Binaural recording
Engineers make a technical distinction between "binaural" and "stereophonic" recording. Of these, binaural recording is more like stereoscopic photography. In binaural recording, a pair of microphones is put inside a model of a human head which includes external ears and ear canals. Each microphone is where the eardrum would be.

The recording is then played back through headphones, so that each channel is presented independently, without mixing or crosstalk. Thus, each of the listener's eardrums is driven with a replica of the auditory signal it would have experienced at the recording location. The result is an accurate duplication of the auditory spatiality that would have been heard by the listener placed where the microphones were. Because of the nuisance of wearing headphones, true binaural recordings have remained laboratory and audiophile curiosities.
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, generating his own ITDs (interaural time differences) and ILDs (interaural level differences).

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 (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. The KEMAR system is another alternative. The more expensive Aachen Head Acoustics unit does automatic equalization and processing to create a more enveloping experience. Other alternatives are the B&K and the 01dB-Metravib acoustic heads. The last one following exactly the IEC959 standard. A simplified version of this, called a Jecklin Disk, uses a 30 cm (11.81") acoustically-absorptive disk between the mics, as a compromise.

Binaural "in-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.
Playback
Once recorded, the binaural effect can be reproduced using only 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. 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 ear from outside. A better design for externalization found in experiments is open-ear one where the drivers are sitting in front of pinneae with ear canal connected to the air. The hypothesis is that when the ear canal is completely blocked, the radiation impedance seen from the ear drum 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. Especially frequencies around 5 kHz are strongly emphasized. Hence, a binaural recording that is listened to through a headphones without any compensation will have a strong and undesired coloration due to interaction between headphone and pinneae. In order to correct this, the binaural recording should properly be equalized. The best playback headphone for binaural recording is one that is transparent down to ear drum i.e one that can directly inject sound to the ear drum as if the sound is being played right in front of an ear drum.

This article is licensed under the GNU Free Documentation License. It uses material from one or more Wikipedia article