One of the best ways to detect and accurately measure movements that small is by using lasers and mirrors, creating a sort of tiny light show inside the microphone.
Kilic ran a fiberoptic cable into the water-filled microphone, with the end of the cable positioned near the inside surface of the diaphragm. He then shot light from a laser out the end of the cable onto the diaphragm.
Normally a diaphragm so thin would be transparent, allowing the laser's light to escape. But the researchers knew that if the diameters of the holes that allowed water to pass through the diaphragm were close to the wavelength of the light from the laser, the holes would interfere with light trying to pass through the membrane. Instead of letting it pass, the holes would reflect the light back toward the tip of the fiber optic cable, effectively turning the diaphragm into a mirror even as it still allowed water to pass.
"It is counterintuitive, because we don't see this happen at our scale," Kilic said. "But at very small scales, with the right size holes drilled through the membrane, it works."
When the diaphragm is deformed ever so slightly by a sound wave, the intensity of the light reflected back into the cable is altered, which is measured with an optical detector.
Now the scientists had a hydrophone that would function at any depth and could detect and measure sound with extreme accuracy. But to be able to capture the full range of volumes they were after a spread of 160 decibels one diaphragm wasn't enough. So they used three.
By giving each one a different diameter, they were able to "tune" each diaphragm to maximize its sensitivity to a different part of the range of volumes they wanted to detect. One was tuned to measure quiet sounds on the library-whisper end of the spectrum, one was attuned more to the loud, TNT explosion end of the range, a
|Contact: Louis Bergeron|