Phase Cancellation

Sound takes time to travel, so a second mic further from the speaker receives the same waveform a fraction later. Summed, some frequencies reinforce and others cancel. The result is a thin, hollow, filtered tone with no obvious fault you can point at. The same thing happens between a mic and a hard reflective surface beside it.

Two habits keep you out of trouble. Follow the 3 to 1 guideline: any second mic sits at least three times as far from the source as the first one is. Check the mix in mono, because the problem hides in stereo and appears the moment a phone speaker sums it. If two tracks are already recorded out of phase, nudging one by a few milliseconds usually fixes it.