[sigstoat]

Yes, I agree that the microphone has to be responsive to ultrasound. That doesn't seem to be the only requirement, though:

"When these tones arrive together at the microphone’s power amplifier, they are amplified as expected, but also multiplied due to fundamental non-linearities in the system"

"In practice, however, acoustic amplifiers maintain strong linearity only in the audible frequency range; outside this range, the response exhibits non-linearity."

That suggests to me that the nonlinear mixing isn't occurring in the MEMS structure, but rather the amplification stage. Perhaps the authors' language is imprecise?

They do say immediately after the last bit:

"The diaphragm also exhibits similar behavior [non-linearity]."

Is just the diaphragm's nonlinearity sufficient for the effect?

[neetdeth]

Ok, that's actually a lot more interesting than I was giving them credit for. I assumed they were just overwhelming the LPF or exploiting some side lobe to create aliasing in the audible range, but that's discussed in section 2.

All I can contribute is that nonlinearity in audio systems is also known as distortion, and it's impossible to eliminate entirely because every system, whether electrical mechanical or even digital, goes nonlinear when it reaches its amplitude limits. Some more gracefully than others.

Seems like it would be difficult to tease out the relative contribution of the MEMS element and the preamplifier because the preamps are typically implemented on ASICs in the same package. So that might be speculation on the authors' part.