Theories for Noise‐Induced Hearing Loss

Since World War II, a considerable amount of information has been gathered on hearing loss connected with intense noise. Most of these data have been collected in industry, but there have been other sources (e.g. aircraft pilots and military personnel). Because in most instances the data have been collected for individuals working eight‐hour days, little information is available for people working shorter or longer shifts each day and some empiricism is necessary to extrapolate results for use in noise regulations. Two main theories have emerged which are used to aid in these extrapolations: (i) the equal temporary effect hypothesis, and (ii) the equal energy hypothesis.

The equal temporary effect hypothesis assumes that the permanent hearing hazard of noise (NIPTS) can be related to the noise‐induced temporary threshold shift (NITTS) caused by the same levels of noise exposure. Some researchers argue that this hypothesis is plausible because it can be related to an observable physiological offset in the ear. Some experiments have shown evidence that both the NIPTS and the temporary threshold shift (NITTS) can he related to the same metabolic process occurring in the inner ear (perhaps fatigue through a depletion of nutrients and a build‐up of waste products). This hypothesis leads to the conclusion that noise which is intense enough to produce temporary loss (NITTS) will also be intense enough to produce permanent loss (NIPTS).

The equal energy hypothesis, on the other hand, assumes that hearing damage is directly proportional to the acoustic energy an individual has experienced in his or her lifetime. Since acoustic energy is intensity multiplied by the time duration, it is also proportional to the sound pressure level times the duration. This equal energy hypothesis suggests that the sound pressure level may be increased by 3 dB for each halving in exposure time per day. This hypothesis does allow a simple, attractive approach to the regulation problem which also seems thoroughly reasonable. This concept has been used by most of the countries around the world in the development of their noise regulations [15].