Rating the hazard from intense sounds: putting theory into practice.

Basilar membrane mechanics appear to be largely responsible for noise induced temporary threshold shift (TTS). Upon noise exposure, the single fiber loses sensitivity and sharpness. The amplitude and configuration (sharpness) of the basilar membrane vibration pattern is reduced. The "best frequency" changes about half an octave. The result is threshold shift and reduced frequency discrimination. The maximal TTS is generally seen one octave above the peak frequency of the stimulus. There appears to be a critical intensity level for noise exposure. Below the critical level little or no hearing damage will develop regardless of exposure time. Beyond the critical level, extensive damage will be caused even after short exposure time. The critical level appears to be a rather absolute measure: in animals, the state of the inner hair cells may change from "no loss" to "extensive loss" upon a stimulus increase of only 3dB. Addition of acoustical energy at a frequency different from that of the original stimulus, may reduce the threshold shift induced by the original stimulus alone. Sound in one ear may also influence the susceptibility of the other. Compared with its isoenergetic impulse noise, a continuous noise generally must be given a 3dB increase (its energy must be doubled) if both stimuli shall produce similar threshold shifts. Factors like frequency distribution, level, duration number, repetition rate and time distribution/presentation pattern of exposure apparently have to be considered in future damage risk criteria. Meanwhile isoenergy, or possibly "A-weighted" isoenergy, is our best indicator for noise hazard.