You’ll need a calibration curve for the microphone. Even of the same model, there is a lot of variance.

MiniDSP makes some calibration mics that run about 60 bucks. I used them as a cheap instrument for some lab work where I needed a calibrated mic a while back and was very impressed with their performance for the price. They ship with a little code that you can use to retrieve the calibration curve from the factory, and I know a lot of people use them for hifi calibration with REW.

Anyone interested in this area should also know that above ~2 kHz it doesn't matter what you do for magnitude equalization because you'll be dominated by sub mm variations in position and direction. The only way to get any amount of repeatability above 2 kHz is with IEMs.

This is dealt with by smoothing the spectrum in a way to preserves the power density. The constructive and destructive interference then cancel each other out.

What does "smoothing the spectrum" mean? What operations are being performed?

I'd guess: Fourier transform, a power density preserving blur convolution, then inverse Fourier transform.

But I am not familiar with the field of signal processing.

That's my guess too but my hunch is that would sound like absolute crap. Intentional IMD. Something to experiment with.

There’s an example of doing this on the readme- scroll way down

There are cheap calibrated mics available. There's one for about $20 from Dayton Audio.

A microphone that is linear to a dB or so is far cheaper than a speaker that is linear to 6dB and room treatment that maintains that.

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.

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.

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