Auditory Effects

Within 0.02 to 0.05 seconds after exposure to sound above the 80 dB level, the middle-ear muscles act to control the response of the ear. After about fifteen minutes of exposure, some relaxation of these muscles usually occurs. This involuntary response of the ear—the auditory reflex—provides limited protection against high noise levels. It cannot protect against unanticipated impulsive sounds; it is effective only against frequencies below about 2000 Hz. And in any case, it provides only limited control over the entrance of noise. These muscles relax a few seconds after the noise ceases.

Following exposure to high-level noise, customarily a person has some temporary loss in hearing acuity and often a singing in the ears (tinnitus). If it is not too great, this temporary loss disappears in a few hours. But if, for example, the TTS experienced in one work period has not been recovered at the start of the next work period, the effect accumulates; permanent hearing damage is almost certain if these conditions persist.

Important variables in the development of temporary and permanent hearing threshold changes include the following:

Sound level: Sound levels must exceed 60 to 80 dBA before the typical person experiences TTS. Frequency distribution of sound: Sounds having most of their energy in the speech frequencies are more potent in causing a threshold shift than are sounds having most of their energy below the speech frequencies.

Duration of sound: The longer the sound lasts, the greater the amount of threshold shift. Temporal distribution of sound exposure: The number and length of quiet periods between periods of sound influences the potentiality of threshold shift. Individual differences in tolerance of sound may vary among individuals. Type of sound—steady-state, intermittent, impulse, or impact: The tolerance to peak sound pressure is reduced by increasing the duration of the sound.

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