Handbook for Sound Engineers

46 Chapter 3

the oval window and the round window. The auditory
portion of the inner ear is the snail-shaped cochlea. It is
a mechanical-to-electrical transducer and a fre-
quency-selective analyzer, sending coded nerve
impulses to the brain. This is represented crudely in Fig.
3-5. A rough sketch of the cross section of the cochlea
is shown in Fig. 3-6. The cochlea, throughout its length
(about 35 mm if stretched out straight), is divided by
Reissner’s membrane and the basilar membrane into
three separate compartments—namely, the scala ves-
tibuli, the scala media, and the scala tympani. The scala
vestibuli and the scala tympani share the same fluid,
perilymph, through a small hole, the helicotrema, at the
apex; while the scala media contains another fluid,
endolymph, which contains higher density of potassium
ions facilitating the function of the hair cells. The basi-
lar membrane supports the Organ of Corti, which con-
tains the hair cells that convert the relative motion
between the basilar membrane and the tectorial mem-
brane into nerve pulses to the auditory nerve.

When an incident sound arrives at the inner ear, the
vibration of the stapes is transported into the scala

vestibuli through the oval window. Because the cochlear

fluid is incompressible, the round window connected to

the scala tympani vibrates accordingly. Thus, the vibra-

tion starts from the base of the cochlea, travels along the

scala vestimbuli, all the way to the apex, and then

through the helicotrema into the scala tympani, back to

the base, and eventually ends at the round window. This

establishes a traveling wave on the basilar membrane

for frequency analysis. Each location at the basilar

membrane is most sensitive to a particular

frequency—i.e., the characteristic frequency—although

it also responds to a relatively broad frequency band at

smaller amplitude. The basilar membrane is narrower

(0.04 mm) and stiffer near the base, and wider (0.5 mm)

and looser near the apex. (By contrast, when observed

from outside, the cochlea is wider at the base and

smaller at the apex.) Therefore, the characteristic

frequency decreases gradually and monotonically from

the base to the apex, as indicated in Fig. 3-5. The trav-

eling-wave phenomenon illustrated in Figs. 3-7 and 3-8

shows the vibration patterns—i.e., amplitude versus

location—for incident pure tones of different frequen-

cies. An interesting point in Fig. 3-8 is that the vibration

pattern is asymmetric, with a slow tail close to the base

(for high frequencies) and a steep edge close to the apex

(for low frequencies). Because of this asymmetry, it is

easier for the low frequencies to mask the high frequen-

cies than vice versa.

Within the Organ of Corti on the basilar membrane,

there are a row of inner hair cells (IHC), and three to

five rows of outer hair cells (OHC), depending on loca-

tion. There are about 1500 IHCs and about 3500 OHCs.

Each hair cell contains stereociliae (hairs) that vibrate

corresponding to the mechanical vibration in the fluid

around them. Because each location on the basilar

membrane is most sensitive to its own characteristic

frequency, the hair cells at the location also respond

most to its characteristic frequency. The IHCs are

sensory cells, like microphones, which convert mechan-

ical vibration into electrical signal—i.e., neural firings.

The OHCs, on the other hand, change their shapes

according to the control signal received from efferent

nerves. Their function is to give an extra gain or attenua-

tion, so that the output of the IHC is tuned to the

characteristic frequency much more sharply than the

IHC itself. Fig. 3-9 shows the tuning curve (output level

vs. frequency) for a particular location on the basilar

membrane with and without functioning OHCs. The

tuning curve is much broader with poor frequency selec-

tivity when the OHCs do not function. The OHCs make

our auditory system an active device, instead of a

passive microphone. Because the OHCs are active and

Figure 3-5. The mechanical system of the middle ear.

Figure 3-6. Cross-sectional sketch of the cochlea.

Eardrum

80 mm^2

Obstacles

Ratio

1.3 to3:1

3 mm^2

window

Cochlea Basilar

membrane

Brain

10 kHz4 kHz 1 kHz100 Hz

Auditory

nerve

Reissner's

membrane

Tectorial

membrane

Outer

hair

cells

Basilar

membrane

Inner

hair cells

Scala

vestibuli

Scala tympani

Scala

media