In order to understand phase cancellation, we first should understand phase, right? Right. And in order to understand phase, we should first understand sound waves.
Sound, like ocean water, light, or radio, travels in waves. Phase cancellation occurs when two waves collide and compete with each other. This is often perceived in the recording process when multiple mics are used to record a single audio source. Here’s what’s going on: as sound waves travel from the instrument being recorded, they are arriving at these multiple microphones and going to the mixing board at distinctly different points in the wave’s cycle, as the sound doesn’t strike the 2 mics at the same precise time. This leads to frequencies in the instrument’s timbre being cancelled out. Home recording studio gear nerds have all kinds of crafty terms for the artifacts created by phase cancellation—recordings can sound “hollow,” “boxy,” “muddy,” “vacant,” “empty,” “dead,” etc.
How might a savvy recordist avoid this? The easiest answer, of course, is to use a single microphone, but one has to be wary of having it perpendicular to a wall—sound waves will simply bounce off the wall, go back into the mic, and then you’ve got phase trouble again. Plus, with many instruments, one generally wants a little bit of natural room reverb picked up by a second mic. This is often true when recording guitar amps or kick drums; it also occurs when using DI boxes in conjuntion with microphones on a bass guitar setup.
So: After that lengthy intro, how do we avoid phase cancellation? With a little trick called the “3 to 1” rule. More info on the “3 to 1″ rule can be found over at HomeStudioCorner.com.
It’s really easy to understand: Your second mic should be distanced from the first mic by a factor of 3x the distance of the first mic to the instrument. For example: if your guitar amp is miked from 1 foot away from the speaker cone, the “room” mic should be 3 feet from the first mic. If your primary kick drum mic is 6 inches from the kick drum, your second mic should be 18 inches from the primary mic. Observing the 3 to 1 ratio generally keeps phase issues to a minimum. However, with all recording “secrets,” the only golden rule is to experiment and do what sounds best to you!
If you find phase cancellation issues on a recording, and don’t have the luxury of re-tracking, you may be able to fix it manually in your DAW or computer recording equipment of choice. Zoom way into the recorded tracks, and slide your “room mic” track to the left or right until the peaks line up with the “close mic” track. Phase problem eliminated. Phase issues become MUCH more obvious when you listen to your tracks in mono.
Side note: as a deliberate effect, a Phaser effects pedal introduces cyclic phase cancellation on purpose—its characteristic “whoosh” works on the same cancellation principle described above.
