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That example is just one frequency from the sound source, so imagine what is going on with all the other frequencies. It is this "phase cancellation" with multiple frequencies at the same time that causes a comb-filtering effect, where there are a series of missing frequencies that create a very unnatural and possibly unpleasant sound. Of course, there will be frequencies that are not cancelled out, but rather amplified because of two identical waveforms at the same point in their cycles being summed together. In other words, both would be at either a dip or a peak at the same time, thereby doubling the power of that frequency. Most frequencies, however, are not completely amplified or cancelled, but are in varying positions within its cycle in relation to each other. There are varying degrees of phase cancellation and amplification going on here, some more noticeable than others. Just remember that anytime the two identical waveforms are not lined up perfectly, there is a certain amount of phase shift going on.
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| Figure 3 - Comb Filter |
| Compliments of © Alexander Magazine |
This now brings us to the 3-1 rule of thumb, the double microphone placement rule followed by engineers all over the world. This rule states the two microphones should be 3 times the distance away from each other as one is from the source. This means that if one microphone is 3 inches from the head of the snare, then the second mic should be 9 inches from the first mic. This is a fairly simple rule, and easy to apply.
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| Figure 4 - 3 to 1 Rule |
| Compliments of © Alexander Magazine |
Having shown you how phase cancellation occurs, you should be able to see how moving one of the microphones can have a drastic impact on the sound of the summed signals. The best thing to do is set up your mics according to the 3-1 rule, then have the drummer play, while you listen to each signal separately. Adjust each one to taste, using whatever compression or EQ you like, and then turn them both on. Repeatedly flip the phase button on either mic preamp and listen to see which summed signal sounds better. The out-of-phase signal will sound unnatural, thin, and lacking low frequencies, while the in-phase signal will regain all it's low frequencies and sound more like the natural instrument.
Phasing problems can also occur with a single microphone when a reflection off a nearby surface gets back to the same mic. Boundary mics can be placed on nearby reflective walls to prevent these phasing problems, since they lay flat and don't allow rear reflections to reach it. Of course, the time the reflection takes to get back to the mic, and its volume will determine the amount of phase cancellation. I say volume because phase cancellation doesn't really occur if the two signals are not closely related in volume. If one is way louder than the other, then the softer one will be drowned out and won't make an impact on the louder one. Many of these other reflections will lose their energy by the time they get back to the boundary mic.
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| Figure 5 - Phase Cancellation Caused By Reflections |
| Compliments of © Alexander Magazine |
Outboard processing and EQ can also cause phase problems because both delay the signal even further. I am not saying you shouldn't use these tools, as they are designed to help you make the signal sound better, but at least be aware of this and possibly compensate by adjusting the position of a mic or adding a short delay to one of the signals (under 10ms should do).
So these are the basics of understanding phase. Like I mentioned before, I used the example of a drum kit (specifically a snare being double miked), but phasing mainly applies to any single source that is miked with more than one microphone with the intention of ultimately blending those sounds together. I would like to think that you all have a deeper respect for this occurrence and will take the time to experiment within your own projects. I guarantee you will be surprised and hopefully better off for what you have learned. Happy Recording!
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