Have you ever experienced a case of microphones squealing feedback before they are loud enough in your PA mix?
Even with great mics and a wise layout, sometimes you could still use more gain-before-feedback (GBF). We’ll take choir miking as our example for discussion. We usually mic choirs with several cardioid condenser microphones, right?
The feedback that occurs here is caused by the microphone(s) hearing itself being amplified in the PA system. The microphone picks up the source in front of it (good) and also picks itself up coming out of the PA (bad).
So, since we wish to have the mics “hear” less of themselves from the loudspeakers, several ideas seem fairly straightforward for increasing GBF:
Turn it down. This is the simplest way to stop a mic from ringing feedback. It’s not real practical if the intent is to hear more of the choir in our PA.
Use fewer microphones. The number of open microphones (NOM) should be as low as possible. In our application of choir miking, use the minimum number of cardioid condensers needed to cover the choir. This sounds better anyway and improves GBF.
Use directional mics aimed at the choir (and away from the loudspeakers). Omni-directional mics can sound great, but in many sound reinforcement applications they may not be practical, as they hear themselves in the PA more easily. Using directional microphones pointed at the choir with their nulls (least sensitive side) facing the PA effectively increases GBF.
Move the mics closer to the choir (and farther away from the loudspeakers). Less gain is now needed on these microphones. And the less gain that is applied to them, the more GBF margin is left.
Use directional loudspeakers, placed and aimed away from the mics. This system design function is not the job of the system operator. A qualified, independent consultant or design-build contractor should be contracted.
Use parametric equalization. OK, so your choir and mics are behind the loudspeakers, you’re using a few good quality, directional condensers, and you understand how to place and aim them—but you still need to turn the choir mics louder in the PA without ringing.
Equalization is often used to carefully cut the feedback frequencies and maximize GBF.