Feedback is not a fault. It is the system doing exactly what it was built to do, arranged in a circle: the loudspeaker excites the room, the room excites the microphone, the microphone drives the loudspeaker. When the energy returning to the capsule equals the energy that left it, at any single frequency, the loop sustains itself and the ring appears.
The quantity that decides how loud a system can go before that happens is gain before feedback. It is a property of geometry rather than of brand: the distance from performer to microphone, the distance from loudspeaker to microphone, the directional pattern of both, the number of microphones open at once, and the room's willingness to return energy. These relationships were set out in the sound system engineering literature decades ago and have not changed, because the physics has not.
The distances do most of the work
Sound pressure from a point source falls with distance by the inverse square relationship, which in decibels means a loss of roughly 6 dB each time the distance doubles. Read backwards, halving the distance from mouth to capsule returns about 6 dB, and every one of those decibels is available as headroom before the loop closes.
This single fact explains the shape of modern musical theatre. A capsule at the hairline sits perhaps 100 mm from the mouth; a float on the stage lip may sit two metres away. The difference in available gain is not marginal, it is the difference between a show that can be understood in the back row and one that cannot. The trade is visual: a performer's face carries a device, and in close staging the audience sees it.
The reverse applies to the loudspeaker. Moving it further from the microphone, or turning its pattern away from the microphone, buys margin by the same arithmetic. This is why front fill placed downstage of the performers is a different proposition from a main hang placed upstage of them, and why the position of a monitor wedge is an acoustic decision rather than a convenience.
Every open microphone costs the whole system
Microphones sum. The established relationship is that potential acoustic gain falls by 10 log of the number of open microphones, which works out at about 3 dB lost each time that number doubles. Two open channels cost roughly 3 dB against one; eight cost roughly 9 dB.
The practical consequence runs against instinct. A chorus of sixteen with sixteen live capsules is quieter, before ringing, than the same chorus on four. Muting what is not in use is not tidiness, it is the cheapest gain available, and it is the reason an operator follows a script rather than leaving everything open.
Where a pattern rejects, and where it does not
A directional capsule buys margin by being deaf in a particular direction, and knowing which direction matters more than knowing the name of the pattern. A cardioid is least sensitive directly behind itself, so a wedge placed on axis behind it is the classic arrangement. A hypercardioid is tighter across the front but is not deaf at the back at all: its nulls sit off to the rear quarters and it retains a rear lobe. Put a wedge directly behind a hypercardioid and the margin that was expected does not appear.
The same logic runs the other way for the loudspeaker. Every pattern is frequency dependent, and control loosens as frequency falls, so a box that behaves obediently at 2 kHz may be close to omnidirectional at 200 Hz. Feedback in the low midrange is often blamed on the microphone when the loudspeaker has simply stopped aiming.
What the equaliser can and cannot buy
Feedback appears first at whichever frequency has the highest loop gain, usually a room mode or a peak in the loudspeaker's response. Narrow cuts at those frequencies raise the usable level, a practice long known as ringing out a system.
The returns diminish quickly and predictably. Each cut removes one ring and exposes the next one down, and after a handful of filters the tonal balance is being dictated by the room's worst resonances rather than by any decision about how the show should sound. Corrective filtering buys a few decibels and spends tonal integrity. That is the trade, and it is worth making deliberately rather than by reflex at half past eleven during a technical rehearsal.
Automatic mixers and what they change
An automatic mixer reduces the effective number of open microphones by attenuating channels that are not carrying signal, recovering part of the 10 log penalty without an operator riding faders. What it cannot do is judge intent: it responds to level, so a quiet aside and a noisy scene change look much the same to it. Where the material is unpredictable, an operator working from a script still outperforms it.
What we cannot verify
Published gain-before-feedback improvements attributed to particular processors come almost entirely from the companies selling them, and test conditions are rarely disclosed in enough detail to reproduce the result. The relationships described here are physical and drawn from engineering literature rather than product documentation, but they describe idealised sources in ordinary rooms, not a specific stage. A system's real margin is found by measuring it in the room it will be used in, with the performers who will use it, at the microphone count the show actually calls for.
The short version
- Feedback is the loop sustaining itself, not a defect in the equipment.
- Gain before feedback is set by geometry, not by brand.
- Halving the source-to-capsule distance returns roughly 6 dB.
- Potential gain falls by 10 log of the number of open microphones.
- Narrow filters raise usable level with sharply diminishing returns.
- Real margin is measured in the room, with the cast, at the show's microphone count.