnetic armature that is pivoted so it can move in the field of the permanent magnet. When precisely centered in the magnetic field there is no net force on the armature, hence the term 'balanced.' As illustrated in the second diagram, when there is electric current through the coil, it magnetizes the armature one way or the other, causing it to rotate slightly one way or the other about the pivot thus moving the diaphragm to make sound.

The JH Audio JH16 custom in-ear monitor utilizes 8 balanced armatures in a triple crossover configuration (4 low/2 mid/2 high). Multiple balanced armatures are often used to provide a higher fidelity sound.

The design is not mechanically stable; a slight imbalance makes the armature stick to one pole of the magnet. A fairly stiff restoring force is required to hold the armature in the 'balance' position. Although this reduces its efficiency, this design can still produce more sound from less power than any other[clarification needed]. Popularized in the 1920s as Baldwin Mica Diaphragm radio headphones, balanced armature transducers were refined during World War II for use in military sound powered telephones. Some of these achieved astonishing electro-acoustic conversion efficiencies, in the range of 20% to 40%, for narrow bandwidth voice signals.

Today they are typically used only in in-ear headphones and hearing aids, where their diminutive size is a major advantage. They generally are limited at the extremes of the hearing spectrum (e.g. below 20 Hz and above 16 kHz) and require a better seal than other types of drivers to deliver their full potential. Higher-end models may employ multiple armature drivers, dividing the frequency ranges between them using a passive crossover network. A few combine an armature driver with a small moving-coil driver for increased bass output.

The earliest loudspeakers for radio receivers used balanced armature drivers for their cones.

Thermoacoustic technology[edit]

The thermoacoustic effect generates sound from the audio frequency Joule heating of the conductor, an effect which is not magnetic and does not vibrate the speaker. In 2013 a carbon nanotube thin-yarn earphone based on the thermoacoustic mechanism was demonstrated by a research group in Tsinghua University.[15] The as-produced CNT thin yarn earphone has a working element called CNT thin yarn thermoacoustic chip. Such a chip is composed of a layer of CNT thin yarn array supported by the silicon wafer, and periodic grooves with certain depth are made on the wafer by micro-fabrication methods to suppress the heat leakage from the CNT yarn to the substrate.

Other transducer technologies[edit]

Transducer technologies employed much less commonly for headphones include the Heil Air Motion Transformer (AMT); Piezoelectric film; Ribbon planar magnetic; Magnetostriction and Plasma-ionisation. The first Heil AMT headphone was marketed by ESS Laboratories and was essentially an ESS AMT tweeter from one of the company's speakers being driven at full range. Since the turn of the century, only Precide of Switzerland have manufactured an AMT headphone. Piezoelectric film headphones were first developed by Pioneer, their two models both used a flat sheet of film which limited the maximum volume of air that could be moved. Currently TakeT produce a piezoelectric film headphone which is shaped not unlike an AMT transducer but which like the driver Precide uses for their headphones, has a variation in the size of transducer folds over the diaphragm. It additionally incorporates a two way design by its inclusion of a dedicated tweeter/supertweeter panel. The folded shape of a diaphragm allows a transducer with a larger surface area to fit within smaller space constraints. This increases the total volume of air that can be moved on each excursion of the transducer given that radiating area.

Magnetostriction headphones, sometimes sold under the label of "Bonephones", are headphones that work via the transmission of vibrations against the side of head, transmitting the sound via bone conduction. This is particularly helpful in situations where the ears must be left unobstructed or when used by those who are deaf for reasons which do not affect the nervous apparatus of hearing. Magnetostriction headphones though, have greater limitations to their fidelity than conventional headphones which work via the normal workings of the ear. Additionally, there was also one attempt to market a plasma-ionisation headphone in the early 1990s by a French company called Plasmasonics. It is believed that there are no functioning examples left.

Benefits and limitations[edit]

Two Sony MDR-V6 headphones; one folded for travel

Headphones may be used to prevent other people from hearing the sound either for privacy or to prevent disturbance, as in listening in a public library. They can also provide a level of sound fidelity greater than loudspeakers of similar cost. Part of their ability to do so comes from the lack of any need to perform room correction treatments with headphones. High quality headphones can have an extremely flat low-frequency response down to 20 Hz within 3 dB. Marketed claims such as 'frequency response 4 Hz to 20 kHz' are usually overstatements; the product's response at frequencies lower than 20 Hz is typically very small. [16]

Headphones are also useful for video games that use 3D positional audio processing algorithms, as they allow players to better judge the position of an off-screen sound source (such as the footsteps of an opponent or their gun fire).


Binaural recordings use a different microphone technique to encode direction directly as phase, with very little amplitude difference below 2 kHz, often using a dummy head, and can produce a surprisingly lifelike spatial impression through headphones. Commercial recordings almost always use stereo recording, rather than binaural, because loudspeaker listening has been more popular than headphone listening.

It is possible to change the spatial effects of stereo sound on headphones to better approximate the presentation of speaker reproduction by using frequency-dependent cross-feed between the channels, or—better still—a Blumlein shuffler (a custom EQ employed to augment the low-frequency content of the difference information in a stereo signal).[citation needed]

Headsets can have ergonomic benefits over traditional telephone handsets. They allow call center agents to maintain better posture without needing to hand-hold a handset or tilt their head sideways to cradle it.[17]

Dangers and volume solutions[edit]

Using headphones at a sufficiently high volume level may cause temporary or permanent hearing impairment or deafness. The headphone volume often has to compete with the background noise, especially in loud places such as subway stations, aircraft, and large crowds. Extended periods of exposure to high sound pressure levels created by headphones at high volume settings may be damaging;[18][19] however, one hearing expert found that "fewer than 5% of users select volume levels and listen frequently enough to risk hearing loss."[20] Some manufacturers of portable music devices have attempted to introduce safety circuitry that limited output volume or warned the user when dangerous volume was being used, but the concept has been rejected by most of the buying public, which favors the personal choice of high volume. Koss introduced the "Safelite" line of cassette players in 1983 with such a warning light. The line was discontinued two years later for lack of interest.

The government of France has imposed[21] a limit on all music players sold in the country:[21] they must not be capable of producing more than 100dBA (the threshold of hearing damage during extended listening is 80 dB, and the threshold of pain, or theoretically of immediate hearing loss, is 130 dB).[22] Motorcycle and other power-sport riders benefit by wearing foam earplugs when legal to do so to avoid excessive road, engine, and wind noise, but their ability to hear music and intercom speech is enhanced when doing so. The ear can normally detect 1-billionth of an atmosphere of sound pressure level,[23] hence it is incredibly sensitive. At very high sound pressure levels, muscles in the ear tighten the tympanic membrane and this leads to a small change in the geometry of the ossicles and stirrup that results in lower transfer of force to the oval window of the inner ear (the acoustic reflex).[24]

Some studies has found somewhat increased risks for temporary hearing damage from listening to music during strenuous exercise, compared to when listening at rest.[25] A Finnish study[26] recommended that exercisers should set their headphone volumes to half of their normal loudness and only use them for half an hour.

Passive noise canceling headphones can be considered dangerous because of a lack of awareness the listener may have with their environment. Noise cancelling headphones are so effective that a person may not be able to hear oncoming traffic or pay attention to people around them. There is also a general danger that music in headphones can distract the listener and lead to dangerous situations.[27]

The usual way of limiting sound volume on devices driving headphones is by limiting output power. This has the additional undesirable effect of being dependent of the efficiency of the headphones; a device producing the maximum allowed power may not produce adequate volume in low-efficiency high-quality headphones, while possibly reaching dangerous levels in very efficient ones.

See also[edit]


  1. ^ a b "earphone". Retrieved 4 January 2014. 
  2. ^ Stanley R. Alten Audio Basics Cengage 2011 ISBN 0-495-91356-1 page 63
  3. ^ The Early Radio Industry and the United States Navy
  4. ^ Utah History To Go. Ruin Followed Riches for a Utah Genius (Will Bagley for the Salt Lake Tribune, July 8, 2001)
  5. ^ Siau, John. "The "0-Ohm" Headphone Amplifier". Retrieved 26 June 2012. 
  6. ^ http://gilmore2.chem.northwestern.edu/tech/dbohn1_tech.htm
  7. ^ "Headphone Sensitivity & Efficiency Calculator". Retrieved 22 November 2014. 
  8. ^ "More Power?". Retrieved 22 November 2014. 
  9. ^ "BS EN 50332 tests for headphones and earphones with portable music players". 
  10. ^ "Understanding Earphone/Headphone Specifications". Shure Customer Help. Shure. Retrieved 30 December 2012. 
  11. ^ "Headphones | AudioBoost - Audio Review, Headphone Review, Amplifier Review, Music Album Review". AudioBoost. Retrieved 2013-09-06. 
  12. ^ a b c Time magazine: custom-made headphones
  13. ^ Ear and Hearing - Abstract: Volume 25(6) December 2004 p 513-527 Output Levels of Commercially Available Portable Compact Disc Players and the Potential Risk to Hearing
  14. ^ In-ear monitors are referred to as "earbuds" by some manufacturers. The box for the "J2" from jbuds describes the product as noise isolating soft silicone earbuds with the latest cutting edge In-Ear design. Also, Koss' The Plug is "a stereo earbud" that "features ... a tubular port structure that is inserted on a soft expandable cushion into the ear canal." Skullcandy also sells "in-ear earbuds" - http://www.skullcandy.com/shop/skin/frontend/default/skullcandy_main-v3/images/ninjistics/in-ear.png
  15. ^ Yang Wei; Xiaoyang Lin; Kaili Jiang; Peng Liu; Qunqing Li; Shoushan Fan (2013). "Thermoacoustic Chips with Carbon Nanotube Thin Yarn Arrays.". Nano Letters 13 (10): 4795–801. doi:10.1021/nl402408j. PMID 24041369. 
  16. ^ cnet reviews: Headphone Buying Guide "Even the flimsiest, cheap headphones routinely boast extremely low bass-response performance—15 or 20Hz—but almost always sound lightweight and bright."
  17. ^ United States Department of Labor. Occupational Safety & Health Administration. Computer Workstation. Checklist. Accessed February 2, 2009.
  18. ^ Ashaorg
  19. ^ NIH.gov
  20. ^ Audiologyonline.com
  21. ^ a b Europa.eu. Consumers: EU acts to limit health risks from exposure to noise from personal music players
  22. ^ "Medical Information Search (Cochlea • FAQ)". Lookformedical.com. Retrieved 2013-09-06. 
  23. ^ A standard threshold of hearing at 1000 Hz for the human ear is 2 x 10−5 Pa (Marsh, Andrew (1999). "Human Ear and Hearing". Online Course on Acoustics. The School of Architecture and Fine Arts, The University of Western Australia. Retrieved 23 August 2010. ); Standard atmospheric pressure is 101,325 Pa. 2 x 10−5 / 100,000 = 0.2 x 10−9, a ratio of less than a billionth.
  24. ^ GSU.edu
  25. ^ "Can Loud Noise during Exercise Damage my Hearing?". Healthy Hearing. Item 4. Retrieved 8 November 2014. 
  26. ^ Airo, Erkko; J. Pekkarinen; P. Olkinuora. "Listening to music with earphones: an assessment of noise exposure," Acustica–Acta Acustica, pp. 82, 885–894. (1996)
  27. ^ Greenfield, Paige (25 June 2011). "Deaf to Danger: The Perils of Earbuds". ABC News. Retrieved 20 June 2013. 

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