Locating objects by sending out sound waves, radio waves or even light waves, and watching for the reflection, is part of many technologies. The first reference of using sound for detection seems to be by Leonardo da Vinci, of a tube sent down in water, while one listened at the open end for sounds of enemy ships. There was also the use of an underwater bell, in addition to the lighthouse, to alert ships of dangers.
The use of sound waves for detection of underwater objects is now advanced, and so is the use of radio waves, in Radar. Sonar, whose development dates from the Titanic tragedy, is an essential part of navigation, and Radar is a vital part of aircraft operations, commercial and military. The quality of performance, however, is nothing to compare with what is found in the natural world, particularly in the case of bats and dolphins.
Laura Kloepper, assistant professor at St Mary’s College in Notre Dame, Indiana described her study of the methods that the bat uses, in a presentation last week, at the annual meeting, in Victoria, Canada, of the Acoustical Society of America with the Canadian Acoustical Association. At the same event, Thomas Neil of the University of Bristol, UK, described how species of moth, the main food of bats, have adapted to evade their predators’ ability to make things out in pitch darkness.
Bats navigate with the help of high pitched clicks that they generate with their larynx or tongues. With the help of echoes, which are received slightly apart by the two ears, the bat is able to create a three-dimensional map of the surrounding space, just like we can with our eyes. As high pitched sound has short wavelength, fine detailing is possible.
The bat, in fact, uses signals of different frequencies — lower frequencies where the signal needs to travel a longer distance and high frequencies, as high as 200,000 cycles a second, when there is need for details. The flying bat, at high speed, is able to navigate through a labyrinth of obstacles and locate and capture fleeing, mite-sized prey.
Another animal well known for echolocation is the dolphin. The dolphin uses an almost identical method, emitting clicks and squeaks, with an organ to pick up the echoes. While the dolphin uses sound not just for navigation but also for communication, the detection and navigating ability is still adapted well enough to make out pebble-sized objects and tell the different texture of rock, metal or animal tissue.
Impressive as the capability is, a further mystery is how the bat and dolphin are able to do all this even when flying or swimming in groups. In thick swarms of bats, or a school of dolphins, each of hundreds, even thousands, of other animals is navigating and foraging with its own staccato of clicks.
How each one is able to tell apart the echoes of its own sounds from sounds of others, or other echoes, has been puzzling. If we understood the methods that animals use, it would help us develop strategies for other fields, like air traffic control or robotics, or even electronic jamming in military operations.
Kloepper, who believes that bats are more proficient, used an arrangement of speakers and microphones to generate dolphin clicks to interfere with the echolocation of lone dolphins. She did find that dolphins modified their own signals, so that they could be told apart. “But they did not have the same level of control of their call as bats do”, she says.
Kloepper puts the difference in control down to the complexity in the sounds the animals use to echolocate. Dolphins produce short-lived and well-separated clicks. The chances are hence low that clicks and echoes would overlap. “Bats, on other hand, have calls that are much longer in duration, so have a higher probability of overlapping with other bats when they are flying in the same airspace”, Kloepper says.
When flying in a swarm, particularly, the bat is likely to be swamped by signals and echoes. How the bat copes is by marking its own clicks with a signature that it can recognise. The typical strategy is to use a particular frequency of sound, and to rapidly switch this frequency. Bats are known to make frequency switches as often as once every fifth of a second, and each one of the bats in the swarm can keep track of its own clicks.
Kloepper has experimented with a novel application of the “unmanned aerial vehicle” or the drone, to verify and investigate this behaviour of the bat, right in the middle of the swarm, in full flight. Normally, the motor that propels the unmanned vehicle would be too noisy for fineries of the bat vocalisations to be captured.
But Kloepper and her group designed unmanned vehicles where the noise of the propellers was blocked from the microphones and the bat sounds could be recorded. In addition to unmanned vehicles, they used a living hawk, mounted with a device to record bat sounds, which could mingle with flying bats.
Working with a swarm of Brazilian free tailed bats, numbering millions, they were able to record and extract individual clicks as the bats swooped down from high altitude, to roost, reaching speeds as high as 128 kmph.
At these speeds, there is also the Doppler shift of frequencies, Kloepper and her student, Morgan Kinniry said that in a paper earlier this year in the journal, Scientific Reports. The paper reports that bats employed both changes in frequency and changes in pulse duration, as wells as combinations of the two.
The method is not far removed from the technique in networking computer devices with Bluetooth. As many intelligent devices like computers, cell phones, headphones, even refrigerators and vacuum cleaners now communicate without wires, there is a need to keep signals apart. Bluetooth is a communication standard where pairs of devices settle on a pattern of unpredictable frequency changes, so that their exchanges stay unique.
Side-by-side with the technology of bats to spot prey in the darkness of the night, some species of prey have evolved evasive strategies. A method that one species of moth uses is to generate its own series of clicks, which “jam” the bats’ signal and put predators off track.
Other species of moths, whose defence is their distasteful flavour thanks to the specific plants that they feed on, advertise the fact with clicks in response to bat clicks. Neil from Bristol describes yet another strategy, of moths evolving body covering that does not create an echo. The moth then becomes essentially invisible.
The method not to reflect sound is to absorb it. A typical place where we need to absorb sound is in recording studios or auditoria to eliminate reverberation. Materials like mats of glass fibre, with micrometre-sized pockets where air cannot transmit sound, are formed, and they are the most common acoustic insulators. Neil found that a category of moths which are deaf to the sounds that bats produce, have body covering that renders the predators essentially as deaf as the prey.
He used a method called acoustic tomography, or measuring the intensity of reflected sound in slices of frequency. “Thoracic fur provides substantial acoustic stealth at all ecologically relevant ultrasonic frequencies,” Neil says, “Moth fur is thin and lightweight, and acts as a broadband and multidirectional ultrasound absorber that is on par with the performance of current porous sound-absorbing foams.”
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