The present disclosure relates generally to a miniature system for sensing and localizing acoustic signals. In particular, the present disclosure relates to a miniature system for sensing and localizing acoustic signals inspired by the directional hearing of the fly Ormia ochracea. 
The fly Ormia ochracea is a parasitoid insect that acoustically locates male field crickets by listening to their calling song at 5 kHz. The female fly, as part of her reproductive cycle, finds cricket hosts as a source of food for her larval offspring. With a separation of only 0.52 mm between auditory organs, less than 1/130 of the calling song sound wavelength, the best available interaural time difference (ITD) and interaural intensity difference (IID) are merely 1.5 μs and less than 1 dB, respectively. However, the fly has super-acute hearing for detecting sounds and localizing sound sources in the absence of visual and olfactory cues. See Robert, Innovative Biomechanics for Directional Hearing in Small Flies, Biol. Bull, 200: 190-94 (April 2001). The key to the fly's phenomenal directional hearing is that its auditory system includes a pair of tympanal membranes that are coupled by an intertympanal bridge, a cuticular structure that pivots about its middle. See R N Miles et al., Mechanically Coupled Ears For Directional Hearing in the Parasitoid Fly Ormia Ochracea, Journal of the Acoustical Society of America, 98(6):3059-3070 (1995).]
As a result of the mechanical coupling, the fly's auditory system operates in a rocking mode in which the tympanal membranes move 180 degrees out of phase, and a bending mode in which the tympanal membranes move in phase. Owing to a suitable contribution from both modes, the fly ear can provide great amplification to minute directional cues from the acoustic inputs so that the magnitude of directional cues at the mechanical response level becomes detectable by the fly's neuron system. The mechanical interaural time difference (mITD) and mechanical interaural intensity difference (mIID) between the eardrum responses are as high as 50-60 μs and 12 dB, respectively.
During the localization process, the fly turns its head front (azimuth θ=0) towards the sound source. The turning speed has been found to be a sigmoid function of azimuth, where the turning speed is a linear function of azimuth θ up to 20°-30°, and it is constant beyond this range. Here the azimuth is an angular measurement in a spherical coordinate system, which describes the angle between the midline (i.e., the vector starting from the midpoint between the centers of active portions of the two tympanal membranes and pointing in a direction that is perpendicular to the plane of the fly ear) of the fly ear and vector starting at the origin of the midline (i.e., the midpoint) and pointing to the sound source.
When the target is outside of the linear range, the fly performs laterization by only determining if the target is towards the left or right and makes a constant turn towards the target. Once the target is within the linear range, the fly performs localization by truly estimating the target position, and turning an appropriate amount of angle to let the midline of the ear point to the target. This laterization/localization scheme helps the fly to achieve an overall directional resolution as accurate as ±2° from the midline. See Mason et al., Hyperacute Directional Hearing in a Microscale Auditory System, Nature, 410 (6829):686-690 (2001).