Temporary threshold shifts produced by exposure to low-frequency noises.

Groups of human subjects were exposed for 8 or 24 h to an octave-band noise centered at 63, 125, or 250 Hz. For a 24-h exposure at 84 dBA, temporary threshold shifts (TTS) increased for 8-12 h and then either decreased or remained constant. Although TTS was less than 20 dB, complete recovery for many of the subjects required as long as 48 h. Accordingly, the higher level exposure which was planned at 94 dBA for 24 h was reduced to 90 dBA for 8 h. For this condition TTS increased throughout the 8-h exposure. TTS from the 90-dBA noise for 8 h exceeded the TTS produced by the 84 dBA; however, recovery from the 24-h exposure required as long as 48 h, whereas recovery from the 8-h exposure required only 12-24 h. Thus the time required for recovery is determined in part by the duration of exposure. TTS was not always maximal 1/2-1 oct above the band of noise, but was maximal in the frequency regions of better auditory sensitivity (350 to 750 Hz). For the 250-Hz condition, TTS increased about 1.5 dB per dB increase in noise level, whereas for the 63- and 125-Hz conditions TTS increased less than 1 dB per dB increase in noise level. More data are needed to specify the relation between TTS and the level of low-frequency noises.