[O__________O]

Not an audiophile, but one way might be tuning forks. That said, I would be super surprised if this was needed for high-end microphones.

[amluto]

A microphone’s ability to reliably identify a frequency is excellent, even if the microphone is cheap, crappy and uncalibrated. It’s almost entirely a function of whatever clock is used to digitize it, and oscillator chips that are just fine are ubiquitous.

The issue is calibrating the amplitude response at a given frequency, and a tuning fork won’t help.

edit: those quartz oscillator chips have a lot in common with tuning forks.

[brokenmachine]

Now I'm imagining using thousands of tuning forks with calibrated knockers.