Headphones

Headphones are a pair of small loudspeakers, or less commonly a single speaker, with a way of holding them close to a user's ears and a means of connecting them to a signal source such as an audio amplifier, radio or CD player. They are also known as stereophones, headsets or, colloquially cans. The in-ear versions are known as earphones or earbuds. In the context of telecommunication, the term headset is used to describe a combination of headphone and microphone used for two-way communication, for example with a telephone.

Contents 1 History 2 Applications 3 Types of headphones 3.1 Circumaural 3.2 Supra-aural 3.3 In-ear headphones 3.3.1 Earbuds 3.3.2 In-ear Monitors 3.4 Headset 3.4.1 Telephone headsets 4 Technology 4.1 Moving-coil 4.2 Electrostatic 4.3 Electret 4.4 Balanced armature 4.5 Orthodynamic 4.6 Other Transducer Technologies 5 Benefits and limitations 6 Dangers and volume solutions 7 See also 8 References 9 External links

History

The telephone earpiece such as the one pictured at the right, was common around the beginning of the 20th century. Headphones developed from the earpiece. Sensitive headphones were the only way to listen to audio signals before amplifiers were developed.^[1]

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, either single ended or balanced armature. The requirement for high sensitivity meant no damping was used, thus the sound quality was crude. They also had very poor comfort compared to modern types, usually having no padding and too often having excessive clamping force to the 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 1,000 to 2,000 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 had dangerous voltages on it. It was normally connected directly to the positive high voltage battery terminal, and the other battery terminal was securely earthed. The use of bare electrical connections meant that users could be shocked if they touched the bare headphone connections while adjusting an uncomfortable headset.

Applications

Headphones can be used both with fixed equipment such as CD or DVD players, home theater, personal computers and with portable devices (e.g. digital audio player/mp3 player, mobile phone, etc.). Cordless headphones are not connected via a wire, receiving a radio or infrared signal encoded using a radio or infrared transmission link, like FM, Bluetooth or Wi-Fi. These are actually made of powered receiver systems of which the headphone is only a component, these types of cordless headphones are being used more frequently with events such as a silent disco or Silent Gig.

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 and monitor their own voice. In studio recordings, musicians and singers use headphones to play along to a backing track. In the military, audio signals of many varieties are monitored using headphones.

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

Types of headphones

The particular needs of the listener determine the choice of headphone. The need for portability indicates smaller, lighter headphones but can mean a compromise in fidelity. Headphones used as part of a home hi-fi do not have the same design constraints and can be larger and heavier. Generally, headphone form factors can be divided into four separate categories: circumaural, supra-aural, earbud, and in-ear.

Circumaural

Circumaural headphones (sometimes called full size headphones) have circular or ellipsoid earpads that encompass the ears. Because these headphones completely surround the ear, circumaural headphones can be designed to fully seal against the head to attenuate any intrusive external noise. Because of their size, circumaural headphones can be heavy and there are some sets which weigh over 500 grams. Good headband and earpad design is required to reduce discomfort resulting from weight.

Supra-aural

Supra-aural headphones have pads that sit on top of the ears, rather than around them. They were commonly bundled with personal stereos during the 1980s. This type of headphone generally tends to be smaller and more lightweight than circumaural headphones, resulting in less attenuation of outside noise.

In-ear headphones

Earbuds

Earbuds or earphones are headphones of a much smaller size that are placed directly outside of the ear canal, but without fully enveloping it. They are generally inexpensive and are favored for their portability and convenience. Due to their inability to provide any isolation they are often used at higher volumes in order to drown out noise from the user's surroundings, which increases the risk of hearing-loss.^[2] During the 1990s and 2000s, earbuds became a common type bundled with personal music devices.

In-ear Monitors

In-ear monitors (also known as IEMs or canalphones) are earphones that are inserted directly into the ear canal. Canalphones offer portability similar to earbuds, and also act as earplugs to block out environmental noise. There are two main types of IEMs: universal and custom. Universal canalphones provide one or more stock sleeve size(s) to fit various ear canals, which are commonly made out of silicone rubber, elastomer, or foam, for noise isolation. Custom canalphones are fitted to the ears of each individual. Castings of the ear canals are made and the manufacturer uses the castings to create custom-molded silicone rubber or elastomer plugs that provide added comfort and noise isolation. Because of the individualized labor involved, custom IEMs are more expensive than universal IEMs and resell value is very low as they are unlikely to fit other people.

Headset

A headset is a headphone combined with a microphone. Headsets provide the equivalent functionality of a telephone handset with hands-free operation. The most common uses for headsets are in console or PC gaming, Call centres and other telephone-intensive jobs and also for personal use at the computer to facilitate comfortable simultaneous conversation and typing. 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.

Telephone headsets

Telephone headsets connect to a fixed-line telephone system. A telephone headset functions by replacing the handset of a telephone. All telephone headsets come in a standard 4P4C commonly called an RJ-9 connector.

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 headset/handset switching. Telephone amplifiers are powered by batteries or AC adaptors.

Technology

Headphone transducers employ one or more of several methods of sound reproduction.

Moving-coil

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 an audio current 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

Electrostatic drivers consist of a thin, electrically charged diaphragm, typically a coated PET film membrane, suspended between two perforated metal plates (electrodes). The electrical sound signal is applied to the electrodes creating an electrical field; depending on the polarity of this field, the diaphragm is drawn towards one of the plates. Air is forced through the perforations; combined with a continuously changing electrical signal driving the membrane, a sound wave is generated. Electrostatic headphones are usually more expensive than moving-coil ones, and are comparatively uncommon. In addition, a special amplifier is required to amplify the signal to deflect the membrane, which often requires electrical potentials in the range of 100 to 1000 volts.

Due to the extremely thin and light diaphragm membrane, often only a few micrometers thick, and the complete absence of moving metalwork, the frequency response of electrostatic headphones usually extends well above the audible limit of approximately 20 kHz. The high frequency response means that the low midband distortion level is maintained to the top of the audible frequency band, which is generally not the case with moving coil drivers. Also, the frequency response peakiness regularly seen in the high frequency region with moving coil drivers is absent. The result is significantly better sound quality, if designed properly.

Electrostatic headphones are powered by anything from 100v to over 1kV, and are in proximity to a user's head. The usual method of making this safe is to limit the possible fault current to a low and safe value with resistors.

Electret

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

Balanced armature

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 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. 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 20% to 40% for narrow bandwidth voice signals.

Today they are typically used only in canalphones and hearing aids due to their diminutive size and low impedance. They generally are limited at the extremes of the hearing spectrum (e.g. below 20Hz and above 16kHz) and require a seal more 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.

Orthodynamic

Orthodynamic, isodynamic or planar magnetic drivers, are composed of a mylar sheet with a metal trace on it. This sheet is the diaphragm. The diaphragm is sandwiched between two magnets which have the same polarity facing each other. As a result the magnets repel from each other and so the whole assembly is clamped together. An electrical signal is passed through the disc as it would be through the voice coil of a moving coil driver and the motion produced generates the sound. Once a popular choice, for many years only Fostex continued making orthodynamic headphones. Recently, also other manufacturers have begun making orthodynamic headphones.

Other Transducer Technologies

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

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 3dB. However, rated frequency response distortion figures do not provide information on what character the sound reproduced at that frequency will be. 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. ^[3]

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).

Although modern headphones have been particularly widely sold and used for listening to stereo recordings since the release of the Walkman, there is subjective debate regarding the nature of their reproduction of stereo sound. Stereo recordings represent the position of horizontal depth cues (stereo separation) via volume differences of the sound in question between the two channels. When the sounds from two speakers mix, they create the phase difference the brain uses to locate direction. Through most headphones, because the right and left channels do not combine in this manner, the illusion of the phantom center can be perceived as lost. Hard panned sounds will also only be heard only in one ear rather than from one side. This latter point is of particular importance for earlier stereo recordings which were less sophisticated, sometimes playing vocals through one channel and music through the other.

Binaural recordings use a different microphone technique to encode direction directly as phase, with very little amplitude difference (except above 2 kHz) often using a dummy head, and can produce a surprisingly life-like spatial impression through headphones. Commercial recordings almost always use stereo recording, because historically 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). While cross-feed can reduce the unpleasantness that some listeners find with hard panned stereo in headphones, the use of a dummy head during recording, with artificial pinnae, can allow on playback through headphones, the experience of hearing the performance as though situated in the position of the dummy head. Optimal sound is achieved when the dummy head matches the listener's head, since pinnae vary greatly in size and shape.

Headsets can have ergonomic benefits over traditional telephone handsets. They allow call center agents to maintain better posture instead of tilting their head sideways to cradle a handset.^[4]

Over time, headphone cables fail. The common scenario in which a replacement might need to be purchased is the physical breakdown of copper wiring at junction points on the cord (at the TRS jack, or at the point of connection to the headphone). These are the sites of greatest and most stressful motion on a cord and so they are typically fitted with some kind of strain relief.

Dangers and volume solutions

Using headphones at a sufficiently high volume level may cause temporary or permanent hearing impairment or deafness due to an effect called "masking." The headphone volume has to compete with the background noise, especially in excessively loud places such as subway stations, aircraft, and large crowds. This leads to the disappearance of the normal pain associated with higher levels of volumes. Extended periods of the excessively loud volume may be damaging;^[5][6] however, one hearing expert found that "fewer than 5% of users select volume levels and listen frequently enough to risk hearing loss."^[7] 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^[8] a limit on all music players sold in the country:^[8] they must not be capable of producing more than 100dBA (the threshold of hearing damage during extended listening is 80dB, and the threshold of pain, or theoretically of immediate hearing loss, is 130dB). Many users decry this as an infringement on personal choice, and use third-party options to reverse the volume limits placed on such devices. Still, other users welcome the government's pro-health stance.

Canalphones and in-ear monitors have been described as being less likely to cause hearing impairment in noisy environments because much of the external noise is physically blocked out due to the noise isolation properties of the in-ear seal. This allows the user to listen at lower volume levels. However, the user can still choose to listen at dangerously high levels.

Other risks arise from the reduced awareness of external sounds—some jurisdictions regulate the use of headphones while driving vehicles, usually limiting the use of earphones to a single ear. The complete isolation from outside noise can be a hazard in itself, as a user could miss the sound of a car horn and walk into traffic with fatal consequences. Losing situational awareness can also lead to theft, particularly in busy environments where bumping into another person would be ignored, e.g., subway stations.

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 actually enhanced when doing so. The ear can normally detect 1-billionth of an atmosphere of sound pressure level,^[9] 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.^[10] Since earplugs reduce the noise in the auditory canal, this protective mechanism is less likely to trigger, and full sensitivity of the ear is maintained. With normal sensitivity, a listener has excellent hearing while listening to helmet speakers through the earplugs. This technique allows excellent hearing of speech, music and most external sounds at sustainable levels without hearing damage.

Listening to music through headphones while exercising can be dangerous. Blood may be diverted from the ears to the limbs leaving the inner ear more vulnerable to damage from loud sound.^[11] A Finnish study^[12] recommended that exercisers should set their headphone volumes to half of their normal loudness and only use them for a half hour.^[11]

See also

• Bone conduction
• Digital audio player
• Headphone amplifier
• In-ear monitor
• Noise-cancelling headphone
• TRS connector

References

External links

Headphone resources

• Headwize Library: An Academically Oriented Resource Site for Headphones and Headphone Listening