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 can pinpoint a sound source far more accurately than humans.
Its secret is a chitin bridge that links the fly鈥檚 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. 鈥淲e call this the rocking mode, and it helps to amplify the directional signals,鈥� says , an engineer at the University of Maryland in College Park.
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Although scientists knew how sensitive Ormia鈥榮 ears are (快猫短视频, 7 April 2001, p 25) no one had managed to create a successful artificial mimic based on fly鈥檚 ears. Now Yu has created a prototype 鈥渇ly mic鈥� just a couple of centimetres in diameter. It can pinpoint the angle of a sound鈥檚 source eight times as precisely as a larger, commercially available microphone sensor.
The 鈥渆ardrums鈥� 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鈥檚 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. 鈥淚n the past people ignored the effect of the cavity, but this helps to improve the fly鈥檚 directional hearing,鈥� she says.
To detect the deformation of the eardrums, the mic鈥檚 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. 鈥淭he dream is to have a device that could detect the acoustic landscape,鈥� says of the University of Bristol, UK, who has studied the structure of the fly鈥檚 ear.
