6.2 HEARING LOSS

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Because of the acoustic characteristics of the outer ear and the mechanical

characteristics of the middle ear, the human ear does not act as a linear

transducer for sound pressure levels. The threshold of hearing as a function

of frequency is given in Table 6-1. This table presents values of the sound

pressure level for a pure tone that a person (below age 18 with no hearing

loss) is just able to hear at the given frequency. It may be noted that the ear

is most sensitive in the frequency range around 3000 Hz, which corresponds

to the resonant frequency of the auditory canal. Because of poor acoustic

impedance matching between the air outside the ear and the outer ear at

frequencies below about 500 Hz, the ear can detect only sounds that have a

sound pressure level greater than about 12 dB for frequencies of 250 Hz and

lower.

For a sound pressure level of approximately 120 dB with a frequency

between 500 Hz and 10 kHz, a person will experience a tickling sensation in

the ears. This level represents the threshold of ‘‘feeling’’ or the beginning of

discomfort due to noise. When the sound pressure level is increased above

approximately 140 dB, the threshold of pain is reached. Continuous exposure

to noise above 140 dB for a few minutes can result in permanent

damage to elements of the ear.

Acoustic Criteria 229

TABLE 6-1 Threshold of Hearinga

Frequency, Hz Lp(threshold), dB Frequency, Hz Lp(threshold), dB

31.5 59.2 2,000 2:4

63 36.0 3,000 _4:1

125 21.4 4,000 _3:6

250 12.1 5,000 0:2

500 6.5 6,000 5:2

1,000 3.6 8,000 17:3

aThe table lists the sound pressure level of a pure tone that a person under age 18

with no hearing loss is just able to hear at the given frequency.

Source: ANSI (1967).

Copyright © 2003 Marcel Dekker, Inc.

Hearing loss is defined as the change (increase) in the threshold of

hearing at a given frequency. There is a naturally occurring loss of hearing

that occurs with age, independent of occupational noise exposure. This

hearing loss is called presbycusis. The shift in the hearing threshold with

age is shown in Table 6-2 for men and women. The hearing loss that occurs

with age is not included in the component of hearing loss associated with

noise exposure.

There is a temporary threshold shift (TTS), in which a person loses

some ability to detect weak sounds, but the ability is regained approximately

16 hours after the noise exposure is removed (Kryter, 1970). Noises having

maximum energies in the low-frequency range (below about 250 Hz) will

produce less TTS than noises having maximum energies in the high-frequency

range (above about 2000 Hz). Exposure to a low-pitched ‘‘rumble’’

noise is less harmful to a person’s hearing than exposure to a high pitched

‘‘screech.’’ TTS cannot be relieved by medication, not even vitamin A: only

getting away from the source of noise into a quieter region will promote

recovery.

There is also a noise-induced permanent threshold shift (NIPTS), in

which a person permanently loses the ability to detect weak sounds. The

frequency range showing the most NIPTS is around 3 kHz, because the ear

transmits sound at frequencies in the range from 1 kHz to 4 kHz most

effectively. If a person is removed from the noisy environment, the

230 Chapter 6

TABLE 6-2 Shift in the Average Threshold of Hearing with Age

(Presbycusis) for Men and Women

Age, years

Frequency, Hz

500 1,000 2,000 4,000

Men Women Men Women Men Women Men Women

25 0 0 0 0 1 0 4 0

30 0 1 1 1 2 1 8 2

35 1 3 2 3 3 4 12 4

40 2 4 4 4 6 5 17 7

45 4 6 5 6 8 8 23 10

50 5 8 7 8 12 10 28 13

55 7 10 9 11 16 13 35 18

60 8 12 11 13 20 15 31 22

65 10 14 13 15 24 18 47 26

Source: Beranek (1960).

Copyright © 2003 Marcel Dekker, Inc.

NIPTS does not progress further, but the ear does not recover. The ear does

not get ‘‘toughened’’ through exposure to noise. Excessive noise always

causes hearing loss, even in teenagers who may feel they are bulletproof.

From work conducted at the Air Force Aerospace Medical Research

Laboratory (Baughn, 1973), it was found that exposure to noise at levels less

than 90 dBA during the normal 8-hour work day would result in 25 dB or

more NIPTS for only 10% or less of the population. The complete results of

the research are shown in Fig. 6-3.

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