INTERRUPTED NOISE EXPOSURES - THRESHOLD SHIFT DYNAMICS AND PERMANENT EFFECTS

A parametric study of the reduction of threshold shift (toughening phe nomena) that takes place during the course of an interrupted noise exp osure is described. 266 chinchillas randomly assigned to one of 32 exp erimental groups were exposed to one of the following: a 400-Hz narrow -band impact noise having a center frequency of 0.5, 1.0, 2.0, 4.0, or 8.0 kHz and peak sound-pressure levels of 109, 115, 121, or 127 dB. T he impacts were presented for 5 d, 24 h/d or for 20 d, 6 h/d. Correspo nding pairs of exposures had equal energy. Group mean noise effects we re estimated from pure-tone thresholds obtained from inferior collicul us evoked potentials and from surface preparation histology. The thres hold shift (TS) toughening phenomena is shown to occur in response to all stimuli that produce a TS and at all audiometric test frequencies. The amount of toughening, which is limited to less than 35 dB, varies with noise frequency and intensity. Based on group mean data the audi tory system is not protected from the permanent effects of an interrup ted noise exposure as a result of the toughening effect but rather dif ferences in permanent effects between the 5- and 20-d exposures are at tributed to the spreading of the exposure energy over an extended peri od of time. (C) 1998 Acoustical Society of America.