Feedback: why it squeals and what actually helps

What is actually happening

The sound travels in a circle: microphone, mixer, amplifier, loudspeaker, room, back into the microphone. As long as more level is lost around that loop than is gained, everything stays quiet. The moment the loop returns exactly as much at one frequency as it consumes, it starts to drive itself.

That is why it always squeals at a single pitch first: at the highest peak of the whole loop. That peak is the sum of everything — the microphone response, the loudspeaker response, the reflections of the room, and the exact spot the microphone is standing in. Half a metre away it is a different frequency. Which is why a list of critical frequencies off the internet is worth nothing.

The four levers, ordered by effect

1. 1Distance to the source. The strongest lever, and it costs nothing. Halve the distance from mouth to microphone and you gain 6 dB — more than any equaliser will ever give you. 20 cm instead of 40 cm is the whole difference between unusable and relaxed.
2. 2Direction. The microphone has to be out of the loudspeaker coverage. It is not distance alone that decides, but how much sound actually arrives where the microphone stands. Loudspeakers ahead of the microphone line, cardioid microphone with its back to the box: often 10 dB and more together.
3. 3Number of open microphones. Every open microphone feeds the same loop. Twice as many open microphones costs 3 dB. Four instead of one costs 6 dB. Whatever is not needed gets closed — this is the lever choirs and panels give away most often.
4. 4Equaliser. Last, not first. A narrow filter on the highest peak buys exactly what that peak stood above the second highest — usually 3 to 6 dB. Then the next one takes over. That is a real gain, but it is limited, and it is the smallest of the four.

Why the equaliser does not carry further

After the first filter the loop is one peak flatter. After the second, the next one. After four or five filters you have not removed peaks any more, you have hollowed out the sound — and the gain per filter gets smaller every time, because further down the peaks sit closer and closer together.

Anyone who needs 20 dB more will not get it from an equaliser. Anyone who needs 4 dB gets it immediately. That is the whole difference — and the reason distance and direction come before the filter, not after it.

How to find the frequencies

By hand: close all microphones but one, then bring the level up slowly until it just begins to ring — not to squeal. Cut that note, narrow, 3 to 6 dB. Bring it up further. Stop after the second or third: what comes after that costs more tone than it buys in level.

With a measurement: the peaks in the response at the listening position are the candidates. They are not the same thing as the loop — the loop includes the microphone and its position — but the strongest room resonances show up in both. Measure the room and you know where it gets critical beforehand, instead of finding out during the service.