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Author Topic: Why Would a Monitor Feed Back Before a Sound is Made?  (Read 7937 times)

Chas Patate

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Why Would a Monitor Feed Back Before a Sound is Made?
« on: July 12, 2011, 12:44:17 AM »

Why would a monitor feed back just by turning up the gain, before there was any sound made into a condenser mic?   It doesn't matter which way the mic is pointing with respect to the monitor from a polar standpoint, as soon as the gain is turned up, before a note is played or sung, there is sharp high frequency feedback.

What is the theory behind this?  You would think that a sound would have to be made first, in order to initiate the feedback loop, but it seems that has never been the case.

Thanks for considering this question - it has defeated my search skills.

Cheers.
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Ales Dravinec

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Re: Why Would a Monitor Feed Back Before a Sound is Made?
« Reply #1 on: July 12, 2011, 01:25:24 AM »

... You would think that a sound would have to be made first, in order to initiate the feedback loop, but it seems that has never been the case.

I figure there's always enough ambient noise to start the 'chain reaction'. Maybe a system's hiss is enough too ?
Interesting question ...

w/r
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Chris Carpenter

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Re: Why Would a Monitor Feed Back Before a Sound is Made?
« Reply #2 on: July 12, 2011, 02:03:37 AM »

I figure there's always enough ambient noise to start the 'chain reaction'. Maybe a system's hiss is enough too ?
Interesting question ...

w/r

That is correct. Unless you're in space, you are going to encounter some ambient noise. Especially if you have a SR system cranked, at which point their is ambient noise created by the sound system itself.

http://en.wikipedia.org/wiki/Ambient_noise_level
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Chas Patate

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Re: Why Would a Monitor Feed Back Before a Sound is Made?
« Reply #3 on: July 12, 2011, 04:03:20 AM »

This applies in more situations than your typical performance hall.  Even in rehearsal spaces that are, well, not exactly silent, but pretty close.  Nothing but typical "silent" room noise, eg computer fans.   Using a Radio Shack SPL meter set on A weighting, fast response, measuring off-the-scale Lo, but occasionally flickering to 50 dB at the mic and at the monitor.

Add quiet monitor hiss that reads 51 dB at the grill just at the feedback threshold.  Yet measuring Lo (off scale) at the mic.

Are we seriously saying that is enough to create a feedback loop?  That is very disappointing. 

Curse you, laws of physics! 
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Bob Leonard

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Re: Why Would a Monitor Feed Back Before a Sound is Made?
« Reply #4 on: July 12, 2011, 08:03:11 AM »

Incorrect gain structure, EQ, effects or compressor settings will cause this. Bypass them all (if you use them) and bring them in line one at a time ensuring you have unity gain. Unity gain is when you set the gain for the channel strip and each additional step through the signal chain does NOT make the signal any louder when the next device is inserted. 0db front to back.
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Brad Weber

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Re: Why Would a Monitor Feed Back Before a Sound is Made?
« Reply #5 on: July 12, 2011, 08:07:57 AM »

This applies in more situations than your typical performance hall.  Even in rehearsal spaces that are, well, not exactly silent, but pretty close.  Nothing but typical "silent" room noise, eg computer fans.   Using a Radio Shack SPL meter set on A weighting, fast response, measuring off-the-scale Lo, but occasionally flickering to 50 dB at the mic and at the monitor.

Add quiet monitor hiss that reads 51 dB at the grill just at the feedback threshold.  Yet measuring Lo (off scale) at the mic.

Are we seriously saying that is enough to create a feedback loop?  That is very disappointing. 

Curse you, laws of physics!
Consider that feedback occurs when the acoustic loop gain exceeds unity, it is a factor of the electroacoustical path gain and not of the source level.  If the sound level hitting a mic from the sound system would be greater than the incoming level, then you have feedback regardless of what that incoming level is.
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John Roberts {JR}

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Re: Why Would a Monitor Feed Back Before a Sound is Made?
« Reply #6 on: July 12, 2011, 10:27:48 AM »

In theory if there was no sound energy at the feedback modal frequencies, there would be no sound to amplify, but the loop time for one pass around the loop is only mSecs. Using nice round numbers, with only +1 dB of positive loop gain and a 10 mSec path delay, the noise floor receives +100 dB of boost every second, so with that much boost the initial conditions don't matter very much. 

JR
 
[edit- Back when I was pimping FLS at trade shows I had a lot of practice making feedback on purpose. With the loop gain set right on the hairy edge of feedback (say a small fraction of a dB gain), it could take several seconds for the feedback to build up.  /edit]
« Last Edit: July 12, 2011, 10:33:36 AM by John Roberts {JR} »
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Re: Why Would a Monitor Feed Back Before a Sound is Made?
« Reply #7 on: July 12, 2011, 12:26:45 PM »

Why would a monitor feed back just by turning up the gain, before there was any sound made into a condenser mic? 
Cheers.

There is sound present, as the others have said.  Put on your headphones, solo the channel in question and listen.  Hear that?  That's the sound that's initiating the "loop".



« Last Edit: July 12, 2011, 12:34:44 PM by dick rees »
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Brian Ehlers

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Re: Why Would a Monitor Feed Back Before a Sound is Made?
« Reply #8 on: July 12, 2011, 12:56:33 PM »

There is sound present, as the others have said.  Put on your headphones, solo the channel in question and listen.  Hear that?  That's the sound that's initiating the "loop".
And that sound, whether acoustic or electronic, is random noise, meaining it consists of "all" frequencies, meaning it WILL excite the frequency with the highest loop gain.
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Tom Reid

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Re: Why Would a Monitor Feed Back Before a Sound is Made?
« Reply #9 on: July 12, 2011, 01:13:57 PM »

Why would a monitor feed back just by turning up the gain, before there was any sound made into a condenser mic?   It doesn't matter which way the mic is pointing with respect to the monitor from a polar standpoint, as soon as the gain is turned up, before a note is played or sung, there is sharp high frequency feedback.

What is the theory behind this?  You would think that a sound would have to be made first, in order to initiate the feedback loop, but it seems that has never been the case.

Thanks for considering this question - it has defeated my search skills.

Cheers.

Years and years ...and years of mixing and I knew what feedback was, how to prevent it, and why it was happening.  But I never got a theory rock solid into my head until I learned about Op Amps.  In an op amp the feedback loop is crucial to the function of the amp.  Plugging in possible gain through the simple op amp cicuit opened my eyes on the nature of an acoustic feedback (loop). 

Sometimes the answer is elsewhere.
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ProSoundWeb Community

Re: Why Would a Monitor Feed Back Before a Sound is Made?
« Reply #9 on: July 12, 2011, 01:13:57 PM »


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