Here's an interesting note that just came to me from a friend at the
American Museum of Natural History.
Bernie Krause
Mic based on fly ear can pinpoint sounds
22 March 2008
From New Scientist Print Edition.
David Robson
IT MAY be inspired by a humble fly, but a sensor just a couple of
centimetres in diameter could turn out to be the best microphone yet
for pinpointing the source of a sound. The device could be added to
hearing aids or mounted on autonomous robotic vehicles to locate
cries for help during disaster relief efforts.
Humans detect slight differences in the timing of sound waves as they
arrive at each eardrum and use this to reconstruct where a sound is
coming from. However, the differences are only noticeable because our
eardrums are at least a few centimetres apart. In contrast, despite
the small distance between its two eardrums, the parasitic fly Ormia
ochracea can pinpoint a sound source far more accurately than humans.
Its secret is a chitin bridge that links the fly's eardrums (see
Diagram). Because they are close together, the difference in the way
the eardrums deform in response to the same sound wave is very
slight. But as they deform, the bridge rocks like a see-saw,
amplifying the tiny differences in the sound wave arriving at each
ear and allowing the fly to detect them. "We call this the rocking
mode, and it helps to amplify the directional signals," says Miao Yu,
an engineer at the University of Maryland in College Park.
Although scientists knew how sensitive Ormia's ears are (New
Scientist, 7 April 2001, p 25) no one had managed to create a
successful artificial mimic based on fly's ears. Now Yu has created a
prototype "fly mic" just a couple of centimetres in diameter. It can
pinpoint the angle of a sound's source eight times as precisely as a
larger, commercially available microphone sensor.
The "eardrums" of her tiny mic are flat diaphragms made from a
flexible polymer called polyamide, which moves even under very small
vibrations. The bridge is a wafer of silicon dioxide. Yu's prototype
also has an air-filled cavity surrounding the eardrums, which helps
to transmit the sound waves from one drum to the other, as does the
real fly. "In the past people ignored the effect of the cavity, but
this helps to improve the fly's directional hearing," she says.
To detect the deformation of the eardrums, the mic's casing contains
two fibre-optic cables, which shine light onto the drums and transmit
the reflected light back to an optical sensor. As the eardrums
deform, the pattern of reflected light changes. A computer uses these
changes to calculate the angle the sound is coming from. Yu believes
that using optical sensors has allowed her to eliminate the noise
that plagued other systems.
She plans to shrink the device further and hopes it will be used in
small airborne vehicles to find people in emergencies. The mic might
also improve hearing aids, as precise directional information,
combined with a sensor for different sounds, would help to filter out
unwanted noise. "The dream is to have a device that could detect the
acoustic landscape," says Daniel Robert of the University of Bristol,
UK, who has studied the structure of the fly's ear.
Wild Sanctuary
POB 536
Glen Ellen, CA 95442
707-996-6677
http://www.wildsanctuary.com
Google Earth zooms: Earth.WildSanctuary.com
|