Why it happens
A microphone picks up sound, the system amplifies it through the speakers and part of that sound enters the microphone again. If the returning signal is stronger than the original, a loop forms and grows until it becomes feedback. It depends on three things: gain (how much you amplify), distance (how close the microphone and speaker are) and frequency (some tones ring first because of the room acoustics).
Prevention: what to do first
- Fewer open microphones. Every open microphone adds noise and risk. Mute the ones not in use at that moment.
- Just enough gain. If a channel needs too much gain to be heard, move the microphone closer to the source instead of turning it up (gain guide).
- Speaker position. The main speakers should sit in front of the microphones, not beside or behind them.
- The right microphones. Dynamic cardioids reject sound from behind better; the singer should use them close to the mouth.
- High-pass filter (HPF) on every voice: it removes noise and energy that adds nothing.
Floor monitors: the critical point
Monitors point toward the stage, right where the microphones are, so they are the main cause of feedback in the service. Place them on the axis where the microphone is least sensitive (with most cardioids, behind the microphone, pointing at the singer) and keep their level as low as possible. If you can, switch to in-ear monitoring: with no speakers on stage, the problem almost disappears.
If you already have feedback
- Pull down the fader of the most likely channel (the microphone closest to a monitor or the one that just opened).
- Mute the microphones that are not in use.
- Calmly find the frequency and apply a small, narrow cut with an equalizer. An automatic feedback suppressor can help, but it should be the last resort.
If feedback returns at every rehearsal, the problem is structural (position, gain or acoustics) and the whole system deserves a review.
Testing the system before the service
A common practice is to “ring out” the system before the service: with the microphones open as they will be in the service, slowly raise the system gain until the first feedback appears, pull a small cut at that frequency, and repeat carefully a couple of times. Do it at low volume and with nobody near the speakers, so you hurt neither people nor equipment.