And thanks Kris for the tip on the MEMS microphones - they close close to what I'd want, although I'm now searching for an analog equivalent of those components.

WT

There are a few, maybe Knowles has one, but they won't be as sensitive as PDM.

Hi Simon - Yes, I saw your replies (and thanks! The links are useful).  I am not yet very familiar with how this site works, so I'm not sure why my question isn't visible....

I did discover that my iPhone with the Røde iXY microphone can pick up frequencies right up to 20 KHz - still not in the ultrasound range but higher than I can hear.  More work required!

@WTPfeffer if you start typing @simo you should see my profile pop up. Click that and it should "mention" me in brown/gold font. Then I should get a notification. Are you willing to try a microcontroller? I think it might be a good way to ensure your sampling hardware can definitely detect 20+ kHz. I'm not sure a PC audio card is designed for ultrasonic (but if you check the specs on yours it may prove me wrong).

Whatever hardware you use, I'd suggest looking at a fast Fourier transform (FFT) algorithm in your software to identify the relative amplitudes in different frequency bands. I don't think you will be able to do this by analysing the raw waveform visually, on an oscilloscope (unless it has frequency analysis functions built-in). 

Happy to keep discussing but I might reply faster if you @mention me. 

Maybe take a look at the TDK ICS41350...or ICS41352

I never ignore a sensor suggestion by @Kris Winer ! Thanks. 

ST analog mic with coverage of the ultra-sound range https://www.st.com/en/mems-and-sensors/imp23absu.html?ecmp=tt19114_gl_enews_dec2020&cid=stmDM37411&bid=370850364&uid=dj/0LdRBZr+DiZm6hzAUjTyIYVqoBFNe...

Hi, did you remove your .stack question about the ultrasonic mic? I put some links in reply but can't see the listing when I go to the stack. Let me know if you can see them.