Building A Better Microphone (With An Orca’s Help)

Listening to what’s going on under the sea matters greatly. Mapping the ocean floor, monitoring whale migrations, repairing the next Deepwater Horizon: each of these tasks relies on or can be aided by an underwater microphone. Eavesdropping on the ocean, though, is more complicated, on a technological level, than listening to the world up above, because the amount of pressure a microphone must withstand differs greatly depending on how deep it dives. In the ocean, descending another 10 meters means adding an atmosphere of pressure; the Marianas Trench, for instance, has a surrounding pressure well over 1,000 times greater than that of the surface.

Stanford researchers mulling this problem decided to study the orca. “Orcas had millions of years to optimize their sonar, and it shows,” Onur Kilic, a postdoctoral researcher in electrical engineering at Stanford, recently said. “They can sense sounds over a tremendous range of frequencies, and that was what we wanted to do.” The orca-inspired microphone he and his team produced is only the size of a pea, but it can plunge to the ocean’s greatest depths and pick up a huge range of sounds, from the faintest whisper to the most deafening explosion.

The first insight that Kilic and a team of engineers (plus one applied physicist) had was this: to make a truly versatile underwater microphone would require the ability to flood the microphone itself. “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 recently told Stanford Report. If you can’t beat 1,000 atmospheres of pressure, join ‘em.

Next, Kilic and his team needed to figure out the mechanics of the sensor itself. The protestations of a certain Caribbean crustacean notwithstanding, it can actually be unbearably quiet under the sea. As Kilic put 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”–or 1/10,000^th the diameter of an atom. Kilic et al. wound up using a laser and a fiberoptic cable to measure these tiny fluctuations. In order to capture the 160-decibel range of sounds he was after, Kilic found it was necessary to use three vibrating membranes, in fact–one for library-style whispers, another for mid-range sounds, and a third for the big bangs.

Kilic’s team, whose research was funded by Litton Systems Inc. (a subsidiary of Northrop-Grumman Corp.), reported their findings not long ago in the Journal of the Acoustic Society of America. If the tiny biomimetic hydrophone winds up in as many places as Stanford suggests it might–assisting everyone from ichthyologists to particle physicists–then the team will have done the orca proud indeed.