A fly-inspired miniature microphone
Hypersensitive 2-millimeter-wide
device could lead to a new generation of miniaturized low-power hearing aids
July 25, 2014
This is a top-side view in a microscope photograph of the
biologically inspired microphone. The tiny structure rotates and flaps about
the pivots (labeled), producing a voltage across the electrodes (labeled).
(Credit: N. Hall/UT Austin)
University
of Texas Austin
researchers have developed a tiny prototype microphone device that mimics
the Ormia ochraceafly’s hearing mechanism. The design may be useful for a
new generation of hypersensitive, millimeter-sized, low-power hearing aids.
The
yellow-colored Ormia ochracea fly, the inspiration for the design, can pinpoint
the location of a chirping cricket with remarkable accuracy because of its
freakishly acute hearing, which relies upon a sophisticated sound processing
mechanism that really sets it apart from all other known insects.
Described
in the journal Applied Physics Letters, from AIP Publishing, the 2-millimeter-wide device
uses piezoelectric materials, which turn mechanical strain (from sounds) into
electric signals that can be used to stimulate sensors in the inner ear. The
use of these materials means that the device requires very little power.
“Synthesizing
the special mechanism with piezoelectric readout is a big step forward towards
commercialization of the technology,” said Neal Hall,
an assistant professor in the Cockrell School
of Engineering at UT Austin. “Minimizing power consumption is always
an important consideration in hearing-aid device technology.
There
are military and defense applications as well, and Hall’s work was funded by
the Defense Advanced Research Projects Agency
(DARPA). In dark environments, for instance, where visual cues are not
available, localizing events using sound may be critical.
Super
Evolved Hearing
Humans
and other mammals have the ability to pinpoint sound sources because of the
finite speed of sound combined with the separation between our ears. The
spacing of several centimeters or more creates a slight difference in the time
it takes sound waves to hit our ears, which the brain processes perceptually so
that we can always experience our settings in surround sound.
Insects
generally lack this ability because their bodies are so small that sound waves
essentially hit both sides simultaneously. Many insects do detect sound
vibrations, but they may rely instead on visual or chemical sensing to find
their way through the fights, flights and forages of daily life.
O.
ochracea is a notable exception. It can locate the direction of a cricket’s
chirp even though its ears are less than 2 mm apart — a separation so slight
that the time of arrival difference between its ears is only about four
millionths of a second (0.000004 sec).
But the
fly has evolved an unusual physiological mechanism to make the most of that
tiny difference in time. What happens is in the four millionths of a second
between when the sound goes in one ear and when it goes in the other, the sound
phase shifts slightly. The fly’s ear has a structure that resembles a tiny
teeter-totter seesaw about 1.5 mm long.
Teeter-totters,
by their very nature, vibrate such that opposing ends have 180-degree phase
difference, so even very small phase differences in incident pressure waves
force a mechanical motion that is 180 degrees out of phase with the other end.
This effectively amplifies the four-millionths of a second time delay and
allows the fly to locate its cricket prey with remarkable accuracy.
Mimicking
the Mechanism
The
pioneering work in discovering the fly’s unusual hearing mechanism was done by
Ronald Miles at Binghamton University and colleagues Ronald Hoy and Daniel
Robert, who first described the phase amplification mechanism the fly uses to
achieve its directional hearing some 20 years ago. In 2013, Miles, and his
colleagues presented a microphone inspired by the fly’s ears.
Inspired
by Miles’s prior work, Hall and his graduate students Michael Kuntzman and
Donghwan Kim built a miniature pressure-sensitive teeter-totter in silicon that
has a flexible beam and integrated piezoelectric materials. The use of
piezoelectric materials was their original innovation, and it allowed them to
simultaneously measure the flexing and the rotation of the teeter-totter beam.
Simultaneously measuring these two vibration modes allowed them to replicate
the fly’s special ability to detect sound direction in a device essentially the
same size as the fly’s physiology.
This
technology may be a boon for many people in the future, since 2 percent of
Americans wear hearing aids, but perhaps 10 percent of the population could
benefit from wearing one, Hall said.
“Many
believe that the major reason for this gap is patient dissatisfaction, he
added. “Turning up the gain to hear someone across from you also amplifies all
of the surrounding background noise — resembling the sound of a cocktail
party.”
The new
technology could enable a generation of hearing aids that have intelligent
microphones that adaptively focus only on those conversations or sounds that
are of interest to the wearer. But before the devices become part of the next
generation of hearing aids or smartphones, more design and testing is needed.
Abstract
of Applied Physics Letters paper
The
parasitoid fly Ormia ochracea has the remarkable ability to locate crickets
using audible sound. This ability is, in fact, remarkable as the fly’s hearing
mechanism spans only 1.5 mm which is 50× smaller than the wavelength of sound
emitted by the cricket. The hearing mechanism is, for all practical purposes, a
point in space with no significant interaural time or level differences to draw
from. It has been discovered that evolution has empowered the fly with a
hearing mechanism that utilizes multiple vibration modes to amplify interaural
time and level differences. Here, we present a fully integrated, man-made mimic
of the Ormia’s hearing mechanism capable of replicating the remarkable sound
localization ability of the special fly. A silicon-micromachined prototype is
presented which uses multiple piezoelectric sensing ports to simultaneously
transduce two orthogonal vibration modes of the sensing structure, thereby enabling
simultaneous measurement of sound pressure and pressure gradient.
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