Like all good development on the cheap,   I have re-purposed a mic design from another acoustics project.   The mics shown are Knowles Acoustic SPM404H.   While they are tested to 10kHZ in the datasheet,   just about all of the mems products go waay into the ultra sonic.   Using a piezo transducer, I have verified there is good response out to 80KHz.  Now... I don't have a good calibrated source so my crude setup (which has peaks and valleys in the spectrum owm its own) tells me I am in the ball park.   The will server the purpose for the time being.   Schematics will be in Github at some point.....   I wish I didn't have to sleep....