SCIENCE GROUP

Dr. Peter Dobbins

Physics Dept. University of Bath

26 November 2004

Dr Dobbins is researching into the way dolphins and other aquatic mammals use sound.

He began his talk by showing a cast of the lower jaw of a bottlenose dolphin, which is like the jaw of other cetaceans, are but unlike the other jaws of other mammals. The teeth have regular separations between them and are in two straight lines at a slight angle to one another. This remarkable structure has evolved to give the creature an efficient receiving mechanism for its sonar activities.

All animals must be able to find their food, find mates, and navigate. Most birds have excellent eyesight and rely on this sense almost exclusively. But underwater, things are not so easy, where vision is practical only at quite shallow depths as light is absorbed quite quickly. At the bottom it can be very dark.

The history of our understanding of how animals use sound began with Abbé Lazaro Spallanzani, who in 1793 poked out bats eyes and found that they could still find their way around, but filling their ears with wax made them bump into obstacles. This work was not followed up until 1908 when Hahn at Indiana University repeated the investigations. After some speculation about echoes from wingbeats, finally in 1934 Pierce and Griffin hear ultrasound from bats. McBride then in 1956 published evidence for echolocation in cetaceans, but the mechanisms were largely not understood. Research in the 1980s showed that their jaws were involved, but Nnot until 1997 was it proposed by Goodman and Dobbins that the teeth played an important part.

Dolphins are fairly easy to work with as they can trained to perform various tasks. They can carry a recorder while they search an area, and we now have data giving signal strengths, frequencies and the direction they point during the retrieval of an object. That they can use their abilities very efficiently is demonstrated by their ability to search a volume 150 yards on a side and depth in 3-4 seconds.

Cetaceans typically have a number of different sounds. They use whistles for communications, but sharp pulses for navigation and searching. As they get closer to an object, they can use higher frequencies, which can resolve smaller objects, but high frequency signals fade more rapidly with distance. Dr Dobbins played a number of examples, and the locating signals consisted of very sharp pulses. Gaps between pulses decreased as the target was approached as the dolphins emit each pulse immediately after hearing the echo from the previous one. Some animals make extremely load sounds, varying from 112dB for harbour porpoises operating in the range 20 - 120kHz, through 180dB for humpback whales in the range 20Hz - 2kHz, to bottlenose dolphins 230dB at 50-130dB, which is close to the theoretical maximum loudness of any underwater signal. (Note that these cannot be compared with dB ratings of sounds in air as a different meaning of dB applies in the two cases.)

The sea is a noisy place, with sounds being generated by shorelines and surface as well as mankind's use of shipping, but dolphins naturally work in a frequency range close to the natural background minimum.

They make these sounds with sets of ‘lips’ close to the blowhole. The clicks of about 50μs duration are then propagated through fatty tissue in a bony case that comprises the bulge on top of the dolphin's jaw and which acts like a wave guide and focussing mechanism to create a strongly directional signal. with sidelobes about 20dB down. At 20kHz the beamwidth is about 15°, whereas at 100kHz it is about 3°.

Most animals have ears to receive sounds, and the presence of two enables them to compare the signals to each ear and so deduce more information about the direction and distance to the source. In crickets, the two ears are on the forelegs and connected by a hollow tube providing interference between the signals to the two legs to aid in direction finding. In mammals, the delay in the signals is created by the nervous system for comparative purposes, and the principle is built into directional microphones that subtract signals from the front and the back of a diaphragm, giving a response biassed strongly towards the front of the microphone, and cancelled at the rear.

Dolphin's teeth are about 1cm apart in a row along the jaw. This distance matches the wavelength of the typical sounds they use for echo location, and when the teeth vibrate in response to the echo, they act like a phased array of receivers, giving a strongly directional response, with sidelobes about 20dB down. At 20kHz the beamwidth is about 15°, whereas at 100kHz it is about 3°. The directivitywhich is even more enhanced by having the two sides of the jawbone set at about 12° to each other. Being able to compare the responses on the jaws enables very small angles to be resolved, although it is not yet clear whether the comparative delays are inserted by the nervous system, by differing transmission times of the tooth nerves, or by cumulative signal transmission along the jawbone, or perhaps a combination of all three.

An even more extreme development is shown in river dolphins, which live in such muddy water that they no longer have any sight. In this case, the teeth get larger towards the front end of the jaw, and show the characteristics of a log periodic array reducing the sidelobes to an almost negligible amount, further reducing the nearfield distortion, but at the expense of broadening the main beam. In addition, they swim on their sides, and this would enable them to remove unwanted loud echoes from surface and riverbed.

We want to know more about the way they process their signals so that we can improve our sonar instruments and remove the need for military establishments to train dolphins to search for mines, which is a very expensive technique. By modifying our own sonars, we would also be able to avoid some of the deleterious effects they currently have on the marine wildlife and stop the cetaceans colliding with ships.

During the questions, a few more points were brought out.

The bottlenose dolphins have two sets of lips to make the clicks, and mix frequencies of about 80 kHz and 120 kHz to get the value they want for a given purpose. Surprisingly, they seem to use the high frequency signals to do global searches, which is odd because these signals would tail off more rapidly in the distance. As they get closer to the target, the signal lowers in frequency, but they may be searching for the resonance of the target fish so they can kill it sonically.

It is now fairly clear that bats have very poor vision capabilities, and it is likely that parts of the brain dealing with spatial reference and navigation is largely driven by the aural system, and not the visual one. However, it is not clear that this is the case for dolphins. They may well still be able to use vision for navigation and spatial awareness. When a dolphin is born it has all the abilities to produce sounds and hear already in place. However, it appears it can take up to 5 years to learn how to use these features fully. During the learning process, it has been noted that dolphins chastise transgressing youngsters, and parents have been known to deliver sonic admonishments to their pups in no uncertain terms.

Although the teeth play an important rôle in signal processing, it is not clear how this is integrated with the ears, nor what the frequency range to which the ears are sensitive.

Andy Pepperdine

2004-12-011-28

Further reading

Books:

W.W.L. Au. The Sonar of Dolphins (Springer-Verlag, 1993)

P.E. Purves & GE Pilleri. Echolocation in Whales and Dolphins (Academic Press, 1983)

D.R. Griffin. Listening in the Dark (Yale University Press, 1958)

Conference Papers:

Journal Papers: