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