Stanford researchers have developed a microphone that can be used at any depth in the ocean, even under crushing pressure, and is sensitive to a wide range of sounds, from a whisper in a library to an explosion of TNT. They modeled their device after the extraordinarily acute hearing of orcas.
For most people, listening to the ocean means contemplating the soothing sound of waves breaking gently on a sandy beach.
But for researchers studying everything from whale migration to fisheries populations, and from underwater mapping to guiding robots trying to repair leaking undersea oil wells, listening to the ocean from the other side – underwater – can reveal volumes of valuable data.
Stanford researchers have developed a highly sensitive underwater microphone that can capture the whole range of ocean sounds, from the equivalent of a soft whisper in a library to an explosion of a ton of TNT just 60 feet away – a range of approximately 160 decibels – and do so accurately at any depth, no matter how crushing the pressure. It also can hear sound frequencies across a span of 17 octaves, spanning pitches far higher than the whine of a mosquito and far lower than a rumbling foghorn.
Existing underwater microphones – called hydrophones – have much more limited ranges of sensitivity and do not perform well at depth, where the ambient pressure can be extremely large, making it difficult to detect faint sounds.
Sonar – using sound to locate and map – is critical to underwater communication and exploration, because radio signals can travel only a centimeter or two before they dissipate in seawater and light can’t penetrate the depths below about 100 meters.
In approaching the challenge of designing the new hydrophone, the researchers first examined some existing listening devices that work well underwater – the ears of marine mammals, particularly orcas.
“Orcas had millions of years to optimize their sonar and it shows,” said Onur Kilic, a postdoctoral researcher in electrical engineering. “They can sense sounds over a tremendous range of frequencies and that was what we wanted to do.”
Kilic is the lead author of a paper about the research published in the Journal of the Acoustic Society of America earlier this year.
What orcas, humans and other creatures perceive as sound consists of small fluctuations in pressure. When someone beats a drum, it is the flexing of the membrane on the drum, first deflecting then rebounding, which causes the sound waves that we can hear. A microphone detects those sounds by means of a membrane or diaphragm inside it that vibrates in response to the pressure waves of sound that reach it.
Air pressure on the surface of the Earth is fairly constant, so in designing a microphone for use on land, engineers don’t have to worry about large variations in air pressure.
But in the ocean, for every 10 meters you descend below the surface, the water pressure around you increases by the equivalent of 1 atmosphere – the air pressure we feel at the surface.
The deepest point on the planet, the Challenger Deep in the Mariana Trench in the South Pacific, lies approximately 11,000 meters (almost 7 miles) below sea level. At that depth, the pressure is approximately 1,100 times the air pressure at Earth’s surface.
“The only way to make a sensor that can detect very small fluctuations in pressure against such immense range in background pressure is to fill the sensor with water,” Kilic said.
Letting water flow into the microphone keeps the water pressure on each side of the membrane equal, no matter how deep.
Kilic and his colleagues fabricated a silicon chip with a thin membrane about 500 nanometers thick – about 25 times thinner than common plastic wrap – and drilled a grid of tiny nano-holes in it, to allow water to pass in and out.
But unlike air, water is virtually incompressible, so having water on each side of the diaphragm damped the amount that the diaphragm could move in response to any given sound waves that struck it.
“The kind of displacements you get of the diaphragm for the quietest sounds in the ocean is on the order of a hundred-thousandth of a nanometer,” Kilic said. “That is ten thousand times smaller than the diameter of an atom.”
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