Underwater 'Thunder' Could Lead to Better Sonar

Sounds generated using a thunder-like mechanism could lead to lighter, thinner sonar projectors for advanced subs and underwater drones.

The new technology relies on carbon nanotubes, microscopic pipes just nanometers or billionths of a meter wide that have displayed a wide range of extraordinary mechanical, thermal and electrical properties. For instance, they are 100 times stronger than steel at one-sixth the weight.

In 2008, scientists in China revealed one more remarkable feature of nanotubes — sheets of them could serve as loudspeakers. When electrical current is run through the fabrics, the nanotubes rapidly get hot, which in turn heats and expands the air around them to produce sound, much like how lightning generates thunder.

In light of this past work, physicist Ali Aliev at the University of Texas at Dallas and his colleagues wanted to see if these sheets could work as loudspeakers underwater as well, potentially for use in sonar. Sonar works by emitting pulses of sound and listening for any echoes. The time delay between the emitted pulses and their echoes can reveal information about the objects the sound waves are bouncing off of, such as how far they are and how fast they are moving.

Still, the scientists were initially not expecting the sheets to perform well when it came to generating sound underwater. While air is relatively easy to heat up, water is less so, which would seem to make it a poor medium to create sounds in a thunder-like way.

Surprisingly, the nanotube sheets proved roughly 100 times more efficient at sound generation than predicted. This is roughly as good as the performance seen with them in air.

Apparently, when the sheets were dipped in water, their water-repellent nature allowed air to come between the nanotubes and the surrounding liquid. This gaseous envelope could readily heat up and generate sound when the nanotubes got hot, sound that then readily conducted itself into the water.

In particular, the nanotube sheets proved about 100 times more efficient at generating the kind of low-frequency sound used in sonar than the much thicker and heavier acoustic projectors conventionally used now.

For instance, a disk a little more than an inch wide (3 centimeters) made of a carbon nanotube sheet just 20 microns thick — a fifth the diameter of a human hair — and 10 millionths of a gram in weight can generate sound in a wide range of frequencies. In comparison, a conventional piezoelectric ceramic disk of the same diameter cannot generate a wide range of frequencies, and would range from 2.5 to 25 millimeters thick and 13.8 to 138 grams depending on which frequency it worked in.

"Since the carbon nanotube film is extremely thin, light and flexible it can be deposited on any  surface to provide negligible addition to the mass of the system," Aliev said. He noted it would prove easy to create large projectors that were, say, a square yard in size and less than a millimeter thick.

To isolate the nanotubes even further from water, the researchers tried encapsulating the fabrics between a variety of sonically conductive sheets — for instance, ones made of glass, metallic foil and thin ceramic plates — and filled the inner chambers with air or inert gases such as argon or helium. These initial setups could improve low-frequency underwater sound generation by more than 10-fold, and the scientists noted there was a lot of room to improve the systems even further — by reducing the thickness of the containers, for example.

The scientists detailed their findings online May 27 in the journal Nano Letters.

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