Distance. Microphones at different distances receive a signal at different times. When combined, the signals are not phase aligned. This causes comb filtering

Distance. Microphones at different distances receive a signal at different times. When combined, the signals are not phase aligned. This causes comb filtering where the combined waves reinforce and cancel each other. Though that is sometimes desirable (e.g. comb filtering is important for spatial ambience such as reverb). Some audio engineers find careful microphone placement solves many potential problems. Drums are notoriously hard because modern recording techniques use many microphones on a kit.