The nulls are set by distance, the depth by level.

When one sound arrives twice, separated by a path difference d, the two copies interfere. The first null is c ÷ 2d and every other null is an odd multiple of it — and that depends on d alone. How deep the nulls go depends on the level difference alone. Two independent axes, and neither term appears in the other formula.

The engine is free and needs no account. Two microphones 35 cm apart in path length put nulls at 491 Hz, 1.47, 2.45 and 3.43 kHz — four of them inside the speech band. Space them at the 3:1 the rule asks for and that is 9.54 dB of separation, which leaves a 6.02 dB comb. Holding it to one decibel needs 24.81 dB — a 17.4:1 distance ratio, not 3:1, and two microphones on one talker essentially never achieve it. At equal level the nulls are not deep but total: the signal is gone, which is the ordinary case for a reflection off a hard table. And equalisation undoes none of it — a boost at a null amplifies the noise floor, because the signal that cancelled is not there. What does work is geometry: under 4.3 cm of path difference the first null clears the speech band entirely, which is why a boundary microphone is pressed to the surface rather than raised off it. Every lane ships a worked example that costs nothing. Writing the report needs credits.