Binaural recording
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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 cross feed 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 techniqueWith 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. |
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