Comb Filtering

The interference pattern that appears when two copies of the same signal meet with a time offset: a series of evenly spaced peaks and nulls, like the teeth of a comb. At 1 ms of delay the first null falls at 500 Hz and repeats every 1 kHz. Hence the 3:1 rule: place the second microphone at least 3× the distance separating the first from the source.