Ultrasensitive underwater microphone inspired by orca ears
By Ben Coxworth
June 24, 2011
Given how poorly light and radio signals are able to travel underwater, sound is still the best medium for wireless undersea communications. Conventional underwater microphones - or hydrophones - have their limitations, however. One of their main problems is that the deeper they go, the less sensitive they become. Scientists from California's Stanford University have now found a solution to that problem, in the form of a hydrophone that is designed to perform like an orca's ear.
On dry land, the air pressure is more or less the same everywhere, so it isn't an issue for microphones. Underwater, however, the water pressure increases by the equivalent of one atmosphere for every ten meters (33 feet) that you descend beneath the surface. In deeper waters, there is so much external pressure on the diaphragms of regular watertight hydrophones, that they are barely able to move in response to the additional pressure waves created by sounds. This drastically reduces the range of sounds that they can pick up, and limits their usefulness.
The new hydrophone solves the pressure difference problem, by letting water flow into the device. Because the pressure on either side of the diaphragm is the same, depth is no longer an issue - as is the case with the ears of orcas.
The diaphragm itself is made up of a silicon chip covered with a membrane, that is about 25 times thinner than ordinary plastic film. A matrix of nano-holes in that membrane allow water to pass in and out. Because water is virtually incompressible, however, the diaphragm isn't able to move much in response to sound pressure waves - some of the quietest sounds that it's able to detect cause it move no more than a hundred-thousandth of a nanometer.
In order to detect these movements, a laser shines from inside the hydrophone onto the interior surface of the diaphragm, then reflects back and is measured by an optical detector. The changes in the intensity of the reflected laser light, caused by the minute flexings of the diaphragm, are translated into sounds.
Just one diaphragm, however, can't capture the 160-decibel range of sounds that the scientists wished to detect, so three separate diaphragms of differing diameters and sensitivities are used. Fiber optic cables run a separate laser beam to each diaphragm, along with a fourth beam used for calibrating the hydrophone. Because the diaphragms are so tiny (the largest being three-tenths of a millimeter across), all three are easily able to fit in the Stanford hydrophone, which is itself only about the size of a pea.
A paper on the technology was published in the Journal of the Acoustic Society of America.