As a sound engineer or as a home recording artist you will sooner, or in the most part later, come across Phase Cancellation. Most likely you have made music or recorded sound without knowing about it, like myself.
So what is it?
Phase Cancellation is when the same sound origin appears in two or more microphones with a slight time delay causing sound-waves to appear inverted in relation to each other, effectively cancelling out each other. If you want to test it yourself, simply add two identical sound-waves in two different channels and invert one of them. Now even if you play them at max volume, you won´t hear a thing. They cancel each other out.
When miking a drum kit in studio you will most likely use more than one microphone like two overheads, two mikes on the snare, or two room mikes. Chances are that when you hear all the microphones together for the first time, the sound will be dull and lacking low and mid tones.
In order to try to fix the problem and improve the sound you need to check the phase. There are more than one way to do this. At the mixing console you can find a button labeled "Ø". This button inverts the sound wave or rather it switches the polarity of the electrical signal making it invert in relation to the others. If you have two overhead microphones, try "Ø" on one of the mikes and hear the difference. You are looking for, or rather hearing for the option that has the most low and mid tones.
When checking the whole kit start with the overheads and then check them against the kick drum. Don´t flip the "Ø" on the kick drum as you will want the speakers to move outwards with every kick. Flipping the kick drum will cause the speakers to move inwards with every kick. So flip both overheads when checking phase relation with kick. Then silence the overheads and check phase with kick in relation to snare and then to the rack and floor toms.
Phase cancellation is especially tricky when recording an artist that wants to play guitar at the same time. Try to record two separate tracks with microphones in such positions that they record as little as possible of the same source. One would be focused on the voice while the other on the guitar.
You can invert the phase within the DAW (Digital Audio Workstation) if you do not have the "Ø" button on your mixer. There are plugins for most DAWs that does it for you.
Phase cancellation can really ruin good sound, and when you use more than one microphone it is really hard to fix. In fact it can never truly be fixed and you will have to live with some cancellation.