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
|Contact: Louis Bergeron|