Page 90 - The Master Handbook Of Acoustics

65
THE EAR AND THE PERCEPTION OF SOUND

For a sound coming
directly from the front of
the observer (azimuth
°
and vertical angle = 0 ), Wavefront
the “frequency response”
of the combination at the
opening of the ear canal
will be that shown in
Fig. 3-15. Instead of fre-
quency response, a curve
of this type is called a
transfer function because
it represents a vector
combination involving
phase angles. FIGURE 3-14
For the sound at the
A wavefront of a sound can be considered as numer-
entrance of the ear canal ous rays perpendicular to that wavefront. Such rays,
(Fig. 3-15) to reach the striking a pinna, are reflected from the various ridges
and convolutions. Those reflections directed to the
eardrum, the auditory
opening of the ear canal combine vectorially (accord-
canal must be traversed. ing to relative amplitudes and phases). In this way
As the transfer function the pinna encodes all sound falling on the ear with
at the entrance to the ear directional information, which the brain decodes as a
directional perception.
canal (Fig. 3-15) and that
of the ear canal (Fig. 3-3) are combined, the shape of the resulting
transfer function impinging on the eardrum is radically changed. Fig-
ure 3-3 showed a typical transfer function of the ear canal alone. It is a
static, fixed function that does not change with direction of arrival of
the sound. The ear canal acts like a quarter-wave pipe closed at one
end by the eardrum exhibiting two prominent resonances.
The transfer function representing the specific direction to the
source of Fig. 3-15 combining with the fixed transfer function of the
ear canal (Fig. 3-3) gives the combined transfer function at the eardrum
of Fig. 3-16. The brain translates this to a perception of sound coming
from directly in front of the observer. 2
The transfer function at the entrance to the ear canal (such as Fig.
3-15) is shaped differently for each horizontal and vertical direction.
This is how the pinna encodes all arriving sound enabling the brain to
yield different perceptions of direction. The sound arriving at the