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Author Topic: volume before feedback and number of mics  (Read 810 times)

Steve Loewenthal

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volume before feedback and number of mics
« on: April 15, 2018, 07:15:46 pm »

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.
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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)"

Ivan Beaver

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Re: volume before feedback and number of mics
« Reply #1 on: April 15, 2018, 07:24:51 pm »

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.
The more open mics you have, the lower the level into each mic before feedback occurs.

Consider this (of course ignoring actual physics).

Let's say you have 1 mic and a speaker, and can add 100dB of gain before feedback.

If you add a second mic, in the same position, then the voltage from both mic combined will be 6dB greater.  This is because you now have double the voltage out of them combined.

So you will have to reduce each mic by 6dB in order to maintain the same 100dB of gain.

So each mic would be at 94dB now.

This is basically the way a Dugan auto mixer works.  As more mics are opened up, the gain of each one is reduced by the same amount of "offsetting" mic gains that are added.
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Ivan Beaver
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Bob Faulkner

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Re: volume before feedback and number of mics
« Reply #2 on: April 15, 2018, 09:16:01 pm »

In a nut-shell, for each doubling of open mics you have, this will reduce the overall system gain before feedback by 3db.
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John Roberts {JR}

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Re: volume before feedback and number of mics
« Reply #3 on: April 15, 2018, 11:20:03 pm »

Google "NOM" number of open mics.....

JR
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Ivan Beaver

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Re: volume before feedback and number of mics
« Reply #4 on: April 16, 2018, 07:40:54 am »

In a nut-shell, for each doubling of open mics you have, this will reduce the overall system gain before feedback by 3db.
Theoretically it would be 6dB.  Because when you double the voltage (mic input into the system) you get a 6dB gain.

But in reality it will vary, because of the physical location of the mics to the speakers, cancellations (lack of total summation) etc.

But 6dB would the the greatest addition you could get (assuming mics of equal output and in the same position)
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Mike Caldwell

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Re: volume before feedback and number of mics
« Reply #5 on: April 16, 2018, 08:00:06 am »



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.

As for your wedding service, I will usually put a wireless lapel on the groom and the pastor, between one of those mics I can usually use one to pick up the bride a little bit/enough to get by. It takes a little fader riding. On a few occasions I have actually had a mic on the bride as well.

I'm not sure they will like a mic stuck on a stand between the bride and groom in all the wedding pictures.
« Last Edit: April 16, 2018, 01:20:59 pm by Mike Caldwell »
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Tim McCulloch

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Re: volume before feedback and number of mics
« Reply #6 on: April 16, 2018, 08:34:06 am »

Hide a wireless lavaliere mic in her bouquet, on the kneeling bench, in the officiant's script binder....
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Don T. Williams

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Re: volume before feedback and number of mics
« Reply #7 on: April 16, 2018, 11:03:51 am »

Sometimes there just seems to be no place a lavalier will work, but a boundary layer (PZM) mic might work well.  I've been able to get much more level (gain before feedback) than I expected using these in place of lavaliers in many situations.  If it has to be wireless, it can get a little complex because of the need for phantom power, but a battery operated phantom power supply can solve that problem. 

Forum members - has anyone tried to operate (as an example) a Beta 91 using a Shure wireless transmitter?  I've never tried this but . . . 

I think Radio Shack had a battery powered PZL that was highly respected for its performance vs. cost.  Might be available on EBay.
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Mike Caldwell

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Re: volume before feedback and number of mics
« Reply #8 on: April 16, 2018, 01:25:35 pm »

I have a lapel mic that I spray painted the mic body, the cable and clip white with some flexible spray paint for plastic.
Put on a white windscreen and you have a bride mic, sometimes you have to get creative on the body pac placement.
« Last Edit: April 16, 2018, 07:54:25 pm by Mike Caldwell »
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Scott Olewiler

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Re: volume before feedback and number of mics
« Reply #9 on: April 16, 2018, 05:23:43 pm »

Sometimes there just seems to be no place a lavalier will work, but a boundary layer (PZM) mic might work well.  I've been able to get much more level (gain before feedback) than I expected using these in place of lavaliers in many situations.

Don,

Typically positioned where, on the ground, on the podium podium?

Most of my wedding  rentals are set-up  and go and I hate leaving open lapel mics when I am not going to be on site to monitor.  Boundary mic might be good alternative.
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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

"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)"

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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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.

DavidTurner

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Re: volume before feedback and number of mics
« Reply #20 on: April 17, 2018, 02:48:04 pm »

Here is a copy/paste from the current version of Sound System Engineering:
If we raise two microphones to the same level in a reinforcement system, we will have to reduce the overall gain 3 dB to avoid the system going into feedback (remember how decibels combine). Since each microphone is sampling the sound field, every time you double the number of open microphones (NOM), we will have to lower the gain 3 dB, Fig. 14-3. Therefore the loss in gain caused by more than one open microphone is: NOM (in dB) = 10logNOM .

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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David Morison

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Re: volume before feedback and number of mics
« Reply #21 on: Yesterday at 02:38:19 pm »

Here is a copy/paste from the current version of Sound System Engineering:
If we raise two microphones to the same level in a reinforcement system, we will have to reduce the overall gain 3 dB to avoid the system going into feedback (remember how decibels combine). Since each microphone is sampling the sound field, every time you double the number of open microphones (NOM), we will have to lower the gain 3 dB, Fig. 14-3. Therefore the loss in gain caused by more than one open microphone is: NOM (in dB) = 10logNOM .

The crucial difference between this and what you posted previously is that you've now included the qualifier "loss in" before the word gain - previously you said the gain itsself was equal to 1/NOM.

Not the same thing at all ;)
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DavidTurner

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Re: volume before feedback and number of mics
« Reply #22 on: Yesterday at 03:00:39 pm »

The crucial difference between this and what you posted previously is that you've now included the qualifier "loss in" before the word gain - previously you said the gain itsself was equal to 1/NOM.

Not the same thing at all ;)

I don’t get your point. What I read in this is that for each doubling of open microphones there is a 3dB loss in potential acoustic gain - in other words, 1/2 the available acoustic power.



Nonetheless, read it how you like. I was just quoting something I read years ago.


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