I am also interested in bats. One idea I have been wondering about was using a SDR- Software Defined Radio receiver with attenuation at the antenna. Some models go down to 40KHz. They are not cheap, but the software will display the whole spectrum of what is coming in, and that is much better than ears. I assume it would be easy to tune it to hear the bats once you see the band. With any simple heterodyne circuit you might hear one species and miss another.
Also, frequency response of microphones is often not tested or listed past 20KHz. It my fall off, or it may not be flat at higher frequencies, yet there may be a nice signal. Some piezo tweeters are listed as reaching 40KHz. I have used a Zoom H4 digital recorder to hear bats after slowing down the recording to 1/10 speed.
Also, frequency response of microphones is often not tested or listed past 20KHz. It my fall off, or it may not be flat at higher frequencies, yet there may be a nice signal. Some piezo tweeters are listed as reaching 40KHz. I have used a Zoom H4 digital recorder to hear bats after slowing down the recording to 1/10 speed.