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Author Topic: volume before feedback and number of mics  (Read 1025 times)

Chris Grimshaw

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Re: volume before feedback and number of mics
« Reply #10 on: April 16, 2018, 05:57:30 pm »

As noted above, adding another microphone in the same place with the same desk settings will put you 6dB closer to feedback.
However, when you add in the different frequency response curves of each different mic in each different position, and the associated phase curves of each, it becomes a complicated comb-filtery mess that averages out at 3dB per doubling of mics.

Chris
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Jerome Malsack

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Re: volume before feedback and number of mics
« Reply #11 on: April 16, 2018, 06:38:02 pm »

Something to consider for the second mic is to send this not to the speakers but to a recording that way you can set Gain with out feedback. 

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Steve Loewenthal

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Re: volume before feedback and number of mics
« Reply #12 on: April 16, 2018, 09:40:52 pm »

Mainly I was just interested in the original question (which I should have worded as "would the maximum SPL before feedback from the speaker" be any different) and not so much about advice on mic placement, using a lav or other wireless on the bride-groom-officiant, etc., as I have already read many threads on that in this forum. (Although I don't recall any mention in those threads of using a boundary mic.) I have done a wedding at this venue before, and if they do it similar to the last one I did there, an SM57 can be somewhat concealed above the couple.

If I am interpreting the answer from Ivan correctly, (and it seems to be the closest to what I was looking for), then it means that my assumption of "no difference in the max speaker spl before feedback" is correct.
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Steve Loewenthal

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Tim McCulloch

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Re: volume before feedback and number of mics
« Reply #13 on: April 16, 2018, 11:46:09 pm »

Mainly I was just interested in the original question (which I should have worded as "would the maximum SPL before feedback from the speaker" be any different) and not so much about advice on mic placement, using a lav or other wireless on the bride-groom-officiant, etc., as I have already read many threads on that in this forum. (Although I don't recall any mention in those threads of using a boundary mic.) I have done a wedding at this venue before, and if they do it similar to the last one I did there, an SM57 can be somewhat concealed above the couple.

If I am interpreting the answer from Ivan correctly, (and it seems to be the closest to what I was looking for), then it means that my assumption of "no difference in the max speaker spl before feedback" is correct.

Translation of Ivan:  If you have 1 mic at the maximum gain before feedback, and you add another mic with the same gain, you will have to reduce your mythical, coincident mics by -6dB in order to stay under feedback.

Chris G suggests that in the real world where microphones do not cohabit in the same space, less than perfect summation of waveforms means more like -3dB.

As JR often points out, feedback is a loop based on a physical distance or time of flight and enough regeneration to sustain the loop.

You can break the loop by changing time or magnitude.  Our usual approach is to go for magnitude and EQ the signal into submission.  Perhaps a few milliseconds added to the output might suffice instead or perhaps reduce the need for radical EQ.  Or when the acoustic output of the people is sufficient, we can actually turn it down below the verge of feedback.... ok, a guy can dream, right?  :D
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Steve Loewenthal

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Re: volume before feedback and number of mics
« Reply #14 on: April 17, 2018, 08:25:33 am »

So I guess I can think about this another way: Instead of my 2 mythical coincident mics, I can substitute a single mic, and just turn up the input gain by 6 db and it should be identical to the 2 mics. Then I can just think of the problem where there is a series of gain stages starting from the input source, then mic preamp gain, then channel fader gain, then possibly a sub-group fader, master fader, amplifier sensitivity knob. I start with an input voltage and end up with an output voltage from the amplifier to the speaker.

Now thinking of it that way, it makes no difference where along that path any gain/amplification occurs. All that matters is what is that the maximum voltage that can come out of the last amplifier in the chain before the feedback loop starts. That voltage out of the last stage will be the same (no more, no less) regardless of the sensitivity of the input source, or anywhere in the chain where the gain occurs.
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Steve Loewenthal

"I'm, just the guy in a band that owns the PA and I'm trying to figure out how it works. (Been trying to learn somethin' about it for about 20 years and I hope somethin' learns me soon)"

DavidTurner

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Re: volume before feedback and number of mics
« Reply #15 on: April 17, 2018, 10:32:00 am »

I've been pondering this for a few minutes, think I know the answer, but want to ask those more knowledgeable.
Will the loudest volume before feedback be different using 2 mics vs 1. Assuming the same speaker placement and mic placement. (break the laws of physics for a moment and let both mics occupy the same space.)

The answer I come up with is that there will be no difference.

While at levels before feedback, given the same source, 2 mics might produce double the output of 1 due to perfect summing, I think this just gets me to the loudest volume before feedback a little earlier.

If the answer is actually different than I think, this might be useful.
BTW, what got me thinking about this is that I have a wedding DJ gig coming up, and will probably be placing a regular mic near the bride for the ceremony. While I have never found any amount of gain before feedback that was enough to overcome a shy bride, I just ride the fader and hope a few extra people can hear the vows.

PAG (potential acoustic gain) = 1/NOM (number of open mics) per Don Davis in “Audio Engineering” and John Ergle in “The  Microphone Handbook”


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Chris Grimshaw

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Re: volume before feedback and number of mics
« Reply #16 on: April 17, 2018, 11:45:07 am »

PAG (potential acoustic gain) = 1/NOM (number of open mics) per Don Davis in “Audio Engineering” and John Ergle in “The  Microphone Handbook”


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I suspect you mean it's proportional to 1/NOM, otherwise that equation fails a basic check of dimensional analysis.

Chris
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DavidTurner

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volume before feedback and number of mics
« Reply #17 on: April 17, 2018, 12:24:18 pm »

Not what I said, what they said. That’s why I quoted the source. It has been decades since I read those texts, but that is how I remember it. That equation really stuck with me. I believe they were speaking about Omni mics however.


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« Last Edit: April 17, 2018, 12:31:58 pm by DavidTurner »
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Stephen Swaffer

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Re: volume before feedback and number of mics
« Reply #18 on: April 17, 2018, 01:05:42 pm »


Now thinking of it that way, it makes no difference where along that path any gain/amplification occurs. All that matters is what is that the maximum voltage that can come out of the last amplifier in the chain before the feedback loop starts. That voltage out of the last stage will be the same (no more, no less) regardless of the sensitivity of the input source, or anywhere in the chain where the gain occurs.

What does matter is the attenuation of the source you intend to amplify by the distance it is from the microphone transducer.  Hence the reason you can usually get a lot more output volume from a quartet using 4 mics rather than sharing one mic (assuming correct mic usage!) -even though the system gbf is lower.

If everyone believed this discussion, microphone marketing people would be out of business.  I don't know how many times I have demonstrated this exact scenario, yet people still want to believe that they can buy a mic that will not feed back and will pick up farther than the next.
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Steve Swaffer

Mike Caldwell

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Re: volume before feedback and number of mics
« Reply #19 on: April 17, 2018, 01:20:57 pm »

My theory on in-correct mic use can be in part from the way people see microphones used in music videos, in the movies and by TV reporters.

All of those uses normally have no gain before feedback issues.
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