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Other sources of audio feedback
October 3, 2010 5:00 PM   Subscribe

Besides the standard speaker-to-mic-to-speaker-to-mic-to-gaaaahhh!, what else can cause audio feedback?

In the last 24 hours, I've done some Google homework to understand the cause of feedback in a sound system, how to prevent, deal with it, etc.

Are there any other factors/things that can cause feedback? I was at a small gig yesterday, and there was a question as to whether the guitarist's metal studded strap was causing feedback (she was doing some vocals, too).

I have a feeling that's not quite right, as it doesn't seem to fit the definition of feedback, but enlighten me, hive mind!
posted by nrobertson to Media & Arts (13 answers total) 2 users marked this as a favorite
 
Feedback describes a situation where there is a loop between audio input and output: mic & speaker, guitar pickup & amp, etc. Sound is transmitted back and forth between the two (one way via the air and the other via wires) and the signal gains in strength until some equilibrium is reached.

A metal-studded guitar strap does not cause feedback directly (there is no loop created between input and output, the strap is not part of either end of the system). I'm having a hard time envisioning how a metal-studded strap could even be involved in a feedback loop, although possibly the strap served as an antennae in some way. Did the person who suggested this indicate what the mechanism was?
posted by ssg at 5:19 PM on October 3, 2010 [1 favorite]


The guitar strap was not causing feedback. At the very most (and this is ridiculously far-fetched) it might conceivably have affected the frequency dynamics of the sound in the vicinity of the microphone, which might have in turn affected the frequency that was feeding back. But that's ridiculously far-fetched.
posted by The World Famous at 5:27 PM on October 3, 2010


In the church basement where my theatre group performs, the metal chairs vibrate at a B-flat. When we hit that note on stage as a big group, it causes some feedback. It's really quite neat. That said, it's lessened with people in the seats, and I doubt that the metal studs, which were attached to fabric and touching your guitarist's body, did anything to create feedback.
posted by moviehawk at 5:48 PM on October 3, 2010 [1 favorite]


Agreed. The strap could not have (in any realistic situation) caused feedback. Feedback is caused when a signal from your system's output (generally, speakers) enters your system's input (generally, microphone) at a loud enough volume to then exit the output again at an EQUAL or LOUDER volume than when it initially exited. A guitar strap... simply does not affect this system. Guitar STRINGS can vibrate in sound waves well enough to cause a feedback loop - this is an effect that is occasionally used in music.

tl;dr - No. It wasn't the strap.
posted by frwagon at 5:53 PM on October 3, 2010


A few things that haven't been mentioned. The nature of feedback is that a pitch is being picked up by the mic, sent out from the speaker, that pitch is then picked up again and repeated, causing a tone which is the feedback. It could have been a problem with the EQ then, but not the strap.
posted by Deflagro at 6:06 PM on October 3, 2010


In the church basement where my theatre group performs, the metal chairs vibrate at a B-flat. When we hit that note on stage as a big group, it causes some feedback.

Since part of the point of the question is what the word feedback actually means, I think it is important to note that you are describing resonance (i.e. the sound you are producing induces vibrations in the chairs and then you hear those vibrations), rather than feedback. The sound is not fed back into anything. There is no loop in this instance and so there is no feedback.
posted by ssg at 6:34 PM on October 3, 2010 [1 favorite]


related research: Metal Machine Music.
posted by ovvl at 6:34 PM on October 3, 2010


Resonance can cause feedback though. Most rooms that are not designed with acoustic treatment have a resonant frequency which amplifies that specific frequency, thus causing feedback. Maybe try finding the resonant frequency of the room and notch it out with some eq.
posted by anonop at 6:57 PM on October 3, 2010


If you were hearing a high pitched squeal that only happened when she wasn't playing her guitar, her pickups may have been going microphonic.
posted by davey_darling at 6:58 PM on October 3, 2010


If it was a low hum/buzz (not the typical high ear-piercing pitch of feedback) it might have been a ground loop.
posted by zsazsa at 7:53 PM on October 3, 2010


A metal-studded guitar strap does not cause feedback directly (there is no loop created between input and output, the strap is not part of either end of the system). I'm having a hard time envisioning how a metal-studded strap could even be involved in a feedback loop, although possibly the strap served as an antennae in some way. Did the person who suggested this indicate what the mechanism was?

Nope. I could ask, but I don't think she'd have anything enlightening. I guess it wasn't the strap since it's so far fetched.

If you were hearing a high pitched squeal that only happened when she wasn't playing her guitar, her pickups may have been going microphonic.

Yeah, I was reading a bit about that earlier, but I don't think that was it from what I observed.

If it was a low hum/buzz (not the typical high ear-piercing pitch of feedback) it might have been a ground loop.

Thanks, it was higher pitched, though. Should have mentioned that initially.

Resonance can cause feedback though. Most rooms that are not designed with acoustic treatment have a resonant frequency which amplifies that specific frequency, thus causing feedback. Maybe try finding the resonant frequency of the room and notch it out with some eq.

Hmm, yeah. It was actually outside under a roof with some pillars on the outside edge.

Anyway...

...it probably was just some plain old microphone feedback, though kind of erratic since it only happened one time, I think.

Are there other known sources of feedback, though? What I read says that mic/pickup to speaker isn't the only way for it to happen.
posted by nrobertson at 8:38 PM on October 3, 2010


Sometimes a bad pickup can cause feedback, due to the actual resonance of the pickup inside the guitar. I've had a guitar that did this at relatively low volumes, but when I replaced the pickup, it worked just fine.
posted by markblasco at 9:44 PM on October 3, 2010


Another possibility: If you're using gear with vacuum tubes in it, a tube itself can become "microphonic" -- meaning that its insides loosen up a bit, turning it into a virtual microphone which sound from the speakers jiggles and causes feedback. The way to check for this is to turn the equipment on, set volume levels to whatever you use, speakers connected, then tap each tube a couple times with the eraser end of a wooden pencil. If you hear anything coming out of the speakers, replace the tube that makes the sound when you tap it. I mention this only for completeness. Most gear these days is solid-state and doesn't have this problem.
posted by exphysicist345 at 8:18 PM on October 4, 2010


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