Diagram 1.
Shure SM-58 Cardoid Pattern
PHASE CANCELLATION
I'm sure we have all experienced this but probably didn't know what was going on.
Microphones have patterns of sensitivity. Some are cardoid, hypercardoid, omnidirectional, bidirectional or Supercardoid.
Most of the mics used in worship performance are cardoid. The Shure SM-58, an industry workhorse, is widely the mic of choice. Most Shure wireless mics come with a SM-58 capsule.
When two or more microphones are placed in close proximity to one another you will start to experience phase cancellation. Have you every had a podium mic on at the same time as the speakers lavalier or headset? The swishing sound you hear when they move about is a great example of phase cancellation. If you look at Diagram 1 you will notice the simplest form of phase cancellation. Sound is represented as a sin wave. If you have two sin waves that oppose each other you will experience cancellation. With multiple mics you can eliminate most of these problems with the POLARITY switch on those channels of your console. If you have two singers 4 ft apart and both are using a hand held cardoid mic, simply reverse the polarity of one of them. You will see better results.
So what if you have one mic and you are fighting cancellation.
Using a single mic around hard reflective surfaces can cause the original sound to be reintroduced to the mic capsule several times, depending on the environment. Mic placement becomes critical at this point.
As you can see in this diagram. The source of sound is initially introduced to the mic capsule as "direct sound" or "1st source". The reflection or indirect sound is slightly delayed due to the longer distance it travels off of a reflective surface. This creates cancellation if the distance traveled by the reflective sound is a multiple of the source distance. For instance, if your mic is 12 inches from your source and a reflection causes the indirect sound to travel 18" before being captured.
Major causes of this type of cancellation are: Plexiglass drum shields or amp shields used to reduce sound levels, hard (reflective) stage , ceiling or wall surfaces.