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a brief, transitory increase in an individual animal's hearing threshold in
response to exposure to sound. All humans typically experience such shifts, such
as the effect that occurs after leaving a noisy room for a quiet location. For a
period of time, hearing sensitivity is decreased such that quiet sounds are not
perceived. TTS recovers so that original hearing abilities return. Minor amounts
of shift (3-5 dB) may recover in minutes; large shifts (40 dB) may recover
overnight, and major shifts (>45 dB) may require days or weeks to recover. Above
65 dB the shift may not fully recover. TTS generally occurs in a limited or
affected frequency band at sound intensities well above hearing threshold
levels. Using NMFS interim guidance (based on human hearing data), the
difference between the threshold of hearing and sound intensities that result in
annoyance (or possibly TTS) in marine mammal is approximately 80 to 100 dB. For
the experiments covered by this assessment, the more conservative value of 80 dB
above threshold will be used throughout. NMFS nevertheless notes that at this
time, exposures that cause PTS or TTS have not been measured for mysticetes or
sperm whales.
Source:
Outline for Acoustics Environmental Assessment
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