We discuss sound pressure levels in terms of frequencies, partly because this is how our ears interpret sound. What we experience as "lower pitched" or "higher pitched" sounds are pressure vibrations having a low or high number of cycles per second, or frequencies.
Hearing sensitivity, and the frequency range over which sound can be heard, varies greatly from species to species. The human ear has evolved to detect frequencies of sounds that are most useful to humans, and has a maximum frequency range of about 20 Hz to 20 kHz. Infrasonic describes sounds that are too low in frequency, and ultrasonic, sounds too high in frequency to be heard by the human ear,
However, for many fish, sounds above 1 kHz are ultrasonic. For those marine mammals that cannot perceive sounds below 1 kHz, much of the signal of an air gun may be infrasonic. These considerations indicate the importance of considering hearing ability when evaluating the effect of underwater signal or noise on marine animals. In this issue of GEO ExPro we give an introduction to the auditory capabilities of marine mammals.
Frequency range of hearing for humans and selected animals. Bats are the land animal with the broadest hearing span (see GEO ExPro Vol. 7, No. 4). The squeaks that we can hear a mouse make are in the low frequency end and are used to make long distance calls, as low frequency sounds travel further than high frequency ones. Mice can alert other mice of danger without also alerting a predator like a cat to their presence, if the predator can not hear their high-frequency distress call. Marine mammals have a mammalian ear that through adaptation to the marine environment has developed broader hearing ranges than those common to land mammals. As a group they have functional hearing ranges from 5 Hz to 200 kHz.
All marine mammals have special adaptations of the external and middle ear consistent with deep, rapid diving and long-term submersion, but they retain an air-filled middle ear and have the same basic inner ear configuration as terrestrial species. Each group has distinct adaptations that correlate with both their hearing capacities and with their relative level of adaptation to water.
An audiogram is a graphical representation of hearing thresholds at several different frequencies, showing the extent and sensitivity of hearing.
To obtain an audiogram, sound at a single frequency and at a specified level is played to the subject, by means of loudspeakers or headphones in air, or underwater loudspeakers in water. A button is pressed when the tone can be heard; the level of the sound is reduced, and the test repeated, until eventually, a level of sound is found where the subject can no longer detect it. This is the threshold of hearing at that frequency. The measurement is typically repeated at different frequencies and the results are presented as the threshold of hearing of the subject as a function of frequency; the subject‘s audiogram.
Typically plotted on a logarithmic frequency axis, audiograms have the appearance of an inverted bell-shaped curve, with a lowest threshold level (maximum hearing sensitivity) at the base of the curve and increasing threshold levels (decreasing sensitivity) on either side.
How to test a dolphin?
The hearing of dolphins or small whales can be tested through traditional behavioral studies or through auditory brainstem response (ABR) experiments.
Most behavioral hearing studies are performed on mammals in captivity, so the behavioral hearing information that is available tends to be for the smaller marine mammals such as pinnipeds (seals, sea lions and walruses), sirenians (manatees and dugongs), and odontocetes (toothed whales, dolphins and porpoises). Very few, if any, behavioral hearing studies have been done with the large baleen whales because they are not kept in captivity, and it is very difficult to perform hearing tests with animals in the wild.
The behavioural studies are in many respects similar to the way we test the human hearing threshold. The animal is schooled to stay underwater while a sound is played. If it hears the sound, it is trained to respond in a particular way, and if it does not hear it, or if no sound is played, it responds differently. Each time a sound is presented, and the animal is right, it is rewarded. The sound is reduced until the animal can no longer hear, and “says” it cannot, at which point the sound level is gradually increased until the animal indicates that it can hear it. By playing lots of different frequencies (pitches), it is possible to determine the threshold point at which the animal can just barely hear for each frequency. In this way the scientists can determine what frequencies and sound levels are audible to different animals.
The ABR hearing measurement, which is also used to measure hearing in human babies just after they are born, is a way to study what a whale hears through the detection and recording of electrical impulses in the brain that occur in response to sound. It is harmlessly measured from the surface of the animal’s skin with gold EEG sensors. The ABR test is powerful because it can be done rather quickly compared to behavioral hearing methods and because it can be performed with untrained or stranded animals.
The hearing functions of marine mammals are also studied by conducting anatomical examinations of dead animals. By examining the air-filled middle ear and fluid-filled inner ear, scientists have been able to estimate the range of frequencies that an animal may be able to hear. Much of our knowledge of mysticete (baleen whales) hearing has come from these anatomical studies.
Odontocetes, like bats, are excellent echolocators, capable of producing, perceiving, and analyzing 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. (see GEO ExPro Vol.7, No. 4 for an explanation of this terminology)
There 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. The 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. The 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. In the next issue of GEO ExPro we draw together public information regarding the marine mammal’s response to seismic activity.