Hello recordists! I am a freak about phase. I see it as one of the most important recording concepts that is often overlooked when recording at home. I feel passionately about this topic, and yet I know that it is a complicated one to discuss. So here I'll try my best to put things in simple terms. Today I’m going to be laying out 3 simple ways to improve the phase relationships in your recordings. They are as follows:
Measure the distance between the source and mics
Use the 3 to 1 rule
Use the polarity switch
Phase relationships are created when you’re recording the same source with more than one microphone. It refers to the difference in time that the sound hits those microphones. In another post I’ll describe the science of this and why we should care, but for the sake of keeping this post more about the practice of improving phase and less about the theory, I’ll just say that when the sound hits the mics at different times in its wave cycle, frequencies can “cancel out”. When this happens with sound the sonic result is that the audio sounds thin and loses much of its clarity.
Anytime you record a source with more than one mic, it is wise to check to see how the phase relationship is. The good news is that in the recording stage, you can always solve phase problems by either moving the mic, or flipping the polarity of one of the microphones.
Before we start, I’d like to give you a couple of definitions that will help me explain some of the concepts. Keep these in your brain-toolbox because they’ll come up later when we discuss phase in further detail.
Amplitude is the fluctuation of atmospheric pressure. When sound is produced, it displaces/pushes around air molecules that sort of bump into each other and then fall away from each other. So when the air molecules are closest together (compression), the amplitude is positive and when they are furthest from each other (rarefaction) the amplitude is negative. When they’re just an average distance from each other, they’re neutral.
Polarity refers to the position of positive and negative amplitudes of a sound wave
Okay, sorta got the idea? These definitions are don't necessarily show us the whole picture, so don't get hung up on it if it seems confusing.
Here are the three methods in more detail so you can ensure that your phase relationships are on the right track:
Measure the distance
If phase refers to a difference in time that the sound hits the mics, then theoretically, if the distance from the source to each mic is the same, the sound will hit them at the same time and they’ll all be in phase. Turns out, this works in practice as well (so long as the musician or source isn’t moving). You can use a tape measure, or you can just use a cable. It doesn’t necessarily matter what the exact distance is, just as long as both mics are that distance away.
Use the Three to One Rule
The three to one rules states that if you’re using two mics to record a source (one close mic and one far mic) the far mic should be triple the distance of the close one. While this doesn’t necessarily help the phase relationship, the level difference is so significant that it minimizes the harmful effects of phase cancellation. Phase cancellation is the most intense when both microphones are receiving the similar signal levels. So, when you do this, be sure to keep the fader of the further mic lower than the close mic, otherwise the three to one rule is obsolete.
Flip the Polarity
Rather than phase which affects the sound because of a delayed time difference, polarity flips the sound’s cycle 180 degrees (it takes the positive amplitude and makes it negative and takes the negative amplitude and makes it positive). There’s no delay in the timing, just in the relationship between or among the waveforms. What this means sonically is that if two signals were perfectly in phase, flipping the polarity will make them perfectly out of phase and vice versa. Flipping the polarity is useful for checking to see if something is in phase. If it is, you’ll hear all of the body leave when you flip the polarity. If it is closer to being out of phase than in phase, you’ll hear a bunch of life and low end appear.
If you want to learn more about the specific scientific details of phase let me know. Otherwise, experiment with it and have fun! For some audio examples of what it sounds like to flip the polarity on a mic, check out the video: