Stanford Researchers Design Hydrophones After Orca Whale Ears
by Owen James Burke
For most of us, listening to the sound of the ocean is a romantic experience, but for a group of Stanford researchers, it is the purveyor of invaluable information. The team has developed an underwater microphone, or hydrophone, with a threshold of 160 decibels and 17 octaves, which is more than twice that of a piano, for example.
Sure, we have radar, sonar, and hydrophones have already been invented. But after looking at the orca whale, whose ultrasensitivity to sound is nearly unmatched, the research team at Stanford believed that we could do better.
“Orcas had millions of years to optimize their sonar and it shows,” said Onur Kilic, a postdoctoral researcher in electrical engineering. “They can sense sounds over a tremendous range of frequencies and that was what we wanted to do.”
Sound on land and sound on water are two very different concepts. On land, there is less ambient noise and current, so sound comes very cleanly into most microphones (assuming you’re not on top of the Empire State building with 30 knot winds). In water, there are constant variations in pressure due to currents and ambient noises from water moving, sloshing and colliding, and the further down you dive, the more pressure you encounter. At the bottom of the Mariana Trench (7 miles below the sea’s surface), small changes in pressure, or sound, are hardly detectable with a microphone designed for land.
So Kilic’s group realized that maybe the hydrophones that scientists have been using below the sea surface were still too tailored to land–being dry on the inside. They then took a filter, plugging nano holes into it in order to let water in and out, creating an equilibrium between the hydrophone and the water outside. Water is not compressible though, so this development produced a rather dampened sound.
One of the best ways to detect movement is with light, so the group got to thinking again, and fit lasers into the camera (still filled with water). Linking a laser to a fiberoptic cable just inside the punctured filter caused a reflection from the holes, sending data back up the fiberoptic cable. The key was keeping the frequency of the light in tune with the frequency of the nano holes. With the aid of the light, the contraption would work at any depth.
But there was still one problem, the range of frequencies and octaves were still not as wide as they needed to be. Taking two more fiberoptic cables and tuning them to low, mid, and high frequency by making them each different sizes was the key, but how to get them into one single device?
Fortunately, these hydrophones are extremely small, and the three together function as one, fitting into a single small housing that is a bit larger than a pea.
Going forward, what does this mean for scientific research? Most any research to do with physics under water should improve, and perhaps we will be better able to survey the ocean floor, so that leaky oil wells can be fixed more promptly, and various breeding, feeding, and migratory waters can be better identified and protected.