Headphones

Headphones let a single user listen to an audio source privately, in contrast to a loudspeaker, which emits sound into the open air for anyone nearby to hear.

Another type, known as earbuds or earpieces,[1] consists of individual units that plug into the user's ear canal; within that category have been developed cordless air buds using wireless technology.

Some headphones are wireless, using Bluetooth connectivity to receive the audio signal by radio waves from source devices like cellphones and digital players.

The device created a listening system through the phone lines that allowed the customer to connect into live feeds of performances at theaters and opera houses across London.

Wireless Specialty Apparatus Co., in partnership with Baldwin Radio Company, set up a manufacturing facility in Utah to fulfill orders.

The common single-ended type used voice coils wound around the poles of a permanent magnet, which were positioned close to a flexible steel diaphragm.

The requirement for high sensitivity meant that no damping was used, so the frequency response of the diaphragm had large peaks due to resonance, resulting in poor sound quality.

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 recent years, impedance of newer headphones has generally decreased to accommodate lower voltages available on battery powered CMOS-based portable electronics.

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).

[37] 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.

Generally, headphone form factors can be divided into four separate categories: circumaural (over-ear), supra-aural (on-ear), earbud and in-ear.

[40][failed verification] 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.

Custom in-ear headphones use castings of the ear canal to create custom-molded plugs that provide added comfort and noise isolation.

Most models of telephone amplifiers offer volume control for loudspeaker as well as microphone, mute function and switching between headset and handset.

Electrostatic drivers consist of a thin, electrically charged diaphragm, typically a coated PET film membrane, suspended between two perforated metal plates (electrodes).

Air is forced through the perforations; combined with a continuously changing electrical signal driving the membrane, a sound wave is generated.

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.

Patent applications from 2009 to 2013 have been approved that show by using different materials, i.e. a "Fluorinated cyclic olefin electret film", Frequency response chart readings can reach 50 kHz at 100 db.

As illustrated in the right 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.

[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.

[47] 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.

[citation needed] Transducer technologies employed much less commonly for headphones include the Heil Air Motion Transformer (AMT); Piezoelectric film; Ribbon planar magnetic; Magnetostriction and Plasma or Ionic.

Due to the small volume of air in a headphone, the plasma or ionic transducer can become a full range driver although the high temperatures and voltages needed makes them very rare.

"[57] The International Telecommunication Union recently published "Guidelines for safe listening devices/systems" recommended that sound exposure not exceed 80 decibels, A-weighted dB(A) for a maximum of 40 hours per week.

[65] Hearing risk from headphones' use also applies to workers who must wear electronic or communication headsets as part of their daily job (i.e., pilots, call center and dispatch operators, sound engineers , firefighters, etc.)

The National Institute for Occupational Safety and Health (NIOSH) recommends sound exposure not exceed 85 dB(A) over 8 hour work day as a time-weighted average.

NIOSH published several documents targeted at protecting the hearing of workers who must wear communication headsets such as call center operators,[68] firefighters,[69] and musicians and sound engineers.

Headphones on a stand
Telephone operator with a single head-mounted telephone-receiver (headphone), 1898
Wireless telegrapher Reginald Fessenden with two head-mounted telephone-receivers (headphones), 1906
Holtzer-Cabot telephone operator head receiver, wireless operator receiver, and telephone receiver, 1909
Sennheiser HD 555 headphones, used in audio production environments (2007)
This adaptor allowed an airline passenger to plug a standard 3.5mm stereo headphone into the twin mono jacks then common on airliners, avoiding the need to pay for a set of headphones from the airline
Headphone cord with integrated potentiometer for volume control
Circumaural headphones have large pads that surround the outer ear.
A pair of supra-aural (on-ear) headphones
In-ear monitors extend into the ear canal, providing isolation from outside noise.
A typical example of a headset used for voice chats
Sony Ericsson Cordless bluetooth headset
Aviation headset [ 46 ]
In-ears are among those good for noise isolation.
A typical moving-coil headphone transducer
Electrostatic loudspeaker diagram
A custom in-ear monitor which uses 8 balanced armatures in a triple crossover configuration (4 low/2 mid/2 high). Headphone designs often use multiple balanced armatures to provide a higher fidelity sound.
Sony MDR-7506 headphones in stowed configuration
A micro audio amplifier for boosting the output power of smartphones etc. to headphones. Used for example to compensate a built-in volume limit in smartphones, the higher volume levels could, however, lead to ear damage.
Product testing - headphones in an anechoic chamber