Onur Kilic, postdoctoral fellow at the Center for Nanotechnology, prepares to test a microphone. Photos by L.A. Cicero |
For most people, listening to the ocean means contemplating the soothing sound of waves breaking gently on a sandy beach.
But
for researchers studying everything from whale migration to fisheries
populations, and from underwater mapping to guiding robots trying to
repair leaking undersea oil wells, listening to the ocean from the other
side—underwater—can reveal volumes of valuable data.
Stanford
researchers have developed a highly sensitive underwater microphone
that can capture the whole range of ocean sounds, from the equivalent of
a soft whisper in a library to an explosion of a ton of TNT just 60
ft away—a range of approximately 160 decibels—and do so accurately
at any depth, no matter how crushing the pressure. It also can hear
sound frequencies across a span of 17 octaves, spanning pitches far
higher than the whine of a mosquito and far lower than a rumbling
foghorn.
Existing
underwater microphones—called hydrophones—have much more limited
ranges of sensitivity and do not perform well at depth, where the
ambient pressure can be extremely large, making it difficult to detect
faint sounds.
Sonar is critical to underwater
communication and exploration, because radio signals can travel only a
centimeter or two before they dissipate in seawater and light can’t
penetrate the depths below about 100 meters.
In
approaching the challenge of designing the new hydrophone, the
researchers first examined some existing listening devices that work
well underwater—the ears of marine mammals, particularly orcas.
“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.”
Kilic is the lead author of a paper about the research published in the Journal of the Acoustic Society of America earlier this year.
What
orcas, humans, and other creatures perceive as sound consists of small
fluctuations in pressure. When someone beats a drum, it is the flexing
of the membrane on the drum, first deflecting then rebounding, which
causes the sound waves that we can hear. A microphone detects those
sounds by means of a membrane or diaphragm inside it that vibrates in
response to the pressure waves of sound that reach it.
Air
pressure on the surface of the Earth is fairly constant, so in
designing a microphone for use on land, engineers don’t have to worry
about large variations in air pressure.
But
in the ocean, for every 10 meters you descend below the surface, the
water pressure around you increases by the equivalent of 1 atmosphere—the air pressure we feel at the surface.
The
deepest point on the planet, the Challenger Deep in the Mariana Trench
in the South Pacific, lies approximately 11,000 meters (almost 7 miles)
below sea level. At that depth, the pressure is approximately 1,100
times the air pressure at Earth’s surface.
Onur Kilic’s miniature underwater microphone |
“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.
Letting water flow into the microphone keeps the water pressure on each side of the membrane equal, no matter how deep.
Kilic
and his colleagues fabricated a silicon chip with a thin membrane about
500 nm thick—about 25 times thinner than common plastic wrap—and drilled a grid of tiny nano-holes in it, to allow water to pass in
and out.
But
unlike air, water is virtually incompressible, so having water on each
side of the diaphragm damped the amount that the diaphragm could move in
response to any given sound waves that struck it.
“The
kind of displacements you get of the diaphragm for the quietest sounds
in the ocean is on the order of a hundred-thousandth of a nanometer,”
Kilic said. “That is ten thousand times smaller than the diameter of an
atom.”
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, and the third was tuned to the
mid-range volumes.
The
diaphragms are so tiny—the largest is three-tenths of a millimeter in
diameter—Kilic could fit all three into a space far smaller than the
wavelengths of the sound they sought to detect. That was critical,
because it allowed the diaphragms to effectively function as one.
“Since
they all measure the exact same signal—just with different degrees of
responsiveness—they work like a single sensor,” Kilic said.
“It
is a very high dynamic range microphone, able to sense everything from
the weakest sounds to those 100 million times stronger.”
All
three diaphragms—along with a separate fiber optic cable for each,
plus another used for calibration—fit easily into the housing of the
microphone, which is barely larger than a pea.
But
the little pea-sized microphone could have a big impact on a wide range
of research, from standard applications such as surveying the ocean
floor to more exotic endeavors in particle physics that use acoustic
detectors to monitor ultra-high-energy neutrinos—almost weightless
particles emitted by the sun—plunging into the ocean.
Michel
Digonnet, research professor of applied physics; Gordon Kino, professor
emeritus of electrical engineering; and Olav Solgaard, associate
professor of electrical engineering, are coauthors of the paper.
The research was funded by Litton Systems Inc., a wholly owned subsidiary of Northrop-Grumman Corp.
