Page 92 - The Master Handbook Of Acoustics

67
THE EAR AND THE PERCEPTION OF SOUND

eardrum is the raw material for all directional perceptions. The brain
neglects (sees through?) the fixed component of the ear canal and trans-
lates the differently shaped transfer functions to directional perceptions.
Another more obvious directional function of the pinna is that of
forward-backward discrimination, which does not depend on encod-
ing and decoding. At the higher frequencies (shorter wavelengths), the
pinna is an effective barrier. The brain uses this front-back differentia-
tion to convey a general perception of direction.
A crucial question at this juncture is, “How about sounds arriving
in the median plane?” The median plane is a vertical plane passing
symmetrically through the center of the head and nose. Sources of
sound in this plane present identical transfer functions to the two ears.
The auditory mechanism uses another system for such localization,
that of giving a certain place identity to different frequencies. For
example, signal components near 500 and 8,000 Hz are perceived as
coming from directly overhead, components near 1,000 and 10,000 Hz
as coming from the rear. 15 This is an active area of research that is
being continually refined.
The pinna, originally suspected of being only a useless vestigial
organ, turns out to be a surprisingly sophisticated sound directional
encoding mechanism.
Sound arriving from directly in front of an observer results in a
peak in the transfer function at the eardrum in the 2- to 3-kHz region.
This is the basis of the successful technique of old-time sound mixers
adding “presence” to a recorded voice by adding an equalization boost
in this frequency region. A voice can also be made to stand out from a
musical background by adding such a peak to the voice response.

Binaural Localization

Stereophonic records and sound systems are a relatively new develop-
ment. Stereo hearing has been around at least as long as man. Both are
concerned with the localization of the source of sound. In early times
some people thought that having two ears was like having two lungs or
two kidneys, if something went wrong with one the other could still
function. Lord Rayleigh laid that idea to rest by a simple experiment
on the lawn of Cambridge University. A circle of assistants spoke or
struck tuning forks and Lord Rayleigh in the center with his eyes