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Sound Reinforcement - Forums for Live Sound Professionals - Your Displayed Name Must Be Your Real Full Name To Post In The Live Sound Forums => LAB Lounge => Topic started by: David Allred on October 16, 2015, 07:17:35 AM
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Is mic feedback limited to the offending speaker only, or does it share the pain to all speakers on the same output path (such as the speakers on stage opposite on a mono or center-panned stereo system)?
Obviously the mic picks up the squeal and then sends it to the other speakers, but that would technically not be "feedback", but rather reinforcement. Right?
For the sake of clarity, let's place the mic 1 foot from the horn's throat. The only other speaker is 30' away, off axis (same orientation). Does the far speaker actually feedback?
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If both speakers are on the same amp output, they get the same signal. Same with any sound passing through it.
Even if they are on different amps but are being sent the same mix, they will both get the same level of feedback signal.
Steve.
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If the system is true stereo, maybe not. If the system is mono all cabinets will produce the sound.
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In a nutshell any cab that you can hear "hello" from when speaking into the offending mic, will also get the feedback signal when that mic feeds back.
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Is mic feedback limited to the offending speaker only, or does it share the pain to all speakers on the same output path (such as the speakers on stage opposite on a mono or center-panned stereo system)?
Obviously the mic picks up the squeal and then sends it to the other speakers, but that would technically not be "feedback", but rather reinforcement. Right?
For the sake of clarity, let's place the mic 1 foot from the horn's throat. The only other speaker is 30' away, off axis (same orientation). Does the far speaker actually feedback?
Are you asking if you cut the signal from the speaker that is 1 foot away from the microphone will the feedback cease if the feedback signal is coming out of both the speaker 1 foot away and 30 ft away?
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Yes, you are correct. Feedback is only happening from the offending speaker and reproduction is happening in the other speakers. That situation could change in seconds however. Why do you want to make the distinction?
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Are you asking if you cut the signal from the speaker that is 1 foot away from the microphone will the feedback cease if the feedback signal is coming out of both the speaker 1 foot away and 30 ft away?
No, that would be the same as just pulling down the volume to stop the feedback.
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Why do you want to make the distinction?
Curiosity about a technical situation. Perhaps trivial.
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Curiosity about a technical situation. Perhaps trivial.
Don is right. Feedback loop is from the nearby speaker, sound from the distant speaker is not being picked up by the mic, the speaker is merely playing the signal created by the actual loop.
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Just to add a different viewpoint (those presented are correct-the sound will come out of all speakers that the mic is routed to).
Let's say you have 1 speaker on (a monitor) and the mic is stable.
In some cases, when you turn on the mains feedback will start.
If you turn off the monitor, the feedback goes away.
So neither speaker alone is causing the feedback. But when both are used (not in every case) the feedback starts.
This is due to the fact that both are contributing to the level at the mic position at the feedback position.
Usually, turning down one or the other will stop it.
Of course the BEST solution is to keep speakers (except monitors of course) from getting into the mics to begin with.
Hence the reason for pattern control on main speakers, to keep the sound where it belongs-on the audience and away from everywhere else.
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In addition to all the speakers reproducing the feedback, any output such as recording or web cast feed will also get the feedback sound if that mic is sent to those outputs.
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About 25 or 30 yrs ago, I recall reading in the YAMAHA SOUND REINFORCEMENT HAND"BIBLE" that feedback happens anytime a sound or frequency coming from the speaker is louder (at the mic) than the original sound source (at the mic). Is that 100% true, in all cases?
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I think the (or at least my) theoretical question is; is the stress/abuse for the far speaker that is reproducing the feedback, in this scenario, the same as it is for the near speaker contributing to the feedback loop? My gut says yes, but not being cyclic in nature, it could be different, although potentially just as damaging in the end.
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I think the (or at least my) theoretical question is; is the stress/abuse for the far speaker that is reproducing the feedback, in this scenario, the same as it is for the near speaker contributing to the feedback loop?
Yes. They are both being fed the same signal voltage and will have the same excursion.
Steve.
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About 25 or 30 yrs ago, I recall reading in the YAMAHA SOUND REINFORCEMENT HAND"BIBLE" that feedback happens anytime a sound or frequency coming from the speaker is louder (at the mic) than the original sound source (at the mic). Is that 100% true, in all cases?
David,
Your statement is not 100% true.
The original source can trigger feedback, but a singer's voice can easily exceed 120 dB at the mic, while feedback could occur at 90 dB (or less).
You can easily confirm that fact using a limiter set to a high ratio, with a lot of input gain, but low output level, you can get speakers feeding back at a conversational level all day long. Quite annoying.
As another side of that coin, heavy limiting will reduce gain while the singer provides input, but when the singer stops singing, the gain rises back up and can cause the mic to feed back.
Feedback is a "loop", when the mic gain is enough to "hear" the speaker (at some frequency) the amplification of that frequency continues to increase until amplification is limited by clipping, limiting, gain reduction, or driver failure.
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About 25 or 30 yrs ago, I recall reading in the YAMAHA SOUND REINFORCEMENT HAND"BIBLE" that feedback happens anytime a sound or frequency coming from the speaker is louder (at the mic) than the original sound source (at the mic). Is that 100% true, in all cases?
Yes ... At least for all frequencies that arrive back at the mic in phase ( or Ray nearly in phase).
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About 25 or 30 yrs ago, I recall reading in the YAMAHA SOUND REINFORCEMENT HAND"BIBLE" that feedback happens anytime a sound or frequency coming from the speaker is louder (at the mic) than the original sound source (at the mic). Is that 100% true, in all cases?
Pretty much correct while a little simplistic. I would have worded it somewhat differently. Feedback has nothing to do with loudness, but loop path gain does. Anytime the same signal passes through a given loop with more than unity gain, such that it is increased with every subsequent pass through through the loop, it will eventually saturate the electronic path. Since each loop through takes a fraction of a second, it doesn't take very long for feedback to be annoying.
An additional qualification is that frequency/wavelength matters. The acoustic path length between speaker and mic, and speed of sound in air, will define the phase relationship between the acoustic path (at the mic) and the electronic path (which is the same phase*** at mic and speaker). For frequency/wavelengths where the returning acoustic sound adds constructively to the electronic path, the signal increases with each pass. For frequencies, where the wavelength results in some cancellation and less than unity gain, those frequencies die off.
*** flipping the polarity of the electronic path will shift the frequency where feedback modes occur, due to the extra half wavelength difference between the two paths.
David,
Your statement is not 100% true.
The original source can trigger feedback, but a singer's voice can easily exceed 120 dB at the mic, while feedback could occur at 90 dB (or less).
yes, if the amp/speaker is not capable of making more than 120dB? at the mic, while conditions exist for feedback, that max level will be dominated by the path... remember it's about gain not loudness.
You can easily confirm that fact using a limiter set to a high ratio, with a lot of input gain, but low output level, you can get speakers feeding back at a conversational level all day long. Quite annoying.
I spent a bunch of years making feedback on purpose to demo FLS EQs and I could make the rig sing at only modest levels by riding the fader for the loop path gain (human limiter).
As another side of that coin, heavy limiting will reduce gain while the singer provides input, but when the singer stops singing, the gain rises back up and can cause the mic to feed back.
+1 common problem for monitors... I've even seen issues from one power amp model that reduced gain when it was hot. Of course the operator pushed up the gain again to maintain monitor levels. then when the amp cooled off, and restored the previously cut gain, instant brain darts occur as the system flies into feedback (it's all about the gain)..
Feedback is a "loop", when the mic gain is enough to "hear" the speaker (at some frequency) the amplification of that frequency continues to increase until amplification is limited by clipping, limiting, gain reduction, or driver failure.
Yes but with a minor qualifications as above.
A given physical set up may have a half dozen hot frequencies where feedback occurs, Changing the path length by moving the mic or speaker can change where the feedback frequencies occur.
Of course this is still a simplification, but should be plenty to chew on for now.
JR
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with digital mixers all having more latency (as compared to analog mixers) does the feedback loop take noticably longer to get going? Perhaps even enough so to prevent some of it from happening where it might have otherwise occurred with an analog mixer?
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with digital mixers all having more latency (as compared to analog mixers) does the feedback loop take noticably longer to get going? Perhaps even enough so to prevent some of it from happening where it might have otherwise occurred with an analog mixer?
No. The latency added by the digital processing is equivalent to an additional foot or two between the speaker and the mic without the level drop a real distance increase would have.
Mac
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with digital mixers all having more latency (as compared to analog mixers) does the feedback loop take noticably longer to get going? Perhaps even enough so to prevent some of it from happening where it might have otherwise occurred with an analog mixer?
Digital path delay is no different than acoustic path delay but will generally be a modest fraction of the combined total.
For analysis of which frequencies feed back based on acoustic path transit time/wavelength, the digital path delay simply adds to the acoustic transit time delay.
For feedback modes between a stage mic and nearby floor monitor where the acoustic path is only several feet long, the additional digital path delay can shift feedback mode locations enough to require different corrective adjustments.
So a digital console may sing at slightly different frequencies but will be just as susceptible to feedback as the analog console.
It's still about the gain....
JR
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Take a listen on a Saturday afternoon to a college football game. Many of them (none of mine of course :) ) have very poorly setup referees microphones. The acoustical delay path in those systems is sometimes over a half second, and there is still feedback.
Sent from my iPhone using Tapatalk
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Take a listen on a Saturday afternoon to a college football game. Many of them (none of mine of course :) ) have very poorly setup referees microphones. The acoustical delay path in those systems is sometimes over a half second, and there is still feedback.
Sent from my iPhone using Tapatalk
Longer acoustic paths support even lower frequency feedback modes, but more path delay does not help (it's still about the gain), while making the delay path short enough can shift the lowest feedback modes up high enough to be less problematic (Remember hot spot monitors that mounted up on the mic stand?). Perhaps the extra delay from a digital path could make hot spot monitors more susceptible to feedback.
JR