Effect of ‘‘conditioning’’ on hearing loss from military noise exposures

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It has been shown with chinchillas as subjects that prophylactic (‘‘conditioning’’) exposures to an interrupted OBN centered at 0.5 kHz for 10 days provide protection against permanent threshold shift (PTS) from high level impulse noise in chinchillas [L. W. Henselman et al., Hear. Res. 78, 1–10 (1994)]. The present investigation studied the effect of ‘‘conditioning’’ with helicopter noise on PTS resulting from high level impulse noise. Thirty‐eight chinchillas were randomly assigned to one of four experimental groups (helicopter noise exposure prior to impulse noise exposure) or a control group (impulse noise exposure only). It was shown that (a) interrupted exposures over a 10‐day period to helicopter noise presented at 112 dB SPL for 1.5 h/day caused TTSs to decrease as exposure days continued at the test frequencies of 0.5 to 8 kHz, and (b) after 4 weeks of recovery, subjects were protected from PTS after ‘‘conditioning’’ with helicopter noise (except the group with prolonged ‘‘conditioning’’). Histological results were consistent with audiological findings and revealed significantly less hair cell loss in the experimental groups that were protected from PTS. The results are discussed in terms of possible application to hearing conservation programs. [Work supported by the U.S. Army Medical Command.]