It might be coincidence, but I know several radio amateurs who are also interested in nature, especially in bats. Maybe that is because a „bat detector“ is also a nice little soldering project.
In this article however, I will explain a way of listening to bats without such a detector.

Bat calls have various frequencies, depending on species and purpose. The spectrum ranges from 20 kHz up to 60 kHz (and sometimes even more). Now I’m not a bat expert, but from my experience a good starting point is a frequency of about 40 kHz (we will pick up on this later).
Conventionally, one would use an ultrasonic microphone and a mixer with a VFO to mix the bat calls down into the audible spectrum – basically, this setup is nothing more than a VLF SSB receiver with a microphone instead of an antenna.

I own a „bat detector“ that works exactly this way (using the ultrasonic receiver of a car parking system – they are cheap and highly available, just in case you’re looking for something like that), but earlier this year I also got a Zoom H1 handheld recorder. It’s intended to record music (which it does quite good at, unless it becomes too loud, like with my band…), and originally I used it for exactly that. But this recorder has the nice trait of being able to record uncompressed PCM audio at 96 kHz sampling rate and 24 Bit depth. Following the nyquist theorem, the highest possible audio frequency to be recorded would be 48 kHz – that’s more than enough to include the bat calls around 40 kHz we want to record!
With these thoughts, I was curious to find out whether the H1 was able to record bat calls directly, without the usage of any converter. At first I was quite doubtful about that, because most electret condenser microphone capsules available on the market don’t go far beyond 20 kHz (end of the audible spectrum for most people). Also, I’d expect such a recorder to have some kind of lowpass filter to cut off ultrasonic sound. But it does work, and even better than I imagined it would!

So, how do you practically do it?
At first you need, of course, a place that has bats. In the area where I live, you have good chances of finding bats around slower water streams or lakes with a lot of trees near to them. If you don’t already know where to find bats, you could also contact a local nature conservation group and ask them. Wind noises can be a big problem, so it would be best to wait for an (almost) windless night.
Secondly, you will need some kind of recorder like I described. It doesn’t have to be the Zoom H1, others will probably do the job just as well, as long as they can record uncompressed audio at a sampling rate of 96 kHz (or more). If you own a mobile computer with a soundcard capable of doing that, you might try that as well – just make sure to use condenser mics, dynamic capsules won’t work.
At last, you need some kind of basic audio editing software. I’m using Audacity, which can be downloaded for free.

Place the recorder on some kind of steady surface to avoid noise by your hands – a bench, handrail or bigger stone will do. If you have one, use a tripod. Turn up the recording volume all the way, unless you should get distortion. As recording parameters, select WAVE/PCM (MP3 compression would eliminate that what we want to record) and 96 kHz sampling rate. If you can, choose 24 Bit depth – the more dynamic range, the better. Now hit the button and record for a few minutes (I wouldn’t recommend more than 10 minutes in one go to avoid getting very large files).
Load the recorded file into your editing programm and lower the playback sampling rate – I chose 8 kHz (in Audacity, that is done like in this picture). Extremely important: Do not resample anything, which would mean to re-calculate the audio track with a lower rate! The data must remain intact and just be played back much slower. With 96 kHz -> 8 kHz we have a ratio of 1/12th the original speed. That means that one minute of recorded audio is now twelve minutes long and a tone of 12 kHz is now 1 kHz. So the bat sounds at ultrasonic 40 kHz become conveniantly-listenable 3,3 kHz.
Done? Almost. To improve quality, use an equalizer to cut off the part of the spectrum that was initially audible. 20 kHz divided by 12 is 1,66 kHz, so cutting off everything below 1,5 kHz should be fine. As the final step, normalize that what is left over.

In my case, the result sounds like this: MP3 file (~400 kB)

I was quite impressed not only by how good this works, but also by the echo you can hear – it sounds almost like in some submarine movie (no surprise, because it’s the same technical principle). Also, it’s amazing what sophisticated systems evolution gave those cute little bats.

Have fun trying this yourself! If you have questions or results you want to share, feel free to comment!