Learning from a fly

A top-side view in a microscope photograph of the biologically inspired microphone. The tiny structure rotates and flaps about the pivots (labeled), producing a voltage across the electrodes (labeled). Credit: N. Hall/UT Austin

Prosternum of Ormia with head removed, showing external ear anatomy. In spite of having "ears" just 2mm apart the fly can work our the direction of the incoming sound. Picture credit - Hoy lab

It is not the first time that I am posting a news on scientists looking at how Nature has solved some problem and then trying to replicate the solution in a product.

Here the problem is how to detect the direction of a sound. We've got two ears and they are conveniently separated by our head that sitting in between them create a spatial resolution that the brain uses to work out the direction of incoming sound.  Sound is made by waves and the waves detected by our ears are more or less displaced because of the distance between them and the displacement depends on the direction of the sound.  If the sound is right in front of us it will reach our ears at the same time, if on the other hand the sound comes at 3 o'clock the 90° difference results in a delay of the sound reaching our left ear with respect to the one reaching our right ear (the delay, for those of you that are really curious, is 0,58 ms, tiny but sufficient to tell the brain where the sound is coming from).

It would be nice in some cases to have a sensor able to tell the direction of the sound but if the sensor is small the delay may become negligible...

Hence the scientists have studied a fly, the Ormia ochracea, that is very good at detecting the sound of crickets, since on that depends its success in reproduction (the fly lays an egg on the cricket that will grow into a larvae using the cricket as meal...). The problem for insects is that they are too small and there is no sufficient separation to use the delay to detect the direction of sound. Indeed, insects do not use sounds to orient themselves, but the Ormia fly is an exception. And yet, the ears of the fly (that are sitting on the pro sternum -the torax), are separated by less than 2 mm, which results in a delay in sounds arriving from 3o'clock of about 5 millionths of a second which is clearly too little for the processing speed of a brain. And yet the fly can detect the origin of sound with very good accuracy. 

The scientists have discovered that the fly has evolved an amazingly precise mechanism based on measuring the phase difference in incoming waves. Even though the delay is less than 5 millionths of a second, in that little time shift there is a phase change in sound that ken be detected by the special structure of the fly ears.

That structure resembles a tiny teeter-totter seesaw about 1.5mm in length. Teeter-totter vibrate such that opposing ends have a 180 degree phase difference and even a tiny phase difference in incoming sound waves leads to a pressure that is different at the two ends of the teeter-totter and this creates a proportional mechanical displacement that is amplifying the signal and let the fly work out the direction of the sound pretty well.

Having discovered this mechanisms, researchers at the University of Texas, Austin, have created a tiny sensors, 2mm in size, that can detect the direction of sound. The created a teeter-totter in silicon and embedded a piezoelectric material that generates an electrical current based on the bending and rotation of the teeter-totter that in turns is reflecting the wave phase change at a distance of 2 mm.

Quite impressive, I would say, but electronics evolution has got us used to this kind of results. What I really like is to see scientists mimicking Nature!

Author - Roberto Saracco

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