Temporary Threshold Shift (TTS) Studies

Exposure to sound may produce elevated hearing thresholds or threshold shifts in mammals. If the hearing threshold returns to baseline levels then it is known as a temporary threshold shift (TTS). Hearing loss depends on the intensity of the sound, the frequency of the sound, and how long the animal is exposed to the sound (duration). The following tables summarize the available data regarding minimum sound exposures that have been found to cause TTS in a few species of marine mammals. The TTS investigations introduce sounds (pure tones, octave band noise or impulsive sound) at varying frequencies, intensities, and durations to determine sound exposures that cause temporary threshold shifts. The measurements were made on a small number of animals. Individual differences in hearing sensitivities are substantial in land mammals. Without additional data it is impossible to know if the same is true of marine mammals, although it would be surprising if they did not have different hearing sensitivities too.

Bottlenose Dolphin (Tursiops truncatus)
Type of Sound Duration Exposure
Frequency
(Hz)
Minimum
Intensity at
Which TTS
Observed
(dB re 1 µPa)
Test Method Reference
Pure tones 1 sec 400 None observed up to 193 dB Behavioral

[1]
3,000
10,000
20,000
75,000
194
192
193
182
Octave band noise 47-54 min 4,000 -
11,000
179 Behavioral
[2]
Octave band noise 30 min 4,000 -
11,000
160 Auditory Brainstem Response (ABR)

[3]
Impulsive     None observed at 221 dB peak-to-peak
(70 kPa peak pressure)
Behavioral

[4]
Impulsive
(Seismic watergun)
    None observed at 228 dB peak-to-peak
(207 kPa peak pressure)
Behavioral

[5]

Beluga Whale (Delphinapterus leucas)
Type of Sound Duration Exposure
Frequency
(Hz)
Minimum
Intensity at
Which TTS
Observed
(dB re 1 µPa)
Test Method Reference
Pure tones 1 sec 400 None observed up to 193 dB Behavioral

[1]
3,000
10,000
20,000
195
192
197
Impulsive     None observed at 221 dB peak-to-peak
(70 kPa peak pressure)
Behavioral

[4]
Impulsive
(Seismic
watergun)
    226 dB peak-to-peak
(160 kPa peak pressure)
Behavioral

[5]

Harbor Seal (Phoca vitulina)
Type of Sound Duration Exposure
Frequency
(Hz)
Minimum
Intensity at
Which TTS
Observed
(dB re 1 µPa)
Test Method Reference
Octave-band noise 20 min 71-141
353-707
707-1414
60 dB above baseline threshold Behavioral
[6]

California Sea Lion (Zalophus californianus)
Type of Sound Duration Exposure
Frequency
(Hz)
Minimum
Intensity at
Which TTS
Observed
(dB re 1 µPa)
Test Method Reference
Octave-band noise 20 min 707-1414
1414-2828
55-65 dB above baseline threshold Behavioral
[6]

Northern Elephant Seal (Mirounga angustirostris)
Type of Sound Duration Exposure
Frequency
(Hz)
Minimum
Intensity at
Which TTS
Observed
(dB re 1 µPa)
Test Method Reference
Octave-band noise 22 min 707-1414 70-75 dB above baseline threshold Behavioral
[6]

Combining the data from all of these studies shows that longer exposures to quieter sounds have the same effect as shorter exposures to louder sounds [7]. The total energy of the sound to which an animal is exposed (a combination of the sound intensity and the duration of exposure to that intensity) is a much better predictor of whether or not TTS will occur than is the sound intensity alone.

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References

  1. Schlundt, C.E., Finneran, J.J., Carder, D.A. and Ridgway, S.H. 2000, "Temporary shift in masked hearing thresholds of bottlenose dolphins, Tursiops truncatus, and white whale, Delphinapterus leucas, after exposure to intense tones." Journal of the Acoustical Society of America 107(6): 3496-3508. 
  2. Nachtigall, P.E., Pawloski, J.L. and Au, W.W.L. 2003, "Temporary threshold shifts and recovery following noise exposure in the Atlantic bottlenosed dolphin (Tursiops truncatus)" Journal of the Acoustical Society of America 113(6): 3425-3429. 
  3. Nachtigall, P.E., Supin, A.Y., Pawloski, J. and Au, W.W.L. 2004, "Temporary threshold shifts after noise exposure in the bottlenose dolphin (Tursiops truncatus) measured using evoked auditory potentials." Marine Mammal Science 20(4): 673-687. 
  4. Finneran, J.J., Schlundt, C.E., Carder, D.A., Clark, J.A., Young, J.A., Gaspin, J.B. and Ridgway, S.H. 2000, "Auditory and behavioral responses of bottlenose dolphins (Tursiops truncatus) and a beluga whale (Delphinapterus leucas) to impulsive sounds resembling distant signatures of underwater explosions." Journal of the Acoustical Society of America 108(1): 417-431. 
  5. Finneran, J.J., Schlundt, C.E., Dear, R., Carder, D.A. and Ridgway, S.H. 2002, "Temporary shift in masked hearing thresholds in odontocetes after exposure to single underwater impulses from a seismic watergun." Journal of the Acoustical Society of America. 111(6), 2929-2940. 
  6. Kastak, D., Schusterman, R.J., Southall, B.L. and Reichmuth, C.J. 1999, "Underwater temporary threshold shift induced by octave-band noise in three species of pinniped." Journal of the Acoustical Society of America 106(2): 1142-1148. 
  7. National Research Council. 2005, "Marine Mammal Populations and Ocean Noise: Determining when noise causes biologically significant effects." National Academies Press, Washington, D.C.