38.2 Structure and Function of the Ear [1]

Back

The main components of the human ear are depicted in Figure 38.1(a). The ear is commonly divided into

three main components: (1) the outer ear, (2) the middle ear, and (3) the inner ear.

The visible portion of the ear is called the pinna. Because of its small size compared with the primary

wavelengths that we hear, the pinna serves only to produce a small enhancement of the sounds that arrive

from the front of the listener as compared to those which arrive from behind; that is, the human sound

reception system has a small frontal directivity. The remainder of the outer ear, which consists of the ear

canal terminated in the ear drum, forms a resonant cavity at about 3 kHz. This resonant or near-resonant

condition allows for a nearly reflection-free termination of the ear canal and thus a good impedance

match of the ear drum to the air in which the sound wave was propagated.

The middle ear consists of three small ear bones, the hammer, anvil, and stirrup. The middle ear serves

as an impedance transformer, which matches the low impedance of the air in which sound travels and in

38-1

© 2005 by Taylor & Francis Group, LLC

which the ear drum is located to the high impedance of the lymphatic fluid of the cochlea beyond the oval

window. Without this impedance-matching transformation, a mismatch would occur, resulting in a loss

of approximately 30 dB.

In the inner ear, the cochlea is the main component where the actual reception of sound takes place.

The schematic extended structure of the cochlea is depicted in Figure 38.1(b). The cochlea, which is

located in extremely hard temporal bone, is divided almost its entire length by the basilar membrane. At

the end of the cochlea, the two canals are connected by the helicotrema, which allows for the flow of the

lymphatic fluid between the two sections. The basilar membrane, which is about 3 cm long and 0.02 cm

wide, has about 24,000 nerve ends terminated in hair cells located on the membrane. The motion of the

oval window is transmitted to the basilar membrane and its associated sensing cells. This motion is

sensed as sound.

Навигация