Brian Gaff wrote:

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OK then, I was under the impression that directivity was due to phase of
the wavefronts on both sides of a diaphragm.


** Correct.

A cardioid mic has openings that make the amplitude and phase of a sound the same on both sides of the diaphragm when arriving from the rear - resulting in no output. If the sound source moves to one side, a significant imbalance rises and the diaphragm moves accordingly.

A figure 8 mic has a null for sounds arriving at right angles to the diaphragm for the same reason.


...... Phil