The sounds of the ocean go far beyond the sounds of waves and gulls that make up the soothing soundtrack people associate with the beach. Listening to what goes on beneath the surface of the water is incredibly useful for researchers studying animal migration, fisheries, or underwater mapping. But, recording these sounds requires highly sophisticated equipment that can function over large distances and at great depth. Researchers have been missing this technology, until now.
Inspired by the ears of Orcas, researchers from Stanford University have developed an underwater microphone that can capture a variety of ocean sounds at a range of 160 decibels, at any depth. The microphone can also pick up frequencies over a space of 17 octaves. Existing underwater microphones (hydrophones) have very limited ranges of sensitivity and struggle at significant depths.
According to the scientists, led by postdoctoral researcher Onur Kilic, Orcas have had millions of years to develop a sophisticated auditory system. The animals can detect sounds over a large range of frequencies, which is what the researchers wanted to replicate. The researchers were particularly interested in the whale’s sonar, the ability to use sound to locate and map the surrounding environment.
What orcas, humans, and other organisms perceive as sound is actually small fluctuations in pressure. A microphone detects these pressure changes using a membrane or diaphragm that vibrates in response to the pressure as the sound waves hit it. Air pressure on the Earth’s surface is relatively constant, so engineers don’t worry much about pressure variations when designing microphones for use on land. But, underwater microphones are a different story. For every 10 meters below the water’s surface, the pressure increases by the equivalent of one atmosphere (the air pressure we feel at the surface).
According to Kilic the only way to make a sensor for a microphone that can detect small fluctuations in pressure over a large range, against so much pressure is to fill the sensor with water. Described in the Journal of the Acoustic Society of America, the researchers designed a microphone that lets water flow into it to keep the water pressure on each side of the membrane equal no matter at what depth it is used.
The researchers made a silicon chip with a thin membrane only about 500 nanometers thick, and drilled a grid of tiny nano-holes in it, to allow water in and out. But, because water is incompressible having water on both sides of the membrane limited the amount it could move in response to sound waves. To circumvent this problem the researchers ran a fiberoptic cable into the water-filled microphone with the ends of the cable positioned near the inside surface of the membrane. One way to detect and measure small movement is using lasers and mirrors to record a light display, in this case inside the microphone. The researchers shot the light from a laser out the end of the cable and onto the membrane.
Typically this kind of membrane would be transparent, allowing the light to escape. But, the researchers purposefully drilled the diameter of the nano-holes that allow water to pass through to be close to the wavelength of the laser light so that the holes would interfere with light trying to pass through the membrane. The holes reflect the light back toward the tip of the fiber optic cable, making the membrane into a mirror. When the membrane is moved by a sound wave, the intensity of the light that gets reflected back is altered and measured by an optical detector.
While the new microphone was capable of functioning at any depth and measuring sound with a high level of accuracy, the researchers decided to take the design one step further. Rather than use a single membrane, they used three. Giving each membrane a different diameter, the researchers tuned each one to maximize its sensitivity to a different section of the range of volumes they wanted to be able to detect. The membranes are so small (the largest is 3/10 of a millimeter in diameter) that all three can fit into a space much smaller than the wavelengths of sound they are used to detect. This allows them to function as one unit.
The resulting microphone is roughly the size of a pea, but could be useful for research ranging from surveying the ocean floor to more advanced applications in particle physics as an acoustic detector that monitors ultra-high-energy neutrinos (weightless particles emitted by the sun) as they plunge into the ocean.
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