Bat Music - Harmonics and horseshoe bats

The really cool thing about horseshoe bats is their use of harmonics. Using their nose leaf they can filter down a note to focus in on a high frequency (around about 80 - 110khz) pure tone (well, nearly pure tone). To understand that you need to understand the fundamental frequency and harmonics.

Most notes you hear emitted by anything are what are termed "compound notes". This means the note is made up of loads of frequencies.

The part of the compound note we "hear" most prominently is the deepest and that is called the "fundamental". Over this are a series of overtones. Where these fall in a nice mathematical pattern of doubling themselves in integers they confer a pleasant sound that makes sense to our ears and we call them harmonics. This is what instruments do.

Where those overtones are more random and fall in an uneven sequence, it doesn't make a pleasant sound. Think of your car. You can definitely perceive a "note" the engine runs at.... you can hear the fundamental.... but its a messy NOISE rather than an organised NOTE.

What's really cool is that every emitter of sound highlights and cuts different overtones and this is what creates "timbre". This is why your voice sounds different to mine.... why an oboe sounds different to a guitar.... a car sounds different to a chainsaw.

Some things can cut out all those overtones and just produce the fundamental. Or focus in on a harmonic. Tuning forks and singing bowls are good examples. When you play harmonics on guitar you are highlighting one overtone and shaving away all the ones underneath it.

Bats in general are masters at using harmonics. It helps them to increase bandwidth and get a more detailed picture of the world. Sometimes you'll see an entire harmonic series filling up your sonogram when the pipistrelles are really going for it.

Most bats seem to be altering their delivery to increase the overtones they can produce. I'm not sure if any bats attempt to isolate harmonics like horseshoe bats do. It's not a perfect system... they put all their energy into the first or second harmonic but there is still a bit of the fundamental there although it's weak. They haven't quite mastered tuning fork!

The reason for all this sophistication is because they specialise in fluttering prey in highly cluttered environments. Most bats have to compromise between range and detail (lower frequencies punch further through the air) but range is pretty pointless for the clutter specialist. That allows these guys to really focus in on using the high detail, high frequency approach.

There are a lot of other interesting functions to their very unique form of echolocation, like picking up the doppler effect, but that's enough info for one day.

Have a listen to their cool, warbley alien call!

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