Why do some microphones screech? And why do the ones at big events like award shows not screech?
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Microphones screech because of feedback – the microphone is picking up the amplified sound from the speakers and amplifying it again.
All amplifiers have some amount of delay, and all rooms have some resonant frequency. The amplifier delay is usually small enough that you don’t notice it, and the room’s resonant frequency is usually tame enough that if you *do* notice it it’s subtle.
When a microphone is picking up the sound from its connected amplifier/speaker though it just amplifies that sound again. The sound it’s hearing loudest is at the resonant frequency of the room/sound system, so that keeps getting amplified until it’s the dominant sound coming out of the speakers – usually an ear-bleedingly unpleasant squeal/screech.
So why don’t the microphones at big events like award shows do this?
Well, normally the room is set up to avoid the problem: The speakers project out over the audience so they’re not amplifying back into the microphone, and the room is full of soft materials like fabric drapes, tablecloths, and of course people – those don’t reflect sound very well, so there isn’t a lot of sound bouncing back to the microphone from the audience.
Big events are also using quality microphones that are highly directional – they’re only picking up what they’re pointed at and within a relatively short range, so they’re not going to pick up the amplified sound even if it is bouncing back at them because it’s not *right in front of the mic* where it can “hear” the sound.
Good setup and equipment isn’t always enough though, so there’s always a sound check where the amplifiers and speakers are set up and tested to make sure they aren’t likely to produce feedback, and there’s also someone at the sound board who knows what they’re doing during the event: If they hear feedback starting they can pull back on the amplifier gain (make the speakers quieter so the microphone isn’t hearing the amplified sound) or pull back on specific frequency ranges (to remove the screech so the microphone doesn’t hear it and try to amplify it again).
Low budget events don’t always have competent sound engineers, or a sound booth that can adjust levels live to cut out feedback, or expensive highly-directional mics, or the luxury of treating the room so it doesn’t reflect a lot of sound.
When someone is giving a speech in your high school gym with a little PA on the floor they might have to make do with turning off or covering the mic when feedback starts, and then maybe turning the volume down a little bit before talking again.
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