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Author Topic: volume before feedback and number of mics  (Read 814 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

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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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A complex question is easily answered by a simple-easy to understand WRONG answer!

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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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