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10


Human diver


Figure 3.77: The scientific data collected in this composite audiogram shows that mysticetes have the most sensitive low-frequency hearing of all marine mammals, with an auditory bandwidth of 5 Hz to 22 kHz. They have good sensitivity from 20 Hz to 2 kHz. Their threshold minima are unknown, but speculated to be 60–80 dB re 1 μPa. The thresholds for odontocetes and pinnipeds are a composite of measured lowest thresholds for multiple species. Odontocetes have auditory bandwidth 100 Hz to 180 kHz. Pinnipeds listening in water have auditory bandwidth 75 Hz to 100 kHz. The vertical axis is relative intensity in underwater dB and the horizontal axis is the frequency of a sound on a logarithmic scale.


Mysticetes Pinnipeds Odontocetes 100 1000 Frequency (Hz)


producing, perceiving, and analysing ultrasonic frequencies. In general, odontocetes have a hearing bandwidth of 100 Hz to 180 kHz, with the most sensitive hearing in the high-frequency range of 10 kHz to 65 kHz where their hearing threshold is 45 to 55 dB re 1 μPa. In the low frequencies below 1 kHz where airgun sound is concentrated, toothed whales have a very high hearing threshold of 80 to 130 dB re 1 μPa. Tere is, however, considerable variability within and among


species. Sperm whales, beaked whales and dolphins are said to sense sound from 75–100 Hz if loud enough. Dolphins are renowned for their acute hearing sensitivity, especially in the frequency range 5 to 50 kHz. Several species have hearing thresholds between 30 and 50 dB re 1 μPa in this frequency range. Te killer whale has a sound pressure level threshold of 26 dB re 1 μPa @ 15 kHz. Mysticetes have a very different hearing capability compared


to toothed whales. Due to their immense size, these mammals cannot be kept in captivity for study like toothed whales. Little information exists on their hearing and further information is unlikely to be obtained in the near future. In the limited


studies done, baleen whales reacted primarily to sounds at low frequencies in the 20 Hz to 500 Hz range. While this is their most sensitive hearing range, the hearing bandwidth for baleen whales is believed to range from 5 Hz to above 20 kHz. In order to provide predictions, models based on anatomical


data indicate that the functional hearing range for mysticetes commonly extends to 20 Hz, with several species expected to hear well into infrasonic frequencies. Te upper functional range for most mysticetes has been predicted to extend to 20–30 kHz. Pinnipeds have a similar hearing bandwidth to toothed


whales, 75 Hz to 100 kHz, but their most sensitive hearing is at middle frequencies of 1 kHz to 30 kHz where their hearing threshold is 60 to 80 dB. In the low frequencies below 1 kHz where airgun signal is concentrated, hair seals have a high hearing threshold of 80 to 100 dB. Man-made sound underwater can cover a wide range of


frequencies and levels of sound, and the way in which a given mammal reacts to the sound will depend on the frequency range it can hear, the level of sound and its spectrum.


10000 100000


(Modified from Office of Naval Research. 2001. Final Environmental Impact Statement for the North Pacific Acoustic Laboratory, May 2001.)


134


Threshold (dB re 1µ Pa)


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