Horseshoe bats’ unique noses and big, flexible ears make them nature’s most dynamic sonar arrays. Engineers built a mechanical version and they’re testing it on a quadcopter drone.
The sonar prototype, designed and built by Virginia Tech mechanical engineering professor Rolf Mueller and his team, doesn’t look like anything you’d find on a warship. It has flexible rubber bat ears and a nose based on the horseshoe bat, a species known for the horseshoe-shaped flaps of skin on its nose.
Those skin flaps are properly known as “noseleaves,” and they help the bat adjust the frequency of its echolocation. Horseshoe bats often hunt in dense thickets, so they have evolved a way to separate the fluttering wings of a moth from the background clutter of branches and even other bats. When they emit sonar pulses through their noses, the noseleaves act as a megaphone to amplify the ultrasound, but they also change their shape to help adjust the pulses.
The frequency of a sonar pulse changes when it reflects off an object, like a tree, a moth, or a bat. Because of their noseleaves, horseshoe bats can tweak the frequency of their sonar pulses so precisely that they can pick up the tiny changes in reflection frequency caused by the flapping of a moth’s wings. Current naval sonar systems aren’t nearly that good at reading fine details in cluttered environments, such as a seafloor littered with obstructions.
Horseshoe bats can also adjust their outgoing pulses to compensate for their own motion in flight, to make sure the reflected frequency will fall into the range they can detect most clearly. Mueller and his team are especially interested in that ability, since sonar arrays mounted on warships are usually moving relative to their targets. He’s testing it by mounting his prototype on a small drone, effectively creating the world’s first working quadcopter bat.
Horseshoe bats’ noseleaves make them what sonar experts call “dynamic emitters.” They’re also dynamic receivers. In the time it takes you to blink, a horseshoe bat can change the shape and angle of its ears about three times. This flexibility helps them filter their incoming sonar echoes to adjust for motion and background clutter. The bats have a sonar array of just two receivers, but they can “see” in much more detail than modern naval sonar arrays with hundreds of receivers.
Mueller, with funding from Naval Sea Systems Command, trained high-tech surveillance on a colony of about 30 horseshoe bats in order to steal the secret of their sonar. Using high-speed video, ultrasonic microphones, and laser Doppler, Mueller and his team studied the motion of the bats’ ears and noseleaves.
Based on that data, they built a digital 3D model of the bats’ noses and ears, then turned that model into a real-world prototype. Four motors control the rubber noseleaves and ears, and Mueller says they move almost as quickly as the real thing.
Image: F.C. Robiller via Wikimedia Commons