Binaural Recordings

It has long been known that it is not necessary to have multiple channels to create convincing 3-D sound -- two channels are sufficient. The trick is to recreate the sound pressures at the right and left ear drums that would exist if the listener were actually present.

A conceptually simple approach is to put two microphones in the ear canals of an acoustic manikin (or even just hold two microphones close to your own ears) and to record what they pick up. When the left and right signals are fed to the left and right headphone units, it is as if the listener were present in the original sound field. In particular, if the manikin and the listener have heads with the same size and shape, the same ITD and ILD information will be present; similarly, if the manikin and the listener have pinnae with the same sizes and shapes, the same elevation cues will be present. Recordings made this way are called binaural recordings, and they can produce quite vivid 3-D sound. In particular, it is possible to use binaural recordings in HCI to produce 3-D sounds for such things as standard system messages.

Despite their economy and effectiveness, binaural recordings suffer from several disadvantages:

They require the use of headphones (but see cross-talk cancelled stereo)
They are not interactive, but must be prerecorded
If the listener moves, so do the sounds
Sources that are directly in front usually seem to be much too close*
Because pinna shapes differ from person to person, elevation effects are not reliable

Improving on binaural recordings requires an understanding of head-related transfer functions.

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