Critical Ratio Measurements Can be Used to Assess Effective Quiet Thresholds in Harbor Seals, Harbor Porpoises, and California Sea Lions.

Effective quiet is a sound pressure level (SPL) that will never cause temporary hearing threshold shifts (TTSs), even after long exposures. TTS is typically measured after the end of a fatiguing sound exposure when some recovery may have occurred during the threshold measurement. Using published data from 24 studies from the SEAMARCO Research Institute, we examined the recovery from TTS of harbor seals (Phoca vitulina), harbor porpoises (Phocoena phocoena), and California sea lions (Zalophus californianus) to assess whether the measurement procedures to determine initial TTSs were compromised by recovery from TTS. When initial TTS1-4 (measured 1 to 4 min post exposure) was < 6.0 dB, recovery occurred in 42, 50, and 57% of sessions (respectively for each species) at 4 to 8 min post exposure. By 8 to 12 min post exposure, the complete recovery rates were 65, 86, and 92% (respectively for each species). A certain amount of recovery from TTS occurs during the post-exposure hearing threshold determination. Rapid recovery from small TTSs makes determination of the effective quiet SPL threshold difficult to measure. An alternative to quantifying unmasked hearing sensitivity to assess small TTSs may be quantifying the critical ratio (CR; the difference in dB between a tonal signal detection threshold and the spectrum level of the masking noise at that frequency) at the beginning and just before the end of the fatiguing sound exposure. If the CR measured just before the fatiguing sound ends is higher than the CR measured just after the fatiguing sound began, then that difference will be the magnitude of TTS that has occurred. Using CR measures will avoid recovery occurring during the measurement process. CR determinations can be made using more trials than during the 1- to 4-min post-exposure hearing tests and should result in more accurate measurements of TTSs. Determining effective quiet SPL thresholds allows the identification of "safe" received SPLs that do not cause TTS. This information will be useful in environmental assessments of anthropogenic sounds. However, even "safe" SPLs may reduce the ability of mammals to hear signals by causing acoustic masking. [ABSTRACT FROM AUTHOR]