[squarefoot]

The interesting parts are those 25KHz transducers which seem identical to the 40KHz used since likely forever in ultrasound remotes and more recently in collision avoidance sensors for robotics. I did a small search and found mostly high powered ones at that frequency, probably ultrasound cleaners spares, or smaller but a lot more expensive transducers compared to 40 KHz ones. Does anyone know of a source for these transducers?

I also wonder if a simpler approach could be used since the purpose appears to be (can't understand the math) generating noise by driving randomly a number of oscillators around the transducers resonance frequency then induce subharmonic vibrations into the MEMS mics through etherodyne operations between these sounds. If that's how it works, then the DDS chips, the Arduino and the code might be swapped with a less random but likely equally functional set of dissonating oscillators modulated by LFOs (all doable with plain old logic gates); not unlike the old school way of generating cymbals metallic sound in analog drum machines. Here's the Boss DR110 relevant schematic as an example.

http://www.sdiy.org/richardc64/new_drums/dr110/dr110a1.html