Ganong's Review of Medical Physiology, 23rd Edition

CHAPTER 13

Hearing & Equilibrium 209

Generally speaking, the
loudness
of a sound is correlated
with the
amplitude
of a sound wave and its
pitch
with the
fre-
quency
(number of waves per unit of time). The greater the
amplitude, the louder the sound; and the greater the fre-
quency, the higher the pitch. Sound waves that have repeating
patterns, even though the individual waves are complex, are
perceived as musical sounds; aperiodic nonrepeating vibra-
tions cause a sensation of noise. Most musical sounds are
made up of a wave with a primary frequency that determines
the pitch of the sound plus a number of harmonic vibrations
(overtones)
that give the sound its characteristic
timbre
(quality). Variations in timbre permit us to identify the
sounds of the various musical instruments even though they
are playing notes of the same pitch.
The amplitude of a sound wave can be expressed in terms of
the maximum pressure change at the eardrum, but a relative
scale is more convenient. The
decibel scale
is such a scale.
The intensity of a sound in
bels
is the logarithm of the ratio of
the intensity of that sound and a standard sound. A decibel
(dB) is 0.1 bel. The standard sound reference level adopted by
the Acoustical Society of America corresponds to 0 dB at a
pressure level of 0.000204
×
dyne/cm
2
, a value that is just at
the auditory threshold for the average human. A value of 0 dB
does not mean the absence of sound but a sound level of an
intensity equal to that of the standard. The 0- to 140-dB range
from threshold pressure to a pressure that is potentially dam-
aging to the organ of Corti actually represents a 10
7
(10 mil-
lion)-fold variation in sound pressure. Put another way,
atmospheric pressure at sea level is 15 lb/in
2
or 1 bar, and the
range from the threshold of hearing to potential damage to
the cochlea is 0.0002 to 2000
μ
bar.

A range of 120 to 160 dB (eg, firearms, jackhammer, jet

plane on take off ) is classified as painful; 90 to 110 dB (eg,

subway, bass drum, chain saw, lawn mower) is classified as

extremely high; 60 to 80 dB (eg, alarm clock, busy traffic,

dishwasher, conversation) is classified as very loud; 40 to 50

dB (eg, moderate rainfall, normal room noise) is moderate;

and 30 dB (eg, whisper, library) is faint.

The sound frequencies audible to humans range from about

20 to a maximum of 20,000 cycles per second (cps, Hz). In

bats and dogs, much higher frequencies are audible. The

threshold of the human ear varies with the pitch of the sound

(Figure 13–9), the greatest sensitivity being in the 1000- to

4000-Hz range. The pitch of the average male voice in conver-

sation is about 120 Hz and that of the average female voice

about 250 Hz. The number of pitches that can be distin-

guished by an average individual is about 2000, but trained

musicians can improve on this figure considerably. Pitch dis-

crimination is best in the 1000- to 3000-Hz range and is poor

at high and low pitches.

The presence of one sound decreases an individual’s ability

to hear other sounds, a phenomenon known as

masking.

It is

believed to be due to the relative or absolute refractoriness of

previously stimulated auditory receptors and nerve fibers to

other stimuli. The degree to which a given tone masks others

is related to its pitch. The masking effect of the background

noise in all but the most carefully soundproofed environ-

ments raises the auditory threshold by a definite and measur-

able amount.

SOUND TRANSMISSION

The ear converts sound waves in the external environment

into action potentials in the auditory nerves. The waves are

transformed by the eardrum and auditory ossicles into move-

ments of the foot plate of the stapes. These movements set up

FIGURE 13–8
Characteristics of sound waves. A
is the record
of a pure tone.
B
has a greater amplitude and is louder than
A. C
has
the same amplitude as
A
but a greater frequency, and its pitch is high-
er.
D
is a complex wave form that is regularly repeated. Such patterns
are perceived as musical sounds, whereas waves like that shown in
E,
which have no regular pattern, are perceived as noise.

Time

Frequency (cycles per unit time)

Pressure change

Amplitude

1 cycle

A

B

C

D

E

FIGURE 13–9

Human audibility curve.

The middle curve is

that obtained by audiometry under the usual conditions. The lower

curve is that obtained under ideal conditions. At about 140 decibels

(top curve), sounds are felt as well as heard.

140

100

0

10

(Tickle in ear)

Threshold of feeling

Frequency (Hz)

102 103 1042 × 104

Intensity level (decibels)

Threshold of hearing–audiometer

Threshold of

hearing–ideal