Bats communicate and orientate themselves outside of our hearing range. With a MEMS microphone you can quickly track down the night owls.

As I sat on the terrace on long and warm summer evenings, I noticed that there was a lot of bat flying in our garden at dusk. This is probably the best hunting season to catch the many insects that local bats feed on. Since I’m not a biologist, I first had to refresh my long-faded school knowledge on the web to learn that there are more than thirty species of bats in Central Europe, many of which are unfortunately threatened with extinction.

The species can usually be identified by their ultrasound calls. Here the frequencies differ, but also the envelope curves of the calls – i.e. the speed at which the call comes on or off. Bats generate the ultrasonic signals to recognize prey and obstacles and to communicate with each other. The frequencies for communication and navigation differ significantly. The frequency of the signals is species-specific between kHz and more than 120 kHz – we humans don’t hear anything any more there.

Therefore there are a lot of devices available for sale, including kits, which the sounds in the human hearing range of 100 Hz to approx. 16 Shift kHz. A method similar to that used for radio receivers is used here: a so-called mixer. If a signal of frequency f1 is multiplied by a sinusoidal signal of frequency f2, then two images of the signal are created at frequency points f1-f2 and f2 + f2. So you can e.g. a 48 kHz signal by multiplication with a 40 Shift kHz sine to 8kHz and thereby make it audible. Of course, not just one frequency point shifts, but the entire spectrum of the signal. So you get a real hearing impression.

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