Better sonar through dolphin teeth
Dental arrays may be optimized for sound in shallow waters.
A model of how dolphins may use their teeth to receive sound could provide clues for improving man-made sonar systems, according to a study published in Bioinspiration & Biomimetics1.
The results, says study author Peter Dobbins of the engineering firm SEA Group Ltd in Bristol, UK, could be particularly useful for improving sonar in shallow water, making it better at tasks such as searching for naval mines.
Dolphins use sonar for navigation and to echolocate prey by bouncing sound waves emitted as high-frequency clicks off objects in their environment. "Dolphins obtain a similar mental 'image' of the surface of a complex object whether they use sonar or vision to look at it," says Elizabeth Taylor, a marine biologist at the National University of Singapore.
Dolphin sonar outperforms any man-made system, particularly in shallow water, where reverberation, water turbulence and suspended sediment make sonar particularly challenging. To discover why dolphins are so adept at echolocating in shallow water, Dobbins devised models based on the theory that the animals receive some sounds using their teeth.
Dobbins modelled each jaw as if it were two straight lines of teeth meeting at an angle of 10-20 degrees, and assumed that most of the sound travelled in the same plane as the jaw — with sound coming from in front of the dolphin's nose, rather than from above, below or to the side. This arrangement, called an endfire array, is commonly used in man-made radio and radar systems, but not in sonar, he says.
The models showed that the endfire-array pattern performs better at close range than the 'broadside' pattern often used in man-made sonar systems. In a broadside array, the sound travels at right angles to the jaw — from directly underneath, for example. Whereas the broadside array loses the ability to determine direction of sound closer than 10 centimetres from the source, the endfire array can still do so.
But the performance of endfire models varied depending on the frequency of the sound. That could pose a problem for dolphins, because they use a range of frequencies from 50 to 150 kilohertz.
To investigate this, Dobbins then used his models to compare the impact of the different tooth arrangements found in marine and river dolphins. River dolphins often live in shallow, muddy waters where sight is of little use, and many river dolphins are virtually blind. Although the teeth are usually evenly spaced in marine dolphins, those of river dolphins are larger and closer together towards the tip of their long jaws. That configuration, Dobbins found, is resilient to frequency changes in the dolphin range.
Strangely, although the system may be useful for man-made sonar devices, whether dolphins actually use it remains controversial. "It's just a wild hypothesis," says Whitlow Au, a marine biologist at the Hawaii Institute of Marine Biology on Coconut Island in Kaneohe Bay. "No one has shown that a dolphin can receive sounds through its teeth."
Au says there are examples of captive dolphins that lost all of their teeth but could still echolocate their prey, even in shallow holding tanks. And, he argues, sounds are unlikely to produce a significant vibration in teeth because the acoustic properties of the teeth are so different from the properties of the water. "The sounds would basically just bounce off of the tooth," he argues.
Dobbins agrees that further work is necessary to determine whether his models accurately reflect dolphin sonar. But regardless of how that debate turns out, he says, the results can already be used to improve man-made systems. "If you are a navy diver in murky water looking for a mine, you certainly wouldn't want a system to stop working once you got close to it," he says.
- Dobbins P. Bioinsp. Biomim., 2 . 19 - 29 (2007).
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