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Want to hear the bat flying in the front yard

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A bat keeps flying almost every night above a certain piece of green yard, right in front of the building, at about the same level with the balcony, so quite close (no more than 10-30 meters, or 30-100 feet in straight line).  The drawback is this is inside the city, so during the night it might be more noise pollution than in the wilderness.

Would like to DIY an ultrasonic receiver, only for the fun of it, so without buying a dedicated ultrasound microphone.  Already having around:

- a few analog electret microphones, mostly from former mobile phones or former headsets
- about 10 of HC-SR04 ultrasonic sensors that can be used as donors for microphones, or even for a directional array
- some piezo discs from singing postcards and toys
- a very old ultrasonic-remote receiver board from an old TV (don't know the model)

Mostly concerned about the available signal to noise ratio (inside a city) and the expected power levels (if there is any hope such microphones would be sensitive enough).  The frequency range is said to be between 20kHz-160kHz, but mostly the signals would be expected to be around 50KHz or so.

Any chances to receive the bat's echolocation signals with microphones like the ones in the list?

I believe electrect microphones are the best.

Piezo discs from greetings cards have a resonant spike at around 30kHz and the HC-SR04 and old remote controls have a peak at 40kHz.

Do you plan to record the sound, or down convert it to the audiable range? Either way you'll need to high pass filter it at 20kHz, or whatever the lowest frequency of interest is. Down conversion can be done the old fashioned way, by heterodynining, or digital signal processing.

From wikipedia it appears to be many types of receivers for bat sounds: https://en.wikipedia.org/wiki/Bat_detector

- heterodyne/superheterodyne (local osc mixed with the Rx signal, so all analog)
- frequency division (amplify to square wave, then use a digital counter/divider to shift down the frequency)
- time expansion (high speed digital sampling and recording then playback the recorder samples but slower, so no real time audio)

Not sure yet how to post process the signal.

At first it would be great only to test if any signal at all can be received, so just the ultrasound mic and an analog amplifier in the 12kHz ... 160 kHz band.

Can test these two in the field, with a DS202 hendheld oscilloscope to see if anything at all can be received.  The DS202 mini oscilloscope has 1MHz analog input band with 1Mohm impedance input, min 20mV/div, max 10MSa/s (also has a signal generator output, 10Hz~1MHz square wave duty adjustable or 10Hz~20Khz Sine/Square/Triangle/Sawtooth wave, might be usable as a local oscillator to improvise a heterodyne but I don't have any connector that fits).

Good point about sensor resonances, thanks!   :-+
(could test for that in the lab at first, to eliminate the worst candidates)

I know you didn't want to buy another microphone, but just to say you can get MEMS mics pretty cheap now, including on carrier PCBs e.g. eBay auction: #114874902126

There are also some homebrew projects already out there:

Rasp Pi http://pibat.afraidofsunlight.co.uk/
Teensy https://www.teensybat.com/

Somebody else PM-ed about a MEMS microphone, too, https://www.digikey.com/en/products/detail/pui-audio-inc/AMM-3742-T-WP-R/14289962 with very good response around 50kHz and above, indeed, but I don't think I have any analog MEMS based mikes.

Might be a few MEMS mikes through the scraped parts boxes, but AFAIK those were with digital output only, and not sure if their sampling rate would be high enough to record at 50kHz or more.  Never tested any of those digital MEMS microphones.  Should look for some analog MEMS microphones, but not sure if I have any.

Tested today some miniature speakers and the piezo discs from singing post cards.  Nothing promising so far.  The speakers doesn't play well at more than a few kHz, and the piezo disks have some strong resonance peak at approx. 7kHz and multiples, but above 30kHz couldn't get any signal with them.   ???

Searching the scrap boxes for electret microphones now.  Would search for any MEMS mikes, too.


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