Page 72 - Master Handbook of Acoustics

Similarly, the ear must provide a way for energy in air to enter the ear’s fluid interior.
The object is to transfer, with maximum efficiency, the energy represented by the vibratory motion
of the eardrum diaphragm to the fluid of the inner ear. The twofold solution is suggested in Fig. 4-4.

The three ossicles (hammer, anvil, and stirrup, as shown in Fig. 4-4A) form a mechanical linkage
between the eardrum and the oval window, which is in intimate contact with the fluid of the inner ear.
The first of the three bones, the malleus, is fastened to the eardrum. The third, the stapes, is actually a
part of the oval window. There is a lever action in this linkage with a ratio leverage ranging from
1.3:1 to 3.1:1. That is, the eardrum motion is reduced by this amount at the oval window of the inner
ear.

FIGURE 4-4 The middle ear provides impedance matching. (A) The ossicles (hammer, anvil, and
stirrup) of the middle ear transmit mechanical vibrations of the eardrum to the oval window of the
cochlea. (B) A mechanical analog of the impedance-matching function of the middle ear. The
difference in area between the eardrum and the oval window, coupled with the step-down
mechanical linkage, matches the motion of the air-actuated eardrum to the fluid-loaded oval window.

This is only one part of the mechanical-impedance-matching device. The area of the eardrum is

2
2
about 80 mm , and the area of the oval window is only 3 mm . Hence, a given force on the eardrum is
reduced by the ratio of 80/3, or about 27-fold.
In Fig. 4-4B, the action of the middle ear is likened to two pistons with area ratios of 27:1