Detecting The Sound of Bats with a Piezo Speaker and SDRplay RSP1A
Over on YouTube user Jan de Jong has uploaded a few screenshots and sounds on a video which shows that he was able to receive the ultrasonic sound of bats by connecting a small piezo speaker to an SDRplay RSP1A.
The piezo speaker used in reverse as a microphone appears to pickup bat echolocation sound waves which are typically between 20 to 200 kHz. The piezo is resonant in the 40 - 55 kHz range and converts sounds from that range into electric pulses that can be received directly by the RSP1A.
I have a simple question about this project. Would the impedance mismatch between the piezo transducer and the SDR or the extremely high voltage produced by the piezo (such as from vibrations or if one had accidentally bumped or tapped the transducer) pose a threat to the input stages of the SDR? I would assume there would be protection from high voltage from static that an outdoor antenna may be subjected to.
I have been fascinated by ultrasound receivers and have built a few with limited success. I do not want to put an expensive SDR (such as the one designed by M Ossmann) at risk for being damaged from doing an experiment like this.
I did have some thoughts about damaging but curiosity took over. I had to know if it would work. Also since I basically short and there is no amplifier, plus a long cable with at least some resistance, I went ahead.
I did tap the piezo to check if it works at all and that does not seem to damage the SDR. I had to use the RSP1A because the normal RTL usb dongle does not go to the 40-60 KHz range.
I can pickup also cicadas and grashoppers at about 8-10Khz. My inspiration for this goes back to the time where i saw in a biology lab a special tape-recorder that was used for this purpose. It would record with high reel speed the Bat-sound and then be played back on lower speed. With the SDR software i finally had a simple way to do more or less the same, at least visualize this.
I did not have too much time over the summer but have made some more recordings. I will post in the next weeks in my youtube again. (without music as some have requested)
I saw this a few days ago, and suggests using an powered electret microphone:
https://hackaday.com/2018/07/12/dual-sensor-echo-locator-gives-high-accuracy-at-low-cost/#comment-4738335
according to the specifications sheet following:
Maximum Input Power +0dBm Continuous
+10dBm Short Duration
http://wera.cen.uni-hamburg.de/DBM.shtml
0dBM 0.100E-02 223.607 mV 316.180 mV 632.360 mV
so now there is something to measure. I hope to see more persons experimenting with this btw.
Another analog input is an LDR or LED to record the “sounds” that are there in sunlight ore moonlight.