Do you live in an area with bats? We do, but they’re so fast that they’re very hard to spot when they’re scudding about after insects at dusk; and, of course, human ears are not equipped to hear the ultrasonic tones that they use to make their echolocation calls, so we can’t hear them either.
A bat primer. Bats are not, as folk legend has it, blind: they can see as well as you or I can. But like you or I, they can’t see in the dark, so they use a rather brilliant system of echolocation to “see” where buildings and the insects they predate on are at night. They make a series of extremely high-frequency calls, and use their big ears to judge how far objects are from them by the amount of time it takes those calls to bounce back to them, which allows them to locate prey and avoid obstacles with great accuracy.
Your human hearing, depending on how old you are (we lose the top frequencies as we age) will range from about 20Hz (cycles per second) to 15-20 kHz (1000Hz). The sounds bats can hear and produce go all the way up to about 110 kHz. Their calls (which are pretty loud, so perhaps it’s a good thing we can’t hear them) aren’t just one tone: depending on breed, they sweep down from a high frequency to a low one; or move the tone around and around a specific frequency.
Over in Germany, Holger and Henrike Körber have turned a Raspberry Pi into a bat detection device. An inexpensive high-sensitivity microphone capable of detecting high frequencies, and some batty software, mean users can make graphical interpretations of bat calls; create histories of bat activity; manipulate those calls to bring them into frequencies they can hear; and identify bat species by call using an algorithmic process.
The Körbers also make available a list of bat literature, so you can deep-dive into echolocation and acoustic identification of bat species.
Head over to FledermausSchutz.de (that’s Bat Conservation in English) to find out what you need and how to get started. If you don’t speak German, you’ll need to run the page through Google Translate; it’s worth doing.