Suppose you're reinforcing a singer/guitarist in mono. There’s one mic on the singer and one on the acoustic guitar.

The vocal sounds funny - sort of hollow or filtered. What's happening? The vocal mic picks up the singer, but so does the guitar mic (See Figure 1). The guitar mic is about one foot farther from the singer than the singer’s mic is. Consequently, the vocal signal in the guitar mic is delayed relative to the vocal signal in the vocal mic. So there are two vocal signals in the mix – direct and delayed.

Figure 1

When you combine a signal with its delayed replica at equal levels, certain frequencies cancel out, depending on the delay. In the frequency response of the combined sounds, there appears a series of notches at frequencies where the sounds cancel. This is called a comb filter effect, because the frequency response looks like the teeth of an inverted comb (See Figure 2, below).

In general, if two mics pick up the same sound source at different distances, and their signals are fed to the same channel, this might cause phase cancellations. These are dips or notches in the frequency response caused by sound waves at certain frequencies combining out of phase. The result is a colored, filtered tone quality. It sounds like mild flanging.

In fact, that's how a flanger works. Using a digital delay that sweeps between 0 and 20 msec, a flanger creates a comb filter whose notches slide up and down the audio spectrum.

The 3 To 1 Rule
To reduce phase cancellations between two mics, follow the 3 to 1 rule: The distance between mics should be at least three times the mic-to-source distance (Again, see Figure 2). For example, if two mics are each 4 inches from their sound sources, the mics should be at least 12 inches apart to prevent phase cancellations.

Figure 2

How was the 3:1 rule determined? It started with the following fact:

When you add a signal to its delayed replica at equal levels, you get severe comb filtering with deep notches. But when you mix direct and delayed signals at different levels, you get less deep notches.

Specifically, if the delayed signal is 9 dB less than the direct signal, the comb-filter notches are only +/- 1 dB, so for all practical purposes they are inaudible.

How do we make sure that the delayed signal, picked up by a distant mic, is at least 9 dB below the direct signal picked up by the closer mic? Put the distant mic at least 3 times farther from the source than the close mic is. Due to the inverse square law, the level drops 9.54 dB when the distance to the source is increased 3 times.

So the 3:1 rule ensures that the level at the distant mic will be down at least 9 dB, so the mixed signals will have comb filtering of +/- 1 dB or less. A ratio of 4:1 or more is even better. The 3:1 ratio is the minimum to avoid audible comb-filter effects.

Suppose the close mic is picking up a loud voice, and the distant mic is picking up a quiet acoustic guitar. You've placed the mics following the 3:1 rule. But you have to turn up the guitar-mic gain a lot because the guitar is so quiet. If so, you might negate the 9 dB separation. That is, the vocal signal in the guitar mic might be less than 9 dB below the vocal signal in the vocal mic, because the guitar-mic's gain is so high.

So there's more to it than just the 3:1 placement. The idea is to get at least 9 dB difference between mic levels for the same instrument. You want at least 9 dB of separation, not exactly 9 dB of separation.

Tips And Techniques
Here are some ways to prevent phase cancellations between mics that are fed to the same channel:

-Mike close, then turn down the excess bass with EQ.
-Spread instruments farther apart.
-Use a pickup on the guitar instead of a mic.
-Delay the vocal mic signal by about 1 msec (millisecond). Then it will align in time with the vocal signal picked up by the guitar mic.
-Use directional mics, and angle the mics away from each other. For example, aim the vocal mic up and aim the guitar mic down. If the close and distant mics are two cardioids aiming opposite directions, the mics can be closer than 3:1 and still get enough separation.
-Use coincident directional mics, aimed up and down, so that the vocal signal arrives at both mics at the same time.

Another tip to prevent phase cancellations: Don't use two mics when one will do the job. For example, use just one mic on a lectern. If you must use two mics mixed to the same channel, place them so their grilles touch, one above the other. That way, there is no delay between their signals, and no comb filtering.

Exceptions
What if two mics pick up the same instrument at different distances and they are NOT mixed to the same channel? You don't get phase cancellations. Instead, you get stereo imaging. The location of the instrument's image between your house speakers depends on the delay between mics, the levels at those mics, and where you panned them.

Suppose one mic is panned hard left and the other is panned hard right. If the delay between mic signals is 0 msec, and the level is the same at both mics, the image will appear in the center between your speakers. If the delay is 0.5 msec, the image will be about halfway off-center. If the delay is 1.5 msec or more, the image will be at one speaker..

You can get phase cancellations even with a single mic in use, if the mic is near a hard reflective surface. We’ll cover this situation next time.


(Bruce Bartlett is a microphone engineer / tech writer for Crown Interational, a recording engineer, and audio journalist.)