Microphones for hearing aids and cell phones need to be small and still function well enough to be useful while limiting the effects of background noise.
Using the sensitive ears of a parasitic fly for inspiration, a group of researchers from the State University of New York at Binghamton, led by mechanical engineering professor Robert Miles, built a new type of microphone the size of a mustard seed that can hear sounds softer than anything currently available.
The design employs a micro-electromechanical microphone with a small diaphragm — only 1 millimeter by 3 millimeters — that rotates around a pivot when sound waves hit it. A tiny optical sensor picks up the motion of the diaphragm. Additional electronic controls and software help to filter out “thermal noise,” which is the vibration of the diaphragm caused by its own internal heat.
The bug that inspired the design is Ormia ochracea. It’s an insect native to the southeast United States and Central America. Ormia ochracea has eardrums that sense sound pressure, just like humans do. The fly’s “ears” aren’t on its head though — they’re near the front legs. A female fly uses her directional hearing to locate singing male crickets, on which she deposits her larvae.
The quietest sound the microphone can pick up, called the noise floor, is about 17 decibels lower than what people get with a pair of low-noise hearing aid microphones. Putting the device in a hearing aid, for example, means that it can pick up less intense sound, which means that it wouldn’t be necessary to “turn it up,” which tends to increase background noise. The microphone is also lot smaller than current designs.
The team will present their results at the International Congress on Acoustics in Montreal in Tuesday, June 4.
Credit: Wikimedia Commons