Feedback: Why It Happens and How to Kill It

Feedback is what happens when a microphone can hear the loudspeaker it is feeding. Sound goes out, some of it comes back in, and on each trip round it is either smaller than it was or it isn't. That one ratio — what returns divided by what left — decides the whole business, and every technique in this lesson is a way of making it smaller.

The assumption on stage is that feedback is caused by volume and cured by an equaliser. Neither holds up. The loop runs all night at every level you ever use, and the master fader only decides which side of one its ratio lands on. Indoors there is a second thing going on that no fader touches — most of what the microphone hears from the PA never travelled there in a straight line. The room is inside the loop.

Every Lap Multiplies, and Unity Is the Cliff

Sing one syllable into an open mic. The PA reproduces it, some fraction of that reaches the diaphragm, and the system reproduces that too, and on it goes. The ratio between what comes back and what went out is the loop gain — the only number in the problem.

Below one, the copies shrink geometrically and the tail is finite. A loop gain of 0.5 adds copies at a half, a quarter, an eighth and downward, summing to exactly one extra syllable smeared over a few milliseconds and heard as a faint thickening, if it is heard at all. At exactly one, every lap is a full-size copy of the lap before, so the note stops decaying and just stays. Above one it grows, and keeps growing until an amplifier or a driver runs out of room. The squeal everyone remembers is that limit being hit — not the feedback itself, but the wall the feedback ran into.

Nothing changes character at the edge — a ratio crosses 1.0, the arithmetic flips from convergent to divergent, and that is the whole event. It is why you never hear feedback coming as "getting louder"; what you hear is decay times stretching. A metre from a wedge grille to a capsule is a lap of roughly 2.9 ms, because the electronics add almost nothing to the acoustic journey, and the loop therefore completes about 350 times a second. At that rate a ratio barely over one goes from inaudible to unbearable inside one syllable.

It Rings on One Note, and the Room Picks Which

Loop gain above one is necessary and not sufficient. The returning copy also has to arrive in step with what is already there — one lap has to work out to a whole number of cycles. A copy that gets back half a cycle late subtracts from the signal that produced it.

For a single path that condition is a comb. A 2.9 ms lap is exactly one cycle of 343Hz, two cycles of 686Hz, three of 1,029Hz — and onward in even steps to the top of the range. Between them the return is partly or wholly opposed to what it is returning to, so it works against itself and never builds.

Outdoors that is close to the whole picture — one straight line plus a ground bounce, a coarse comb you could nearly predict with a tape measure. Indoors there are hundreds of paths. Every reflection is another route from box to capsule with its own length and its own comb, and they all sum. What comes out is not a comb at all but a jagged landscape of peaks and dips with no simple spacing, and whichever peak reaches unity first is the note you get.

That peak belongs to the room and the system jointly — to neither one alone. Change the mic and it moves; nudge the mic and it moves; fill the room with bodies and it moves. There is no such thing as a microphone that feeds back, and pushing through the first peak only hands the job to the second, at a different pitch.

Indoors, Half the Geometry Stops Paying

The outdoor lesson gave two distance levers: halve the source-to-mic distance for 6 dB, double the speaker-to-mic distance for another 6. That is free-field arithmetic — indoors only the direct sound obeys it, and the two levers come apart.

One survives intact. A singer's mouth is centimetres from the capsule, far inside critical distance, so closing that gap pays in full in any room — the cheapest reliable decibel available to a band, and it needs no equipment. The other lever expires. The room lesson put a number on where: past critical distance the reverberant field is one flat level, so a mic already out in it gains nothing by moving further from the box. Flying the mains higher stops helping once what reaches that mic is mostly room.

Mic directivity is worth less indoors too, which catches people out. A cardioid turned so the box sits in its null is rejecting a direction — and a reverberant field does not have one. Against diffuse sound that capsule collects roughly a third of what an omni would, which is 4.8 dB, and it collects that third whichever way you aim it. The lever that does keep working is the other end: coverage tight enough to put the sound into bodies rather than plaster, so less of it ever becomes reverberant field in the first place. Unlike repositioning one microphone, that helps every open channel at once.

Ringing Out Is Real Gain, and It Runs Out Fast

Ringing out means provoking each peak deliberately and notching it. The filters have to be narrow because the peaks are narrow, and anything wide enough to feel gentle takes real programme material down with it — you will hear that long before the audience hears feedback.

It works, inside limits tighter than most bands expect. A careful pass is worth single digits of decibels rather than tens, and individual notches want to be a couple of dB deep, not twenty. Returns fall away quickly, because pulling the tallest peak down only exposes the next, and near the top of the pile the peaks sit close together in height. By the third or fourth filter you are trading audible programme material for a decibel of ceiling, and the tell that you have run out is the instability going broad — no single pitch any more, just a system that will not settle.

The larger limit is that you tuned the wrong room. Every one of those peaks is assembled out of reflections — and a full house can halve a room's reverb time. An empty-stage ring-out is a starting point the first song partly invalidates, which is also why a mic nudged twenty centimetres along its stand during changeover can undo an hour of careful work.

The Six Decibels You Never Spend

Systems get sized with an equation that predicts how much gain a given arrangement of source, mic, speaker and audience can support, and it carries a literal minus six on the end. That six is part of the design figure rather than caution bolted on afterwards, and it is a floor, not a target. The outdoor lesson already described how a stage sounds once that margin is gone. Indoors you want the margin for a second reason as well — every peak in that jagged landscape was assembled out of reflections, and the reflections keep moving all night as the room fills and thins.

Open microphones are how bands spend that margin without noticing. Each doubling of open channels costs about 3 dB, as the outdoor lesson set out — and indoors they do not all cost the same. A close mic on a snare sits well inside critical distance for its own source, so what it returns is mostly snare. A pair of overheads a metre up is largely in the reverberant field, and what it hands back is mostly room and PA. Two of those can cost more headroom than four close mics, and a vocal channel left open through an instrumental spends its full share while contributing nothing.

Working Underneath the Ring Point

• Close the source-to-mic gap before anything else — it is the one free-field lever a room cannot take away from you, and it still pays 6 dB per halving indoors.
• Re-check the ring point once the room has people in it — the peaks you tuned out belonged to an empty floor, and an audience rewrites most of them.
• Stop notching when a filter buys less than the one before it — past that point the filters cost more clarity than the ceiling they buy back.
• Mute or gate anything nobody is using — the doubling arithmetic counts open channels, not useful ones, and that count is entirely yours.
• Suspect the ambient mics before the close ones — overheads and room mics live in the reverberant field, so they feed the loop more PA per decibel of instrument.
• Fire the boxes into bodies, not plaster — every reflection you avoid comes out of every open channel at once, which moving one microphone can never do.
• Treat a new ring frequency as information — a different note means something in the room moved, and finding it usually beats reaching for another filter.
• Keep the last 6 dB in reserve — a full room is not the room you rang out, and that margin is what absorbs the difference.