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In this thread I referred to binaural sound as stereophony sound. Binaural
sound is the correct term.

Binaural sound is most often recorded using a model of a human head which
has accurately proportioned or approximately proportioned human ears
appropriately positioned on the sides of a more or less realistically shaped
head of human form. Recording microphones placed inside the dummy head hear
the sounds traveling down the dummy's ear canals. The listener must use
headphones or inside the ear canal speaker plugs. The effect of human ears
in collecting sound is included in the recordings. Listening to the sound
recorded from the mics using the external ear characteristics a second time
would spoil the effect. In addition to things already mentioned the listener
has to have the head erect, forward and motionless.  Otherwise a very
confusing aural sensory experience will result.

One listening to a binaural sound recording can hear not only the lateral
location of the sound source, as heard by the dummy's head but with three
dimensional location of the sound source that is limited only by the ability
of the listener in ordinary situations to hear where the sound comes from.
For most listeners that ability is better than we may realize. Most people
can determine where sound is (or seems to be) with respect with before or
behind, above or below, and left and right.

Just as things like mirrors can alter our visual perceptions, so that we see
something that is in front of us to be somewhat behind us for example, our
audio perceptions are often altered. Hard sound reflecting surfaces can
alter can change where a sound source is heard to be. In a room we might
hear a voice to be from a person on our right when the person is down a
hallway that connects to the room from which runs towards the left but which
enters the room in front of us. The voice has traveled down the hall,
crossed the room, reflected from the room wall on the right and then reached
our ears from that direction. Binaural sound will not correct such
illusions. The way we would have heard the sound had we been where the
recording dummy head was should be the way we experience the binaural
recording--echoes, distortions, and all.

For binaural to work well the recording and playback equipment should be
better then the human audio system. Frequency response should be flat from
sub-audible to super-audible frequencies. The way that the human audio
system locates sounds in space requires an amazing processing of subtle
changes to sound caused by the shape of the ears. These clues include very
small time delays and other modifications to secondary sounds formed when a
frequency complex directional sound wave front interacts with the listeners
ears as the sound is coupled via our vertically and laterally unsymmetrical
ears into the ear canal before heading down the ear canal towards the ear
drum (tympanic membrane).

Binaural perception would seem to require that when listening to the
recording the sound wave is being recreated inside our own ear canals. That
would seem improbable, how could a wave form be recreated in an ear canal
using a simple non-directional sound source? Binaural is known to work so is
the suggestion that playback must recreate the sound wave in the ear canal
incorrect? By the time sound enters a sufficient distance into the ear canal
the required shaping and directional clues seem to have been reduced to a
sound wave adequately recorded by non-directional means aside from its being
received by the microphone in a particular ear. As far as sound is
concerned, the ear canals are a round tube. The playback equipment generates
sound using a device conveys no physical directional information aside from
the sound being inserted into the appropriate ear.

Listening to binaural sound recording using loud speakers even if they were
located directly beyond our respective left and the right ears would be a
satisfactory binaural experience. The directional clues provided by the
shape of our ears would alter the recorded sound that is already binaurally
encoded by the recording dummy's ears. The consequence is something that we
could not properly interpret as positional information.

When the Web is searched using the term binaural with other appropriate key
words and phrases a wealth of information is found. This includes photos of
the heads of recording dummies and other information and lore and binaural
recordings from short clips to full length binaural recordings in various
formats. Some of the recordings are very old binaural recordings. Software
exists with capabilities to create binaural like effects in sound that is
already recorded.

Ralph Bradley
Family Voices, family archive services