Technical guide
Why the microphone squeals and how to prevent it
Feedback isn't bad luck: it's a loop. If you understand the loop, you know where to break it, and it is almost always broken before the event, not during it.
The mechanism
A microphone that hears itself
Feedback — also known as howlround — happens when the sound coming out of the speaker gets back into the microphone with enough energy to come out amplified again. The cycle repeats many times per second, and within tenths of a second it turns into that squeal that makes two hundred people reach for their ears.
The key is that the squeal always appears at a specific frequency: the one at which the combination of microphone, room and speaker has the most gain. That is why feedback sounds like a note, not like noise. And that is why it can be tackled surgically instead of turning down the overall volume.
The four factors in the loop
There are only four variables: the system's volume, the distance between speaker and microphone, the orientation of both, and the room's acoustics. Every solution involves adjusting one of those four. If the first three are properly sorted in the set-up, the fourth rarely becomes a problem.
Prevention
What gets sorted before anyone arrives
Speakers in front of the microphone
The golden rule. Every reinforcement speaker must sit in front of the line where the open microphones are. A speaker behind the lectern is feedback waiting to happen.
Bring the microphone closer to the mouth
Every time you halve the distance between the microphone and the mouth, you gain voice level without turning anything up. That is why a headset mic can handle far more room volume than a stand mic a hand's width away.
Make the polar pattern work for you
Directional microphones reject what comes from behind or from the sides, depending on their pattern. Pointing that rejection zone towards the speaker costs nothing and works.
Open only the channels in use
Four open microphones at a round table give far more loop gain than one. Muting what isn't in use is the most effective measure at conferences and official ceremonies.
Distribute the reinforcement
Several speakers close to the audience at low level send less energy back towards the stage than two big cabinets pointing into the room from the front.
Test at real volume
A system tested in an empty, silent room reveals nothing. You should turn it up until you find the feedback point, back off and leave some headroom. That is literally measuring the available ceiling.
In the moment
What to do if it squeals in the middle of a speech
First of all: don't pull the master down abruptly. The audience notices the cut more than the squeal, and whoever is speaking loses their voice for a few awkward seconds.
What a skilled technician does is identify the frequency and cut it on that channel's equaliser with a narrow filter. A tiny portion of the voice's sound is lost and several decibels of headroom are gained. If there is no technician, the alternatives in order of effectiveness are: bring the microphone closer to the mouth, step out of the speaker's line, turn that channel down a little — not the master — and, as a last resort, mute the microphones that aren't in use.
What almost never works
Covering the microphone with your hand: it worsens the polar pattern and usually causes more feedback. Moving the microphone away from the mouth: it reduces the voice and forces you to turn the channel up. Raising the speaker without changing its angle: if it is still pointing at the microphone area, the loop is still there.
Tools that help
Digital consoles include filters and sometimes automatic feedback suppressors. They help, but they are no substitute for correct placement: if the set-up is wrong, the suppressor keeps notching out frequencies one after another until the voice sounds hollow. It's a patch, not a method.
Tricky cases
Where feedback crops up most often
Ceremonies in courtyards and cloisters
Hard surfaces, vaulted ceilings and lots of reverberation. The solution is to bring the reinforcement closer to the audience and use close-miking, not to turn up the volume.
Round tables and conferences
Several microphones open at once is the most feedback-prone scenario. At conferences it is solved with channel management and, when the format allows, push-to-talk table microphones.
Events with a standing audience very close to the stage
The reinforcement ends up almost level with the microphones. What helps here is well-angled monitors rather than turning up the room.
Karaoke and roaming microphones
The microphone ends up wandering in front of the speakers. With karaoke, it's best to set a singing area and stay within it.
PA systems in large spaces
When the same system covers a warehouse or a square, some sound coming back is unavoidable. Distributed PA with many points at low level is the standard solution.
Wireless mics with the gain set wrong
A wireless system with the bodypack gain set too high feeds back much sooner. It is adjusted on the transmitter, not just on the desk.
Keep reading
Related links
Frequently asked questions
Feedback FAQs
Why does it only feed back sometimes?
Because the loop depends on the microphone's position and the room. As soon as the speaker moves, turns or gets close to a loudspeaker, the conditions change and the squeal appears.
Do lavalier (lapel) microphones feed back more?
Yes, generally. Because they are further from the mouth, they need more gain, and more gain means less margin before feedback. A headset mic is safer for long speeches.
Does better equipment prevent feedback?
It helps, because microphones with a better polar pattern and speakers with controlled dispersion give more headroom. But a badly positioned set-up will feed back with any equipment.
Can feedback be eliminated with a processor?
It can be greatly reduced with well-set filters. Automatic suppressors help as a safety net, but if they are overused the voice loses its natural sound.
Do I need a technician to prevent this?
At an event with several microphones, yes. Most of a technician's work during a speech consists precisely of opening, closing and adjusting channels so that this doesn't happen.
Speeches, a ceremony or a panel discussion?
These are the set-ups where having a technician makes the biggest difference. Tell us about your event and we'll put forward a plan.
