What makes you think it's not microphone noise? Looks exactly like we are hitting the noise floor to me. Easy to confirm via an FFT anyway. For transducers I've seen which had high higher order distortion at lower levels, the level still didn't quite stay constant. Even if it just went up by 5dB between 70 and 90dB like on the HEDD Type 07 here. Would be fun/interesting to build a flat plate coupler with like 35 PUI 5024HD 10mm microphones arranged in a 5x7 grid (roughly ear-sized) lol. Combined noise floor could be in the 0dBA neighborhood. I'm sure the comb filtering and directivity would be interesting aswell, haha. Nowadays we have transducers with like -100dB THD at 100dB through DSP and acoustic feedback magic, so something crazy like that may even be necessary to gather additional insight in that case. It's something I've had in the back of my head as a project for some time now - might try to build it within the next months.