6.1 THE HUMAN EAR

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To gain an appreciation of the damaging effects of sound on the human ear,

one must understand the physical construction of the ear. The human ear is

a remarkable acoustic system. The ear is capable of responding to sounds

over a frequency range from about 16–20Hz up to frequencies in the 16–

20 kHz range. In addition, the ear can detect acoustic pressures as low as

20 mPa at a frequency of 1000Hz and can withstand acoustic pressures as

large as 2000 Pa for short times.

The acoustic particle velocity for sound in air at 208C (688F) for an

acoustic pressure of 20 mPa may be calculated from Eq. (2-9):

u ј

p

_c ј р20Юр10_6Ю

р413Ю ј 48:4_10_9 m=s ј 1:9_10_6 in./sec ј

0:165 in/day

The corresponding particle displacement for a frequency of 1000Hz may be

found from Eq. (4-43):

_ ј

u

2_f ј р48:4Юр10_9Ю

р2_Юр1000Ю ј 7:70_10_12 ј 7:70pm ј 3_10_10 in

The diameter of the nitrogen molecule is about 380_10_12m or 380 pm

(Reid and Sherwood, 1966). The human ear can detect particle displacements

that are almost 1/50 of the diameter of a nitrogen molecule.

The human ear is one of the more intricate and complex mechanical

structures in the body. As shown in Fig. 6-1, the ear consists of three main

parts:

1. The outer ear, consisting of the pinna or visible ear, which acts as

a horn to collect sound, and the meatus or auditory canal, which

is terminated by the tympanic membrane or eardrum.

2. The middle ear, which involves three small bones: the malleus or

‘‘hammer’’, the incus or ‘‘anvil,’’ and the stapes or ‘‘stirrup’’.

These bones of the middle ear serve to transform the pressure

variations in the air in the outer ear into mechanical motion. The

eustachian tube in the middle ear serves to equalize the pressure

between the outer and inner ear volumes.

3. The inner ear, which contains the semicircular canals, the fluid

gyroscope associated with maintaining balance of the body, and

the cochlea, which analyzes, converts, and transmits information

about sound from the outer ear to the brain through the auditory

nerves.

226 Chapter 6

Copyright © 2003 Marcel Dekker, Inc.

The auditory canal acts as a resonator tube to increase the sound

pressure level of the sound striking the visible ear by 10 dB to 20 dB at

the eardrum, depending on the frequency of the sound. The resonant frequency

of the auditory canal is on the order of 3 kHz, so the acoustic

pressure increase is more pronounced in the 2–4 kHz octave bands. The

approximate length of the auditory canal is 25–30mm (1–11

4 in).

The mechanical motion of the eardrum is transmitted and amplified by

about 25 dB through the three-bone linkage in the middle ear. The hammer,

attached at one end directly to the eardrum, is normally locked to the anvil.

The anvil drives the stirrup, which is mounted into and sealed around the

periphery of the oval window by a network of elastic fibers. When the ear is

subjected to very intense sound, the contact between the hammer and the

anvil is broken, so the three-bone set acts as a safety device to prevent

damage to the oval window.

The main part of the inner ear is the cochlea, which is a bony tube

about 34mm (1.34 in) long, filled with liquid and coiled like a snail’s shell.

The cochlea makes about 23

4 turns around a central hollow passage that

contains the nerve fibers going to the brain. The cochlea is illustrated in

Acoustic Criteria 227

FIGURE 6-1 Cross-section of the human ear. (From Engineering Principles of

Acoustics, D. D. Reynolds, 1981. By permission of Allyn and Bacon, Inc.)

Copyright © 2003 Marcel Dekker, Inc.

Fig. 6-2. There is a bony projection or shelf and a membrane called the

basilar membrane that runs the length of the cochlea. The basilar membrane

divides the cochlea into two chambers, the upper chamber or scala vestibuli,

and the lower chamber or scala tympani. There is a small opening at the end

of the cochlea, called the helicotrema, which provides a connecting passage

between the upper and lower chambers. The basilar membrane varies in

width from 0.2mm (0.008 in) at the oval window to about 0.5mm

(0.020 in) at the end of the cochlea chamber.

The organ of corti is mounted about halfway along the spiral of the

cochlea on the basilar membrane. The organ of corti is made up of about

30,000 hair cells, arranged in four rows, which are attached to the tectorial

membrane in contact with the upper surface of the organ of corti. Any

movement of the basilar membrane supporting the hair cells will cause the

hair cells to bend. The bending of the small hairs produces the nerve

impulses in the neurons that are transmitted to the brain. This is the component

of the ear that can become permanently destroyed through long-

228 Chapter 6

FIGURE 6-2 Internal details of the cochlea. (A) Cross-sectional view through one

turn of the cochlea. (B) The cochlea is shown ‘‘rolled out’’ straight, instead of its

actual coiled configuration. (From Engineering Principles of Acoustics, D. D.

Reynolds, 1981. By permission of Allyn and Bacon, Inc.)

Copyright © 2003 Marcel Dekker, Inc.

term exposure to loud noise. The hair cells become fatigued due to exposure

to prolonged bending stress, and the cells die. When the hair cells die, they

cannot be rejuvenated or resurrected. The person suffers permanent hearing

loss when the hair cells die. No amount of ‘‘pre-conditioning’’ will

strengthen the hairs to resist exposure to loud noise.

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