what causes tweeting of a mic in front of it's speakers ?

[boran]

not nececearily highly technical, but I wondered if my theory on this was correct.

whenever a microphone comes in front of one of it's speakers a very loud tweet starts (I think everyone will have heard this once at least) now I was wondering how this is caused,
my theory is that the small sound between the speaker and mic gets amplified and then into mic and then amplified etc, so u essentially start an eternal loop (also would explain why it increases in loudness over time) if i'm wrong or not entirely correct feel free to correct me.

also, on stage artists have speakers pointed at them (to get an idea what the hell they sound like to the audience I think) why are these exempt from this tweeting behavior ? (timings ?)

thanks for any clarification.

Boran.

 

[f95toli]

I think your explanation is correct.
And the speakers an stage are probably connected to a device called a "feedback destroyer", it is a common tool which reduces this effect.

 

[uart]

Well that's why they usually call that "tweet" as feedback

 

[ManBOO YA]

Why monitors (the speakers facing the musician) don't feedback is that the mics used by musicians are unidirectional (takes sound from one direction) so they don't recieve the sound from the monitor which is located behind the mic.

 

[MPankau]

In very general terms you guys are correct.

Microphones are necessarily unidirectional - but the have cardioid, and other various response patterns.

Feedback is created by the loop between the microphone and the speaker - most people DO NOT use a feedback 'destroyer' as they can trash a good sounding mixer pretty easily, but rather use graphical eq's and parametric eq's in combination to elminated problem frequencies.

Hope this helps a little bit.

MP

 

[nineball9]

One can have fun with video feedback too. Just plug the output of a camcorder into a TV, and then use the camcorder to image the TV's screen. At certain angles, one can get a regression to infinity image.

 

[dkozloski]

Back in the old days of records and turntables I could get mechanical feedback going from the low frequecy response of my Wharfdale W90's sitting on the floor back to my Empire turntable and Marantz amp that if it went unchecked would feel like an earthquake. The first time I encountered it I thought the world was coming to an end. For a fix I set the turntable on 2 inches of foam rubber.

 

[MPankau]

Originally posted by: dkozloski
Back in the old days of records and turntables I could get mechanical feedback going from the low frequecy response of my Wharfdale W90's sitting on the floor back to my Empire turntable and Marantz amp that if it went unchecked would feel like an earthquake. The first time I encountered it I thought the world was coming to an end. For a fix I set the turntable on 2 inches of foam rubber.

Okay, now that has to be one of the coolest things I've heard all week.

MP

 

[rectifire]

Originally posted by: nineball9
One can have fun with video feedback too. Just plug the output of a camcorder into a TV, and then use the camcorder to image the TV's screen. At certain angles, one can get a regression to infinity image.

Just thought of very first way to accomplish the same thing. Get two mirrors and face them at each other. Cool effect!

 

[MystiKal]

FYI, those speakers on stage facing the performers are there so they can hear the music. The speakers facing the audience and usually off to the side, in front of the stage and firing away from the performers. If you ever DJed or performed on stage in a similar setup, you'll know that you can't really hear the sound to well. Plus, the speakers firing at the audience are at higher decibals...so, the position of these speakers reduces feedback. But monitor speakers can also be fed into a feedback destroyer.

If anyone DJs, try using a monitor speaker (like a single self-amplified speaker or something right on the DJ stand) and you will notice the difference and be able to hear the music as it sounds to the ppl listening to it.

 

[beaconengr]

Feedback is caused when a particular sound arrives back at the microphone at a level greater than the original sound entered. This is why environments that have reflective materials on the ceilings and walls have a much greater tendency for feedback. Certain microphones resist feedback better than others. Feedback destroyers can increase the level at which feedback occurs, but not by much. Also, they do this by cutting out the offending frequencies, but at the same time taking all the music that is reproduced at this frequency with it. Some feedback destroyers have filters centered at 1/10 of an octave which helps a lot, but offending frequencies tend to bunch up together, and if you set 10 filters to fixed, you have removed an entire octave of music. One mentioned that equalizers are better, but this is not true. You are doing exactly the same thing. You are using the equalizeer to attenuate the offending frequency, and also cutting out the music. Equalizers are not meant for this purpose.

 

[Sahakiel]

1. you forgot about destructive interference.

2, long live the op-amp.