Don Romano (alt)Knowles SPH0641 MEMS digital microphone. That's a very good frequency response for the human hearing range. You could use this as a calibration microphone. I reckon the bump at 25 kHz is the resonance frequency of the diaphragm. That's probably a compromise. Too high, and the diaphragm would be too stiff, too low and it would be audible. It's also a decent compromise for the ultrasound range, since it leaves the 36 - 70 kHz flat to within +/- 1 dB.
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