For the prototype I will be using a MEMS type microphone (Knowles Acoustics).   Virtually all of the mems mics will respond into the ultrasonic bands.    I am now characterizing the current design at >20KHz.

The challenge with high frequency sensors is diffraction.   The wavelength is comparable (or much smaller) that the characteristic dimension of the sensor and can yield "lumpy" response parameters.     In fact, most of the data provided by the vendors (i.e. Knowles) shows this effect!   I am currently building the prototype mic with the parts I have on hand.    This will be a baseline to get started.    We can improve the mic down the road...