Headphones

Two Sony MDR-series professional headphones; one folded for travel

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 earphones, earbuds, stereophones, headsets or, informally cans. 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.

History

Old telephone earpiece

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

Brandes radio headphones, circa 1920

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. Impedance varied, but 1,000 to 2,000 ohms was common, which suited both triodes and crystal sets.

When used with early powered radios, the headphone was normally connected to the positive high voltage battery terminal, and the other battery terminal was securely earthed. The use of bare electrical connections meant some 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.). Some cordless headphones do not need to be 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.

Headphones are widely used for listening to audio sources for recreation. 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 connectors are 1/4" and 3.5 mm stereophonic jack plugs and sockets. The older 1/4" standard is used on professional equipment, and is often used on fixed equipment. Sony introduced the smaller, cheaper, and now widely-used, 3.5 mm "mini" stereo connector in 1979, adapting the older monophonic 3.5 mm connector for use with its Walkman portable stereo tape player. Adapters are available for converting between 1/4" and 3.5 mm devices.

Headsets

Main article: Headset (telephone/computer)

South Australian Premier John Bannon using a headset.

Headphones that include a microphone are more commonly known as Headsets, and are more often targeted for communication than recreational listening.^[2] Communication headsets include mainly three types, wired telephone, computer and mobile telephone. Telephone headsets are usually connected to a fixed-line (PSTN) telephone terminal, replacing its handset, so users can talk on the phone while they work on other things. Telephone headsets are very commonly used in Call Centers and offices, but are sometimes used at home. Computer Headsets come in two types, standard 3.5 mm plugs for connecting to sound-card of a PC, or USB connection. Both types are very commonly used for VoIP communication, but USB headsets tend to have better sound quality. Gamers might choose to use a headset in order to both hear the game sounds and talk to their fellow gamers. Computer users who employ speech recognition software often use a headset. Mobile phone headsets come in two types, wired and wireless. Wired mobile phone headsets are sometimes called as "Mobile Handsfree", and are basically a pair of earphones with a microphone module connected to the cable. Wireless mobile handsfree also comes in different types, and the most common type nowadays is the Bluetooth headset that rides on the ear. Wireless and wired intercom units used by the coaching staff of professional sports and for broadcast control and event management often employ a headset attachment.

Types of headphones

Circumaural headphones have large pads that surround the outer ear.

There are many different types of headphone designs, with the listening situation and the needs of the listener determining what type of headphone will be used. Generally, headphone formfactors can be divided into four separate categories: circumaural, supra-aural, earbud, and in-ear.

Circumaural

Circumaural headphones have circular or ellipsoid earpads that fit around the ears. This type of headphone is commonly used in recording studios and among audio enthusiasts.

Supra-aural

A pair of supra-aural headphones

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.

Earbuds

Earbuds or earphones, such as these that ship with the Apple's iPod, rest in the outer part of the ear canal.

Earbuds or earphones are headphones of a 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. However, due to their inability to provide isolation, they are not capable of delivering the same dynamic range offered by many full-sized headphones and canalphones for a given volume level. As a result, 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.^[3] During the 1990s and 2000s, earbuds became a common type bundled with personal music devices.

Canalphones

Canalphones extend into the ear canal, facilitating greater dynamic range than earbuds as well as isolation from outside noise.

Canalphones, also known as in-ear monitors, or IEMs, are earbuds 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. Universal canalphones are typically marketed to casual listeners and are relatively inexpensive, though some offer very high audio quality.

Custom canalphones are fitted to individuals. Castings of the ear canals are made, usually by an audiologist. 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.

Earcup differences

The housing, or earcup, of a headphone will be either open or closed. Open headphones, sometimes marketed as “open air” headphones, have an open grille on the back of the earcup, exposing the driver to the outside and allowing sound waves to propagate away from the ear freely. This backing type does not isolate the listener from outside sounds; in addition, sound through open headphones can be heard by others in the vicinity of the user. They usually have less distortion due to the lack of earcup resonance. Closed headphones have a sealed backing, which attenuates sound waves propagating away from the ear. As a result, listeners away from the headphones cannot hear the produced sound easily. In addition, sound from outside is attenuated by the sealed backing, providing a level of isolation to the listener. A sealed chamber is often claimed to have the negative effect of distorting sound in certain frequencies due to resonances within the earcup housing, however, bass frequencies are stronger in a sealed chamber headphone design.

Technology

A typical moving-coil headphone transducer

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 loudspeaker diagram

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

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

Balanced armature transducer with armature torqued and exerting a 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 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 'sound-powered' telephones for military use. Some of these achieved astonishing electro-acoustic conversion efficiencies in the 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 (<20Hz, >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. Some combine an armature driver with a small moving-coil driver for increased bass output.

Orthodynamic

Orthodynamic, or magnetostatic drivers, are either composed of a thinly pressed disc made of tightly coiled fine aluminium wire affixed to a mylar sheet or of a printed circuit. This disc is the diaphragm. The diaphragm is then 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 manufacturers such as Yamaha for their headphones, the technology has fallen generally into disuse as companies increasingly favour moving-coil designs. Fostex though, continues to manufacture orthodynamic headphones.

AMT/Piezo/Plasma

AMTs and Piezoelectric drivers are very uncommonly used (when compared to those above) diaphragm technologies, however both have been utilised as headphone drivers in the past. Current production models utilising the technologies total only one for each, although the TakeT piezoelectric headphone is technically a piezoelectric-AMT hybrid design because of the shape of its diaphragm. There has also been one attempt to market a plasma-ionisation drive headphone.

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 potentially provide a level of sound quality 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. ^[4]

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 centre 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 import 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 positron 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 an 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.

Over time, headphone cables may 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 can 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, airplanes, 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 a limit on all music players sold in the country: 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 external noise is physically blocked out, allowing 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.

See also

• Noise-cancelling headphone
• Bonephones
• Headphone amplifier
• TRS connector
• Digital audio player
• In-ear monitor
• Headset (telephone/computer)

References

External links

Headphone Resources

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