New microphone has scientists abuzz

Modelled on a fly’s ear, a tiny microelectromechanical microphone could be used for hearing aids, mobile phones and noise control systems.

Using the sensitive ears of a parasitic fly for inspiration, a team of researchers have created a new type of microphone that achieves better acoustic performance than what is currently available in hearing aids. The scientists presented their results at the 21st International Congress on Acoustics, held this week in Montreal.

Ronald Miles, Distinguished Professor of Mechanical Engineering at Binghamton University, studies the hearing of Ormia ochracea, a housefly-sized insect that is native to the southeast United States and Central America. Unlike most other flies, Ormia ochracea has eardrums that sense sound pressure, as do our ears, and they can hear “quite well,” says Miles. The female flies use their “remarkable” directional hearing to locate singing male crickets, on which they deposit their larvae.

Previously, Miles and colleagues Daniel Robert and Ronald Hoy described the mechanism by which the fly achieves its directional hearing, despite its small size. Now Miles and his group have designed a microphone inspired by the fly’s ears.

The new design uses a microelectromechanical microphone with a 1 x 3 mm diaphragm that is designed to rotate about a central pivot in response to sound pressure gradients. The motion of the diaphragm is detected using optical sensors. To minimise the adverse effects of resonances on the response, Miles and his colleagues used a feedback system to achieve so-called active Q control.

“Q control basically is an electronic feedback control system to introduce electronic damping,” Miles explained. “You don’t want a microphone diaphragm to ring like a bell. It turns out that in order to achieve a very low noise floor - which is the quietest sound that can be detected without the signal being buried in the microphone’s noise - it is important to minimise any passive damping in these sensors. If you do that, the diaphragm will resonate at its natural frequency. We are the first group to show that you can use this sort of electronic damping in a microphone without adversely affecting the noise floor of the microphone.”

Indeed, the noise floor of the fly-inspired microphone is about 17 decibels lower than what can be achieved using a pair of low-noise hearing aid microphones to create a directional hearing aid. The new design could be used in applications ranging from hearing aids and mobile phones to surveillance and acoustic noise control systems, Miles says, and “could easily be made as small as the fly’s ear.”