Shine a light in a gecko's ear, and you'll see light coming out the other side. This quirk of biology may one day give us robots that can amplify and locate sounds without requiring any energy to do so. Researchers at the Technical University of Munich have solved a longstanding bio-physical question. The problem was, "How do geckos, birds and other small animals hear?" The researchers just published the answer in Physical Review Letters, and they believe that it can one day help us build less energy-dependent robots.
Mammals locate the source of a sound by judging the delay between a sound reaching one ear and then reaching the other. This is possible when the ears are relatively far apart, but gets increasingly difficult as the head gets smaller and the ears get closer together. Small animals like lizards and frogs have a tunnel that connects their ear drums — the membrane-covered organs that vibrate in response to sound waves. This tunnel creates an internal sound wave, different from the external waves coming into the ear. A popping balloon would cause the external sound wave; the pop's echo through the tunnel would create the internal sound wave. The animal hears both of the waves at once, sensing a superposition of the two.
The difference in those waves should be fairly small, but the tunnel amplifies it. The comparison can work two different ways. If the external sound is at a lower frequency than the fundamental frequency of the ear drum, the tunnel increases the time difference between the external and internal waves. If the external frequency is higher than the fundamental frequency of the ear drum, the tunnel increases the difference in amplitude between the internal and external waves — in other words, it in increases the difference in the loudness of the sound the ears hear.
These internally coupled ears are interesting, in terms of engineering, because they are purely structural. "The amplitude difference occurs solely through the coupling of the two ears," Leo van Hemmen, the leader of the team studying the structure and one of the authors of the paper, explained in a statement. "I can very well imagine applications in robotics, because this kind of amplification doesn't need energy." Robots that need to respond to audio signals, but can't use a lot of power, can be given these internal tunnels between a couple of "ear drum" membranes, and locate sounds the same way lizards do, without needing anything to power its "ears".
Image: Prof Dr Frieder Mugele, University of Twente