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Binaural recording is a method of recording sound that uses two microphones, arranged with the intent to create a 3-D stereo sound sensation for the listener of actually being in the room with the performers or instruments. This effect is often created using a technique known as "Dummy head recording", wherein a mannequin head is outfitted with a microphone in each ear. Binaural recording is intended for replay using headphones and will not translate properly over stereo speakers.
The term "binaural" has frequently been confused as a synonym for the word "stereo", and this is partially due to a large amount of misuse in the mid-1950s by the recording industry, as a marketing buzzword. Conventional stereo recordings do not factor in natural ear spacing or "head shadow" of the head and ears, since these things happen naturally as a person listens, generating their own ITDs (interaural time differences) and ILDs (interaural level differences). Because loudspeaker-crosstalk of conventional stereo interferes with binaural reproduction, either headphones are required, or crosstalk cancellation of signals intended for loudspeakers such as Ambiophonics. For listening using conventional speaker-stereo, or mp3 players, a pinna-less dummy head may be preferable for quasi-binaural recording, such as the sphere microphone or Ambiophone. As a general rule, for true binaural results, an audio recording and reproduction system chain, from microphone to listener's brain, should contain one and only one set of pinnae (preferably the listener's own) and one head-shadow.
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With a simple recording method, two microphones are placed 18 cm (7") apart facing away from each other. This method will not create a real binaural recording. The distance and placement roughly approximates the position of an average human's ear canals, but that is not all that is needed. More elaborate techniques exist in pre-packaged forms. A typical binaural recording unit has two high-fidelity microphones mounted in a dummy head, inset in ear-shaped molds to fully capture all of the audio frequency adjustments (known as head-related transfer functions (HRTFs) in the psychoacoustic research community) that happen naturally as sound wraps around the human head and is "shaped" by the form of the outer and inner ear. The Neumann KU-81, and KU-100 are the most commonly used binaural packages, especially by musicians. A simplified version of binaural recordings can be achieved using microphones with a separating element, like the Jecklin Disk. Not all cues required for exact localization of the sound sources can be preserved this way, but it works also well for loudspeaker reproduction.
In the late 1960s, Aiwa and Sony offered headphones with a pair of microphones mounted on the headband around two-inches above the ears. These allowed pseudo-binaural recordings to be made.
Miniature binaural "in-ear" or "near-ear" microphones can be linked to a portable Digital Audio Tape (DAT) or MiniDisc recorder, bypassing the need for a dummy head by using the recordist's own head. The first clip-on binaural microphones using the recordist's own head were offered by Core Sound LLC in 1989. The first completely "in-ear" binaural microphones using the recordist's own head were offered by Sound Professionals in 1999.
Once recorded, the binaural effect can be reproduced using headphones or a dipole stereo. It does not work with mono playback; nor does it work while using loudspeaker units, as the acoustics of this arrangement distort the channel separation via natural crosstalk (an approximation can be obtained if the listening environment is carefully designed by employing expensive crosstalk cancellation equipment.)
The result is a listening experience that spatially transcends normally recorded stereo, since it accurately reproduces the effect of hearing a sound in person, given the 360° nature of how human ears pick up nuance in the sound waves. Binaural recordings can very convincingly reproduce location of sound behind, ahead, above, or wherever else the sound actually came from during recording.
Any set of headphones that provide good right and left channel isolation are sufficient to hear the immersive effects of the recording, and anyone who has even a cheap set of headphones can enjoy the recordings. Several high-end head set manufacturers have created some units specifically for the playback of binaural. In addition, a number of headphone amplifier companies have created hardware that takes advantage of these special recordings. However, these in-ear-canal phones tend to suffer from poor externalization i.e. inside-head localization. It is also found that even normal headphones suffer from poor externalization, especially if the headphone completely blocks the ear from outside. A better design for externalization found in experiments is the open-ear one, where the drivers are sitting in front of the pinnae with the ear canal connected to the air. The hypothesis is that when the ear canal is completely blocked, the radiation impedance seen from the eardrum to the outside has been altered, which negatively affects externalization.
There are some complications with the playback of binaural recordings through normal headphones. The sound that is picked up by a microphone placed in or at the entrance of the ear channel has a frequency spectrum that is very different from the one that would be picked up by a free-standing microphone. The diffuse-field head-transfer function, that is, the frequency response at the ear drum averaged for sounds coming from all possible directions, is quite grotesque, with peaks and dips of 10 dB. Frequencies around 5 kHz in particular are strongly attenuated as compared to side-presentation. If headphones were designed to have a flat response, then they would sound much too 'bright' when used to listen to stereo recordings and for this reason, most headphones are designed to have a notch around 5 kHz. This can only ever approximate to a particular listener's ears, meaning that there can be no such thing as a 'flat' pair of headphones - they have to match the listener's ears, unlike speakers which aim to produce a flat free field response. For binaural listening though, the headphones need to have a flat response to the ear-canal entrance (not the eardrum) since it is at the ear-canal entrance that the recording microphones are placed. While it is in theory possible to equalise any headphones for a flat response at the entrance, a better approach is to use headphones designed without the notch in the first place.
The fact that most headphones are actually designed to have a notch in the 5 kHz region is not widely understood, and this may have led to errors in the assessment of binaural recordings, since flat headphones are not normally available, and 'high quality' headphones incorporate the notch just like any others, though perhaps with a better approximation to that needed by the average ear. Male and female ears are of course different in size, and the outer ear gets bigger with increasing age, raising the possibility that most headphones are only really suited to young men. If headphones were designed to be flat, with a choice of equalisations available in headphone amplifiers, then headphone listening could be a more controlled experience, whether in binaural or stereo mode.
The history of binaural recording goes back to 1881. The first binaural unit, the Théâtrophone, was an array of carbon telephone microphones installed along the front edge of the Opera Garnier. The signal was sent to subscribers through the telephone system, and required that they wear a special head set, which had a tiny speaker for each ear.
The novelty wore off, and there wasn't significant interest in the technology until around forty years later when a Connecticut radio station began to broadcast binaural shows. Stereo radio had not yet been implemented, so the station actually broadcast the left channel on one frequency and the right channel on a second. Listeners would then have to own two radios, and plug the right and left ear pieces of their head sets into each radio. Naturally, the expense of owning two radios was, at the time, too much for a broad audience, and again binaural faded into obscurity.
Binaural stayed in the background due to the expensive, specialized equipment required for quality recordings, and the requirement of headphones for proper reproduction. Particularly in pre-Walkman days, most consumers considered headphones an inconvenience, and were only interested in recordings that could be listened to on a home stereo system or in automobiles. Lastly, the types of things that can be recorded do not have a typically high market value. Recordings that are done in studios would have little to benefit from using a binaural set up, beyond natural cross-feed, as the spatial quality of the studio would not be very dynamic and interesting. Recordings that are of interest are live orchestral performances, and ambient "environmental" recordings of city sounds, nature, and other such subject matters.
The modern era has seen a resurgence of interest in binaural, specifically within the audiophile community, partially due to the widespread availability of headphones, and cheaper methods of recording. A small grassroots movement of people building their own recording sets and swapping them on the Internet has joined the very few CDs available for purchase.