Don Romano (alt)Bode plots and polar patterns for simulation of 2-element array of omnidirectional microphones, with their outputs subtracted and delayed to create a single cardioid microphone, with a low-shelf filter added to linearise the frequency response.
It looks like you want to keep them really close together to move the comb filter out of the audible range. The drawback of doing this is the need for even more amplification on the low end.
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