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.
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