Protection from noise induced hearing loss: is prolonged 'conditioning' necessary?

The effect of prior 'conditioning' noise exposures on the protection from subsequent higher level exposures was studied using four groups of chinchillas. The three experimental groups were 'conditioned' using a 0.5 kHz octave band noise at 95 dB SPL for 6 h a day. The first group was exposed to the noise once and allowed to recover for nine days prior to the second exposure. The second and third groups were exposed for ten and twenty days respectively. The first group showed only small reductions in threshold shift (TS) following the second exposure. The other two groups showed significant reductions in TS with repeated exposures. Following the last 'conditioning' exposure, all three experimental groups were allowed to recover for five days before exposing them to the same noise at 106 dB SPL for 48 h. Threshold shifts recorded following the 106 dB exposure were compared against those recorded in a control group exposed only to the higher level. Each of the three experimental groups developed significantly less permanent threshold shifts than the control group. However, there were no significant differences among the three experimental groups and the differences in hair cell losses were insignificant.