The fly Ormia ochracea can determine the direction and location of its prey, a chirping cricket, with remarkable accuracy for a creature so small. Larger animals typically use interaural time, or the difference in time between when a sound wave hits one ear versus the other, to determine the location of a sound. The ears of the fly are less than 2 mm apart, however, a distance 50 times smaller than the wavelength of the sound emitted by a cricket. Its hearing therefore relies upon a unique, sophisticated sound processing mechanism. This mechanism inspired researchers at the University of Texas in Austin to develop a prototype for a novel hearing device for use in humans.

Although the difference between a sound wave's time of arrival at each O. ochracea ear is a negligible 4 millionths of 1 s, the phase of the sound wave shifts slightly between the first and second ears. The fly's ear contains a structure, only 1.5 mm in length, that detects this phase shift and uses two modes of vibration to amplify the interaural time and sound level differences so that the location of the sound can be determined.

The tiny prototype developed by Michael Kuntzman and Neal Hall mimics this structure and its properties. The device consists of a thin, flexible, silicone beam suspended by two pivots, resembling a miniature see-saw. Four springs wrap around the perimeter of the beam and connect to the left and right ends of the beam. In response to sound waves, the beam and spring structure has a 'rocking' mode of vibration, in which the beam rotates about the pivots, and a 'flapping' mode of vibration, in which both ends of the beam alternate their motions. The device also contains four sensing ports made of piezoelectric materials, which turn mechanical strain into electric signals. These ports measure the phase displacements that result from the sound wave (i.e., the flexing and rotation of the beam) to amplify the interaural time and sound level.

In experiments, the device replicated the fly's ability to detect sound direction and compute the angle of sound incidence via the piezoelectric sensing ports (Appl. Phys. Lett. 105, 033701; 2014). The researchers say that future versions of the device will only need two sensing ports and may include a third 'twisting' mode of vibration, which would make it possible to simultaneously sense gradients along both horizontal and vertical axes.