Page 89 - Master Handbook of Acoustics

FIGURE 4-15 A measured example of the sound pressure (transfer function) at the opening of the ear
canal corresponding to sound arriving from a point directly in front of the listener. The shapes of
such transfer functions vary with the horizontal and vertical angles at which the sound arrives at the

pinna. (Mehrgardt and Mellert.)

For the sound at the entrance of the ear canal to reach the eardrum, the auditory canal must be
traversed. As the transfer function at the entrance to the ear canal (see Fig. 4-15) and that of the ear
canal itself (see Fig. 4-3) are combined, the shape of the resulting transfer function impinging on the
eardrum is radically changed. Figure 4-3 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 and exhibiting two prominent
resonances.

The transfer function representing the specific direction to the source (see Fig. 4-15) combining
with the fixed transfer function of the ear canal (see Fig. 4-3) gives the transfer function at the
eardrum of Fig. 4-16. In this example, the brain translates this to a perception of sound coming from
directly in front of the observer.