Microphone Techniques: Phasing Issues with Multiple Microphones

The use of two microphones can introduce problems owing to phase discrepancies between the mics—and that potential increases with the number of microphones in concurrent use. In essence, phasing problems occur when a sound reaches different mics at different times. Telltale signs are different notes from the same source sounding at different volumes, or bass response that is either overly strong or excessively shallow.

Here are a few tips in minimizing phasing problems when using multiple mics:

Move the mics. The first line of defense is to just get into the studio with headphones on and move one or more of the problem mics until the phasing issue is resolved.

Check the cables. If a cable is accidentally wired out of phase, it can cancel out the signal from a neighboring mic. Make certain that the mic cables are wired with continuity (i.e. pin 1 on one end goes to pin 1 on the other end, and so forth).
Follow the 3:1 Rule. If possible, microphones should be three times further away from each other than from the source. As an example, microphones placed 5 inches away from a sound source should be at least 15 inches apart from each other. (This does not apply to the coincident stereo miking techniques we’ll discuss in a future issue.)

Minimize the number of microphones in concurrent use. The more open mics you have, the greater the potential for phasing issues. While it might be tempting to put a separate mic on each component of a drum kit, for example, the tradeoff is the amount of time it might take to eliminate phasing complexities. Less can be more in situations where you have difficulty getting phasing under control.

Separate the sound sources. With the exception of stereo recording, the general idea behind using multiple microphones is to isolate the sound sources. Phasing issues provide another reason to isolate the sources. Solutions include simply spreading the mics apart, putting them in separate rooms or isolation areas, or using baffles, gobos and the like to provide additional separation. In the case of two mics on the same instrument, it is sometimes beneficial to devise a baffle that goes between the mics.

Minimize reflective surfaces. Hard surfaces like wood floors, smooth walls, windows and mirrors are common culprits in phase issues because they reflect sound back into the microphone. If your recording does not sound right, try moving the performer and/or mic. Also experiment with damping those reflections with blankets, towels, baffles and the like.

Avoid boxing mics in. Microphones typically need a little breathing room in order to avoid reflection. Omni pattern mics placed in a corner, for example, sound like they’re, well, in a corner. Similarly, placing the back of a cardioid too close to a surface or corner can sonically block the rear ports, thereby distorting the effective polar pattern of the mic. Also, exercise care when using baffles and gobos because these mechanisms do not completely absorb sound and can actually cause reflections when too close in proximity to the mic.