There has always been a problem with recording sounds underwater. Although sound carries in water even faster than it does on land, technicians are forced into a Catch-22 situation. They can seal the microphone, allowing the inside to be filled with air. As sound waves slightly change the pressure of the water outside, the seal vibrates, which causes the air inside to compress and vibrate. The microphone passes the pattern of those vibrations along to speakers, which let people hear the underwater sound. The problem is, unlike air pressure, which stays fairly constant, water pressure changes dramatically depending on the depth, which means the microphone has to deal with immense ranges of pressure if it’s to be used at different depths. This is often too tough to do.
Letting water flow into the microphone freely through holes in the seal would allow the pressure to equalise at every depth. Vibrations would still be heard as sound, and the microphone could be used anywhere. The problem is, air compresses readily. Water, on the other hand, is almost impossible to compress. The water within the microphone would either need to feel immense pressure to vibrate the way air does, or the microphone would have to be much more sensitive to changes in pressure than air microphones are. It would have to sense movement of less than the diameter of an atom to pick up quiet sounds.
Such movement can only be picked up using lasers shot from the base of the microphone towards the seal, and reflected back. The membrane would be so thin, though, that it would be transparent the lasers would simply shoot out into the water. Researchers have found an inventive way to overcome that. The membranes on the microphone would have tiny holes drilled into them. These holes would allow water to flow through, but would be around the size of the wavelength of light shining out from inside the microphone. Adding holes to an otherwise transparent membrane actually gets the light to reflect back on itself. The holes interfere with the way the light shines out into the ocean, and that interference bounces the light back.
When a sound from the ocean vibrates the membrane of the microphone, the light that is reflected back changes in intensity. An optical sensor measures that change in intensity, translates it into the rapidity and intensity of the vibrations of the membrane, and transmits that pattern of vibrations back to the listener. And the underwater world is suddenly audible. [via Stanford]
Image: Kevin Lafferty/US Geological Survey