Measurement of impulse noise-induced temporary threshold shift in endangered and protected animals—Two case studies

Abstract

Sonic booms at levels typical for supersonic airlines (<3 psf) are considered harmless to hearing based on laboratory studies. However, data on laboratory animals are difficult to generalize to other species. Wildlife present a particular problem because experimental efforts to induce permanent threshold shift (PTS) cannot be conducted for ethical or management reasons. Instead, ‘‘significant’’ temporary threshold shift (TTS) has been proposed as a conservative damage risk criterion (DRC). With this in mind, two series of experiments were conducted using simulated N waves at levels up to 6 psf, rise time down to 0.4 ms, and 100- or 300-ms duration—one on seals and sea lions and a second on the desert tortoise. Level of the least-detectable auditory brainstem response (ABR) was used to estimate best sensitivity before and after exposure. Small immediate threshold shifts (∼5 dB) were detected in pinnipeds and larger (∼20 dB) shifts in desert tortoises. The greater impact on desert tortoises may be explained by their greater best sensitivity at low frequencies. No PTS was detected. Thresholds for TTS were therefore detected in both species and may be useful in establishing DRCs for animals. [Work supported by NASA, Contract No. NAS1-20101 and USAF, Contract No. F33615-89-D-4003.]