Bat detectors offer you a way into the hidden world of high frequency bat calls.
Different bat species
at different frequencies and their calls are usually a mixture of very high and lower sound waves. The job of the bat detector is to turn these ultrasonic waves into sounds that people can hear by slowing down their frequency.
Have a listen to this Lesser Horseshoe bat's calls (by Keoka)
The human hearing range is between 20 Hz and 20 KHz so we'd have no chance of hearing these calls without a bat detector. (In this case, it was set to 107 KHz!)
The three main types have names that sound like they're straight out of a sci fi movie. Here's some information on how they work:
This is the type that I've used and I'll never forget the first time I got to hear some common pipistrelles echolocating.
It was even possible to tell when they'd caught an insect by the clicks getting faster and faster and then hearing a sound like someone had blown a raspberry.
Heterodyne detectors receive incoming bat calls and then combine them with an internally produced high frequency sound wave. Through some technical wizardry, these two frequencies are then added to and subtracted from each other.
You won't be able to hear the sum because the frequency will be too high. The audible clicks, clonks or warbles are the difference between the sound waves.
TIME EXPANSION DETECTORS
It's a pity these don't actually stretch time. I could think of a few situations in which that would come in pretty handy. What they do is slow down the frequency of the bat calls that they receive to a speed that makes them audible to humans.
And the way they do this is by recording the inaudible bat sounds and then playing them back at a slower speed.
This is one of the big advantages of this type of detector as the recorded sounds can be loaded onto a computer and analysed. They're fairly expensive though, especially when they're combined with a heterodyne component.
With this type of detector, the received high frequency bat calls are divided by 10. So common pipistrelles which echolocate the loudest at 45 kHz would have their calls reduced to 4.5 kHz, which is well within the human hearing range.
Bat detectors can add a whole new dimension to bat watching. There are many different types to choose from so if you're thinking of buying one, let your level of interest, skill and budget guide you in choosing the best type for your purposes.
And if you're technically minded, maybe you'd like to try making your own.
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