A team of researchers at The University of Texas at Austin (UT Austin) has developed a tiny prototype device that mimics the hearing mechanism of a parasitic fly, the yellow-colored Ormia ochracea. This development may be useful for a new generation of hypersensitive hearing aids. Described in the July 22 online edition of Applied Physics Letters (DOI: 10.1063/1.4887370), the 2-mm-wide device uses piezoelectric materials, which turn mechanical strain into electric signals. The use of these materials means that the device requires very little power.

The space between the ears of insects is typically so small that sound waves essentially hit both sides simultaneously. However, the O. ochracea has an unusual physiological mechanism in which the sound phase shifts slightly when the sound goes in one ear and when it goes in the other. The fly, whose ears are less than 2 mm apart, has an ear 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 a 180° phase difference, so even very small phase differences in incident pressure waves force a mechanical motion that is 180° out of phase with the other end. This effectively amplifies the four-millionths of a second time delay the O. ochracea experiences in its hearing.

Neal Hall, an assistant professor in the Electrical and Computer Engineering Department at UT Austin, and his graduate student Michael Kuntzman built a miniature pressure-sensitive teeter-totter in silicon that has a flexible beam and integrated piezoelectric materials. By using multiple piezoelectric sensing ports, the researchers enable numerous vibration modes which then amplify the interaural time and level differences such as the fly experiences. 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 the researchers to replicate the fly’s special ability to detect sound direction in adevice essentially the same size as the fly’s physiology.

A photograph of the biologically inspired microphone taken under a microscope, providing a top-side view. The tiny structure rotates and flaps about the pivots (labeled), producing an electric potential across the electrodes (labeled). Credit: N.Hall/UT Austin.