Onur Kilic's microphone prototype
Stanford researchers spent years studying marine animals in an effort to develop a better underwater microphone. The orca had the frequency range they wanted, 17 octaves, or 160 decibels. The orca ear works as such: sound brings the tympanic bone, and especially its thin tympanic plate, into vibration. The ossicles in the air-filled middle ear cavity form a bridge from the tympanic plate to the periotic bone connecting the vibrating plate to the oval window and the inner ear. (The anatomy of the killer whale middle ear). In other words, the many ear plates are tuned to various frequencies.
Another major issue, the team needs the microphone to work at incredible depths in the ocean. For example, the Challenger Deep in the Mariana Trench. Approximately 7 miles below the surface with a pressure of around 1,100 atmospheres.
To conquer these obstacles, the team at Stanford lead by Onur Kilic, a postdoctoral electrical engineering researcher, built a thin silicon membrane diaphragm. The membrane is 500 nanometers thick, three-tenths of a millimeter in diameter, and nano-scale holes were drilled in a grid. This allows water to penetrate the membranes and equalize the pressure all around it. The membrane does not deform under these conditions at all. A laser from a fiber optics cable is directed onto the membrane. As the membrane deforms due to sound pressure, the light beam changes in intensity as it is reflected back onto a optical detector. The sound waves can then be seen.
The single membrane could not detect the 160 db range. The novel approach was to use three membranes that are turned to different frequency ranges. The membranes only differ by the diameter size (largest being three-tenths of a millimeter). All three diaphragms fit in a space smaller than the wavelengths they are designed to sense. Kilic says, "Since they all measure the exact same signal – just with different degrees of responsiveness – they work like a single sensor." Each diaphragm has its own fiber optic element. All components fit into a space smaller than five millimeters in diameter.
Deep sea exploration and military operations are soon to change. Great work Onur Kilic.
Eavesdropper
