Page 68 - The Master Handbook Of Acoustics

43
THE EAR AND THE PERCEPTION OF SOUND

Pinna

Semicircular
canals

Middle Round Cochlea
ear window
Eardrum
Auditory
canal
FIGURE 3-1

The four principal parts of the human ear: the pinna, the auditory canal, the middle ear,
and the inner ear.

The Pinna: Directional Encoder of Sound

In ancient times, the pinna was regarded as either a vestigial organ or
a simple sound-gathering device. True, it is a sound-gathering device.
The pinna offers a certain differentiation of sounds from the front as
compared to sound from the rear. Cupping your hand behind the ear
increases the effective size of the pinna and thus the apparent loud-
ness by an amount varying with frequency. For the important speech
frequencies (2,000 to 3,000 Hz), sound pressure at the eardrum is
increased about 5 dB. This front-back differentiation is the more mod-
est contribution of the pinna.
Recent research has revealed that the pinna performs a very crucial
function in imprinting directional information on all sounds picked
up by the ear. This means that information concerning the direction to
the source is superimposed on the sound content itself so that the
resultant sound pressure on the eardrum enables the brain to interpret
both the content of the sound and the direction from which it comes.