Have you ever watched a sitcom or a soap opera on TV, and the sound swirled around as two characters passed each other? That's phase cancellation. It happens when two or more microphones are picking up the same source from different distances. On TV it's more apparent and changes because the mics are moving.
Sound consists of waveforms like ripples in a pond. Two separate microphones will pick up the same waveform at different times and points between the peaks and valleys. Sound is slow. It only moves around 1,180 feet per second. Roughly 1 millisecond of delay for every foot of distance. Depending on the type of sound, we start hearing discrete echoes at 15 milliseconds. That's only 15'!
When two identical and delayed signals are blended(mixed)together, some frequencies will be cancelled. Recording musical instruments is similar. The resulting sound is "hollow" or "thin." High frequencies can sound "swishy" or "wishy-washy." If the instrument you're recording sounds "off," try changing the placement of a mic or change mics. Experiment.
By the way, I have Mr. Mackey from South Park stuck in my head. He's saying, "Phase cancellation's bad, mmmmkaayyyy."
Any instance when two or more mics are used, there is bound to be some phase issues. Fortunately, there are some methods to ease the audible effects of phasing. Here are a few guidelines to follow. As I've stated previously, there aren't any hard and fast rules pertaining to music production. However, the following are a couple of rules. Rules are made to be broken. Once you understand the rules, you also understand how to break them to your advantage.
The first rule is fairly simple. Put the microphones as close together as possible. Putting two mics together in an XY or Blumlein configuration requires the capsules to be next to one another. These are also known as coincident pairs. By putting the capsules close together, they receive the sound at approximately the same time. Therefore, very minimal phasing. The drawback to the an XY coincident pair is that the recording can sound fairly sterile. It's a good quality stereo image, but it lacks pizzazz. Other near-coincident pairs (NOS, ORTF) offer similar sounds with a wider stereo image. You can read about those in my post about stereo miking techniques.
The second rule is also quite simple. It's the 3:1 Distance Rule. Simply put, microphones should be at least three times as far apart as they are from the source. Let's take an acoustic guitar for example. If one mic is 6" away from the guitar, the second mic should be 18" away from the first mic. In the case of an orchestra, the mics should be at least 30' apart if they are 10' away from the orchestra.
Close-miking with a directional microphone is also effective in combatting phasing. When a mic is within 6" of an instrument, the bleed from other instruments is low compared to the primary instrument. The drawback is that this can make the instrument sound unnatural. Proximity effect is another side-effect to close-miking. With directional mics, there is a low frequency build-up as it gets closer to the instrument. That's why those smooth jazz radio DJ's sound so rich. They get up close and personal with their mic.
Another method for minimizing phase cancellation is to use a microphone's pickup pattern to your advantage. Let's return to our acoustic guitar example. Instead of recording the guitar in stereo, now we want to capture her singing while she's playing. We'll still use two microphones on two different sources. However, the vocal mic will pick up some guitar and vice versa. There are ways to cut down on the amount of leakage between mics. One method I like to use, in this scenario, is a bipolar(a.k.a. Figure 8)mic. Since this mic rejects very well from the sides, point the side towards her mouth while the front is picking up the spot on the guitar I like. The same holds true for the vocal mic. It can be a cardioid pattern. I'll position the microphone so that its rear is aimed towards the direction of the guitar.
So far, I've only mentioned two microphone situations. What happens when there are more than that? Being a drummer, I believe that getting a good drum sound is the cornerstone to a good recording. Once the drums sound good, the rest falls into place. Did you know that on a typical five-piece drum kit, I'll use as many as 13 mics? I can't always follow all the rules at the same time. Or can I? Let's run down the different mics, shall we?
Two mics on the kick drum. One inside and one outside. The inside is there to capture the sound of the beater against the batter head, while the outside mic captures the boom of the resonant head. Each are close-miking their spots about 18" apart while only 2-4" from their source. So far, so good.
Two mics on the snare. One on top to get the stick hit and tone. One on the bottom for some added "snap." They are 6-8" apart while being 1-2" from their source. Another close miking situation here. The bleed from the kick drum is minimal. And I like to face the rear of the top mic towards the hi-hat to prevent the snare mic from picking up too much hi-hat.
Three mics for the rack and floor toms. Again, this is a close-miking technique. Any bleedthrough of other instruments is minimized by facing the rear towards cymbals.
One mic on the hi-hat. This one is different. Often, you'll see hi-hats miked right up close towards the top hat. I point mine away from the hi-hats and the rest of the drum kit. Sometimes as far as 3' away. The reason is that cymbals resonate outwards from their sides. I want more of the sizzle of the hi-hat and less stick hitting the hi-hat. So, I point the hi-hat mic 1-3' away towards where the sound will be. Try it! Very little bleed from the kit.
Two mics for overheads. These two mics pick up the overall drum kit. Generally, I'll space them apart 6-9' while they are 2-3' above the kit. I've also been known to place them beyond the kit to pickup more cymbals than toms. Same principle as the hi-hat mic here.
Finally, two room mics. Depending on the size of the room, I'll place these mics 10' away from the kit and 30' apart. I've had one 8' off the floor and the other 3" off the floor. If my room isn't that big, another miking rule comes into play.
The Reflective Surface Rule is similar to the 3:1 Distance Rule. In a single microphone application, the mic should be at least twice as far from the nearest reflective surface than it is from the source. For example, a single vocal mic is about 6-12" away from the singer and should be at least 3' from the floor or wall. Sound will reflect off hard surfaces back into the microphone causing phase issues.
In the case of room mics, it's perfectly acceptable to break that rule. The more room sound, the better. In fact, reverse the Reflective Surface Rule and your drums will sound bigger. They should be twice as far from the source as they are from the nearest reflective surface.
One method is not as much phase-related as it is polarity-related. In professional audio, different pieces of equipment are connected with XLR connectors. These are 3-pin connectors. One pin is grounded while the other two carry the audio. One is 180 degrees out of phase - otherwise known as reverse polarity. Not all equipment is wired the same. By that, I mean, some manufacturers design equipment to send the positive signal on pin 2(a.k.a. pin-2-hot)while others send the hot signal out pin 3. For example, Shure SM58's used to be wired pin-3-hot. Ampex and Tascam tape machines were pin-3-hot. Those signals will be out of polarity with equipment that is wired pin-2-hot. Most of today's equipment is pin-2-hot. Back in the 1980's, you had to know which. In any case, try flipping the polarity of one mic and listen to what that does for your sound.
Lastly, when everything is recorded, there is one extra measure that I will go to for my drums. Back in the days of tape, this wasn't possible. With today's digital technology, we can go an extra step. I'm talking about track alignment. Pro Tools does this exceptionally well since I'm able to edit at the sample level. Basically, I will align the various drum tracks to be phase aligned to each other, realizing that there is bound to be some bleeding of the instruments.
Starting with the kick drum mics, I'll zoom into the sample level and move forward the outside kick mic to the inside mic. Then, I'll adjust the overheads to those. The snare, hi-hat and tom mics will be adjusted to the overheads. The room mics are left alone. If done properly, the drums become more open.
That's all the noise I have for this post. I hope it was all in phase(coherent). Ha! Sometimes, I crack myself up!
Rock. Roll. Repeat.