Page 90 - Master Handbook of Acoustics

FIGURE 4-16 The transfer function of Fig. 4-15 at the opening of the ear canal is altered to this
shape at the eardrum after being combined with the transfer function of the ear canal. In other words,
a sound arriving at the opening of the ear canal from a source directly in front of the observer (see

Fig. 4-15) looks like Fig. 4-16 at the eardrum because it has been combined with the characteristics
of the ear canal itself (see Fig. 4-3). The brain discounts the fixed influence of the ear canal from
every changing arriving sound.

The transfer function at the entrance to the ear canal 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 eardrum is the raw material for all
directional perceptions. The brain neglects the fixed component of the ear canal and translates 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 directly depend on spatial encoding. At higher frequencies (shorter
wavelengths), the pinna is an effective barrier; sounds from behind have relatively lower high-
frequency levels. The brain uses this front-back differentiation to convey a general perception of
direction.

The ear can also weakly perceive vertical localization. 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 technique 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.
Sound arriving from directly in front of a listener results in a peak in the transfer function at the
eardrum in the 2- to 3-kHz region. This is partly the basis of the technique of imparting “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.