Binaural recording

From Academic Kids

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Dummyhead.jpg
Dummy head being used for binaural recording; the second microphone is obscured.

Binaural recording is a method of recording audio which uses a special microphone arrangement. The term "binaural" has often 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. In truth, binaural recordings are the best way to reproduce stereo with headphones. Typical stereo recordings are mixed for loudspeaker arrangements, and do not factor in natural crossfeed or sonic shaping of the head and ear, since these things happen naturally as a person listens.

Recording technique

With a simple recording method, two microphones are placed seven inches (18cm) 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 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. The KEMAR system is another alternative. The more expensive and accurate Aachen Head Acoustics unit does automatic equalization and processing to create a more enveloping experience. A simplified version of this, called a Jecklin Disk, uses a 30cm (12")acoustically-absorptive disk between the mics, is a compromise.

In the past, a number of microphone sets were available that offered a customized "in-ear" microphone system, which could be linked to a portable DAT or MiniDisc recorder, bringing the ability to not only create binaural recordings to the masses, but to create mobile recordings as well. Unfortunately, these have been discontinued and are only available in the used electronics market.

Playback

Once recorded, the binaural effect can only be reproduced using headphones. 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 crossfeed (unless the arrangement is carefully designed, and using expensive crossfeed 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 records 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. As with any playback, higher quality headphones will do a better job of creating the illusion. Several high-end head set manufacturers have created some units specifically for the playback of binaural. Etymotic Research's ER-4B canal phone actually sits inside the ear, much like a hearing aid. The B model is tuned and equalized to enhance binaural playback. In addition, a number of headphone amplifier companies have created hardware that takes advantage of these special recordings.

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 free-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. Especially frequencies around 5kHz are strongly emphasized. A good headphone for normal usage is designed to have a frequency response that compensates for the distortion caused by the pinneae, shoulders, head, and ear channel. A binaural recording already has these corrections in the frequency response, and a headphone should not affect the frequency spectrum again. Hence, a binaural recording that is listened to through normal headphones will have a strong and undesired coloration. Binaural recordings that are intended to be played back through standard headphones should therefore be recorded with (omnidirectional) microphones outside the ears, approximately at the location where the headphone loudspeaker will be sitting.

History

The history of binaural recording goes back to 1881. The first binaural unit was an array of carbon telephone microphones installed along the front edge of the Paris Opera. 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 broadcasted 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 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 crossfeed, 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 small collection of CDs that one can find available for purchase.

See also: binaural tones.de:Binaural

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