A direct relationship exists between TTS and PTS. Noise levels that do not produce TTS after two to eight hours of exposure do not produce PTS if continued beyond this time. The shape of the TTS audiogram resembles the shape of the PTS audiogram.

Noise-induced hearing loss is generally first characterized by a sharply localized dip in the hearing threshold limit (HTL) curve at frequencies between 3000 and 6000 Hz. This dip commonly occurs at 4000 Hz (Figure 6.3.1). This dip is the high frequency notch.

The progress from TTS to PTS with continued noise exposure follows a fairly regular pattern. First, the high-frequency notch broadens and spreads in both directions. While substantial losses can occur above 3000 Hz, the individual does not notice any change in hearing. In fact, the individual does not notice any hearing loss until the speech frequencies between 500 and 2000 Hz average more than a 25 dB increase in HTL on the ANSI-1969 scale. The onset and progress of noise-induced permanent hearing loss is slow and insidious. The exposed individual is unlikely to notice it. Total hearing loss from noise exposure has not been observed.

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