Beamforming in bat biosonar for flight in clutter

Beamforming is common in sonar/radar but requires multiple transducers in an array. We suggest that echolocating big brown bats use beamforming with only two ears. When bats fly along constricted paths surrounded by dense, extended clutter, they maintain head aim frontally with small “looks” to the sides. Left and right echoes are highly correlated. When arriving at the bat’s ears simultaneously, these could evoke perception of phantom objects straight ahead, thus preventing accurate assessment of the upcoming path. Inferior colliculus neurons tuned to biosonar frequencies are sensitive to interaural level differences from + 10 to −20 dB, corresponding to external-ear directionality from 0° to 75°. Interaural time sensitivity is hundreds of microseconds, but maximum external-ear differences are only ±50 μs. When these level differences are converted to azimuth, about half of the sampled neurons are tuned to either first or second harmonics and pointed ±5–7° tightly to the front. Other sampled neurons are tuned only to 1st-harmonic frequencies and pointed from ±15° to ±75° to the side, providing a basis for beamforming by amplitude differences. A beamforming sound camera successfully guides movement along narrow, cluttered paths. (Work supported by ONR.)