How NOT to hold a large number of open microphones backstage

D

Daniel Postilnik

Junior
Jan 14, 2011
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San Francisco, CA
Hi folks, I thought I would share an experience I had with a certain acoustical phenomenon that a number of JV people might not have had a chance to encounter.

I was working on a large show that required 20 wireless handheld microphones (Shure UHF-R series, SM58 capsules) to be open at all times due to the nature of the show.

The monitor engineer was working on getting all 11 mixes on stage EQ'd and stable for use by all of these microphones. While the microphones were being held backstage behind a curtain, he had some trouble getting rid of a low-level consistent "oooooooo" in a number of the wedges that sounded like the edge of low-mid frequency feedback even though no mic was near them. I was thinking "Wow, our GBF has really suffered from having this many open mics."

After a good amount of hacking at graphics with no results, he decided to take a peek backstage, where he discovered the problem. The microphones had all been lined up capsule-to-capsule, which apparently creates one very large, high sensitivity microphone.

I don't know the technical term for this phenomenon, but when the mics where flipped so they alternated capsule/butt, the low-level wailing in the monitors went away. The Varsity-level technicians who were handling the microphones didn't know not to do this (granted, this was a third-world Caribbean country), so I thought some of you on this forum might benefit from knowing about it.

Cheers!
 
Re: How NOT to hold a large number of open microphones backstage

Caleb Dick said:
Like a gate, but increasing gain reduction as level is decreased, rather than a hard mute threshold.
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Exactly. I routinely use the expander option of the gate/expander function on the StudioLive. Set the threshold where you need it on each mic channel. Keeps things cleaner without the shock of the open/closed response of a hard gate. Your 20 "open mics" then are not all open all the time, but only open when sound is present at the mic above your set threshold. Sort of a "poor man's auto-mixer".
 
Re: How NOT to hold a large number of open microphones backstage

If I understand correctly, the mics were lined up as:

!!!!!!!!!! (the dot being the capsule)

Doesn't this create a "reverse line array"?
 
Re: How NOT to hold a large number of open microphones backstage

Mercenary Audio - Microtech Gefell KEM 970 Line Array Microphone

Just like speaker line-array technology, the Gefell KEM-970 defies the inverse square distance rule whereby sound is normally attenuated by -6dB as one doubles the distance away from the source. The KEM-970 measures a mere –3dB drop in gain allowing greater freedom of ‘near to far’ movement.
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Chris
 
Re: How NOT to hold a large number of open microphones backstage

When needing to operate multiple open mics, in an acoustic space it is useful to have an automatic mixer in the mic paths, to reduce feedback sensitivity.

An AM using the Dugan gain sharing algorithm looks like one open mic as far as feedback is concerned.

JR
 
Re: How NOT to hold a large number of open microphones backstage

John Roberts said:
When needing to operate multiple open mics, in an acoustic space it is useful to have an automatic mixer in the mic paths, to reduce feedback sensitivity.

An AM using the Dugan gain sharing algorithm looks like one open mic as far as feedback is concerned.

JR
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You know this JR but a point of clarification for anyone else. The Dugan automixer or even the Shure SCM810 automixers are great as long as it is for speech only. They don’t work very well for music. I use them all the time for musical theater but only for the dialog parts.
 
Re: How NOT to hold a large number of open microphones backstage

Kevin Maxwell said:
You know this JR but a point of clarification for anyone else. The Dugan automixer or even the Shure SCM810 automixers are great as long as it is for speech only. They don’t work very well for music. I use them all the time for musical theater but only for the dialog parts.
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I made the ASSumption that 20 hand held mics would be used for speaking or perhaps solo singing parts.

Tracks should not suffer from acoustic feedback so don't need to be run through the AM.

Indeed AM generally has time-constants optimized for speech, I think Dan has done work to get better results with music, I never tried to since IMO that is a different problem, where you don't necessarily want the sources to alter the gain sharing (mix) with how loud they play.

JR
 
Re: How NOT to hold a large number of open microphones backstage

John Roberts said:
I made the ASSumption that 20 hand held mics would be used for speaking or perhaps solo singing parts.

Tracks should not suffer from acoustic feedback so don't need to be run through the AM.

Indeed AM generally has time-constants optimized for speech, I think Dan has done work to get better results with music, I never tried to since IMO that is a different problem, where you don't necessarily want the sources to alter the gain sharing (mix) with how loud they play.

JR
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I think we have talked in the past and I know you designed an automixer or was it auto mixing capabilities into a mixer for Peavey. So I know you know what you are talking about.

I was on the auto mixing for live sound panel with Dan at AES in NYC 2009. I talked to Dan the night before at dinner about the music issue. He had an automixer that had a music setting and it required a separate mic on stage (ambient) for it to work. To the best of my knowledge none of his newer models have music handling capabilities.

I have also never seen the advantages of trying to use an automixer for music but I just like to warn people so they don’t miss use it and then think that automixers don’t work.
 
Re: How NOT to hold a large number of open microphones backstage

Kevin Maxwell said:
I think we have talked in the past and I know you designed an automixer or was it auto mixing capabilities into a mixer for Peavey. So I know you know what you are talking about.

I was on the auto mixing for live sound panel with Dan at AES in NYC 2009. I talked to Dan the night before at dinner about the music issue. He had an automixer that had a music setting and it required a separate mic on stage (ambient) for it to work. To the best of my knowledge none of his newer models have music handling capabilities.

I have also never seen the advantages of trying to use an automixer for music but I just like to warn people so they don’t miss use it and then think that automixers don’t work.
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Yup as the song goes, "I know a little 'bout it"... (one patent and Peavey's first commercial AM design 15 years ago or so).

In classic web forum style, we are not really addressing the OP's comment, just spewing stuff we know. :-)

Namely holding a bunch of open mics back stage. In that case, flipping the polarity of every other mic, will create a first order cancellation of sufficiently distant/long audio wavelengths. HF room sound will not cancel as neatly but should be attenuated back stage.

JR
 
Re: How NOT to hold a large number of open microphones backstage

Scott Helmke said:
RF intermod - the closer the transmitters are to each other, the more it happens. Probably there was a lot of RF weirdness with all those mics stuck together.
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This is very often an issue, and happens whether the console channels are open or not. If you look at the RF spectrum while you pick up all the mics in one hand you will be amazed by what you see.

Mac
 
Re: How NOT to hold a large number of open microphones backstage

mackerr said:
This is very often an issue, and happens whether the console channels are open or not. If you look at the RF spectrum while you pick up all the mics in one hand you will be amazed by what you see.
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It's why we keep a bunch of metal bread pans handy for testing large systems. Things get too weird with all the mics just laid out on countertops.
 
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