Killer Whales' Ears Inspire New Undersea Microphones
The new hydrophone based on orca ears.
CREDIT: Onur Kilic
By copying the ears of killer whales, scientists have designed underwater microphones that could one day help track migrating whales, guide robots toward leaking undersea oil wells and monitor exotic cosmic particles plunging into the ocean.
Conventional underwater microphones, known as hydrophones, have very limited ranges of sensitivity. They cannot perform well at great depths, where the great ambient pressure of the ocean abyss can make it hard to discern faint sounds.
Now researchers have developed an ultra-sensitive hydrophone that can detect a wide spectrum of underwater sounds, from the weakest ones to those 100 million times stronger. "The decibel range of the sensor ranges from 20 decibels to 180 decibels in water — this is equivalent to a microphone that can record a whisper in a quiet library and the sound from 1 ton of TNT exploding 60 feet away," researcher Onur Kilic, an applied physicist at Stanford University, told TechNewsDaily.
At the same time, this new hydrophone can work at virtually any depth, no matter how crushing the pressure. It also can hear sound frequencies from 1 hertz to 100 kilohertz, spanning pitches far higher than the whine of a mosquito and far lower than a foghorn.
[Read also ‘Arctic Voyage Sets Off to Track Mysterious Whales.’]
A new kind of hydrophone
To design the new hydrophone, Kilic and his colleagues analyzed ears they knew already worked very well underwater, such as those of killer whales, also known as orcas.
"Orcas had millions of years to optimize their sonar and it shows," Kilic said. "Their sense of hearing is perfectly adapted to the noisy environment in the ocean, and that was what we wanted to do."
The ears of orcas and humans are both based on thin membranes that wobble back and forth when hit by sound waves, data that gets transmitted as electric impulses to the brain. Hydrophones use thin membranes or diaphragms as well.
"The first thing that comes to mind when thinking about underwater sound and animals are whales," Kilic said. "I decided to build our sensors adapted to ocean noise just like orcas and then improve its frequency range beyond what orcas can sense."
In the ocean, the amount of ambient pressure can vary greatly. For every 33 feet (10 meters) that one descends below the surface, water pressure increases by about 1 atmosphere, the air pressure felt at the surface. At the deepest point on the planet, the Challenger Deep in the Mariana Trench in the South Pacific ― about seven miles (11 kilometers) below sea level ― the pressure is approximately 1,100 times the air pressure at Earth's surface.
"The only way to make a sensor that can detect very small fluctuations in pressure against such immense range in background pressure is to fill the sensor with water," Kilic said. Doing so keeps the water pressure on each side of the membrane equal, no matter how deep.
Kilic and his colleagues fabricated a microchip with a silicon membrane about 500 nanometers thick, or about 25 times thinner than common plastic wrap. They next drilled a grid of tiny holes in the membrane to allow water to pass in and out.
To detect the wobbles of such a membrane in response to sound, the researchers shine a laser on this reflective sheet with a fiber-optic cable. Since the diameter of the holes in the membrane are close to the wavelength of light from the laser, the holes interfere with the light trying to pass through the membrane, reflecting it toward a detector. When the membrane gets deformed by sound waves, the intensity of the light alters, which the detector can pick up.
"The kind of displacements you get off the diaphragm for the quietest sounds in the ocean is on the order of a hundred-thousandth of a nanometer," Kilic said. "That is 10,000 times smaller than the diameter of an atom."
To capture the full range of sound volumes the researchers were after, the new hydrophone uses three membranes, not just one. By giving each one a different diameter, the scientists made each diaphragm sensitive to a different range of volumes. They are at most 300 microns in diameter, or about three times the width of a human hair.
The new hydrophone could have an impact on a wide range of research, from standard applications such as sonar and studying fisheries populations to more exotic endeavors in particle physics, ones that use acoustic detectors to monitor ultra-high-energy particles called neutrinos zipping through the ocean.
The sensors are quite complex in structure and the technologies used are not amenable to mass production. "We are, however, working on simplifying the technology without sacrificing its performance," Kilic said.
Kilic and his colleagues detailed their findings in the April issue of the Journal of the Acoustical Society of America.