Why do some microphones screech? And why do the ones at big events like award shows not screech?


Why do some microphones screech? And why do the ones at big events like award shows not screech?

In: 13

The screech is called feedback and it often happens when a microphone is placed in front of, or too close to, the speaker where the sound picked up by that microphone is coming out. It’s like a repeating loop, where the microphone picks up the sound already coming out of the speaker which then produces more noise. This happens again and again almost instantly, so it gets louder and higher and more unpleasant. At awards shows, the mics are usually either fixed in place or the speakers are well in front of the stage so there’s no chance for them to get too close to each other.

That screech is feedback. You speak into the microphone and noise comes out the speaker. Feedback happens when the mic pickups the audio coming out of the speakers and creates a FEEDBACK LOOP. In the movies when the mic screeches at the big conference when the guy is adjusting doesn’t happen. Guitarists do this on purpose. They put their guitar right up into the grill of the amp. The pickups in the guitar pick up the amps output and loop it through creating the high pitched feedback.

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.

provided a [great explanation](https://www.reddit.com/r/explainlikeimfive/comments/15t228l/comment/jwhmdyb/?utm_source=share&utm_medium=web2x&context=3) for most of the factors.

One that they left out is feedback elimination processors. High end venues have equipment in place that work like an equalizer with a lot of very narrow, robot-operated sliders to automatically cut the volume of any frequencies where feedback starts to happen.

They don’t need an audio engineer to constantly listen for feedback and do something about it. The equipment notices faster and deals with it more efficiently than a human can.