Page 70 - The Master Handbook Of Acoustics

45
THE EAR AND THE PERCEPTION OF SOUND

15
Relative sound-pressure level - dB 5
10

0

5

10
0.1 0.2 0.5 1 2 5 10 20
Frequency - kHz
FIGURE 3-3
The transfer function (frequency response) of the ear canal. This is a fixed component that is combined with
every directionally-encoded sound reaching the eardrum. See also Figs. 3-15 and 3-16. (After Mehrgardt and
2
Mellart. )

4,000 Hz. There is a secondary resonance nearer 9,000 Hz of lower
peak pressure. 2

The Middle Ear
Transmitting sound energy from a tenuous medium such as air into a
dense medium like water is a serious problem. Without some very spe-
cial equipment, sound originating in air bounces off water like light off
a mirror. It boils down to a matter of matching impedances, and in this
case the impedance ratio is something like 4,000:1. Consider how sat-
isfactory it would be to drive the 1-ohm voice coil of a loudspeaker
with an amplifier having an output impedance of 4,000 ohms! Clearly
not much power would be transferred.
The object is to get the feeble energy represented by the vibratory
motion of a rather flimsy diaphragm, transferred with maximum effi-
ciency to the fluid of the inner ear. The two-fold solution is suggested in
Fig. 3-4. The three ossicles (hammer, anvil, and stirrup) form a mechan-
ical linkage between the eardrum and the oval window, which is in inti-
mate 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