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Author Topic: Polarity reversal for live sound  (Read 14080 times)

Jon R Smith

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Re: Polarity reversal for live sound
« Reply #10 on: April 09, 2011, 01:18:00 am »

I have to mix live for small acoustic acts, often with no soundcheck, but my mixer doesn't have the ability to reverse polarity on each channel.

Any suggestions for a way to do this on the fly in case feedback issues arise and I want to try reversing the polarity without muting the channel?

I am imagining a box between the snake and the mixer where I could flip a switch.

Conversely, any analog mixers out there with this built in?

Thanks in advance,
Hugh

I've used it with some success on bluegrass band that will use a LDC for a vocal dance yet will insist on a SDC condenser right below it. What it helped me with was getting ride of the stereo reverb effect that mics too close together can cause.

Also, better GBFB which was what the OP was asking about. I think...  ;)
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John Roberts {JR}

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Re: Polarity reversal for live sound
« Reply #11 on: April 09, 2011, 08:33:01 am »

I got the idea from the Sound Reinforcement Handbook by Davis and Jones, section 11.11.2.  "Reversing the polarity can reduce feedback because, instead of having the direct and reinforced sound waveforms adding together and thereby exceeding the threshold of feedback, they subtract from one another."  This is me quoting a book, not claiming understanding.  If having the ability to do this isn't go to solve (m)any problems, let me know!

The application is for inexperienced acts, (and inexperienced me) starting with a rung out system. Then an artist who plays a reflective instrument (guitar, melodeon, even mando) who plays off the mic most of the time so the trim is turned up to get enough gain, suddenly steps forward and there's feedback in that one position.  So they step back in surprise and it goes away.  Or sometimes they play so the instrument isn't reflecting sound from the monitors into the mic, and then shift the instrument position so there is feedback.  So my thought was since there is position-dependent feedback, polarity reversal might be useful.

Hugh

Bzzt... not to quibble but the book is presenting an over simplified version of reality. For the polarity reversal to create a cancellation of the reinforced signal from the direct they would need to be coincident in time. In the real world, the sound coming from the speakers must travel over several feet before reaching the mic, which delays the signal X mSec. The result is some frequencies will be in phase and some will be out of phase due to the speed of sound in air and the wavelength of the frequencies.  There will be multiple peaks and dips as the fixed delay creates different amounts of phase shift for different frequencies, and higher frequencies will rotate through being in phase several times.

Polarity inversion, shifts where these peaks and dips occur by one half wavelength. The former peaks become dips and vice versa. Feedback occurs where these peaks coincide with system loop gain greater than unity. For a perfectly flat system, you will get similar feedback in both positions just at different frequencies. For a system with narrow response peaks, this could help, or make things worse, depending on where the peaks end up. 

Yes position matters, wrt moving closer toward or away from a mic, in different ways. The meat sock standing at the mic alters the loop gain/frequency response of the sound being picked up by the mic from the speakers. For best results ring out the system with meat socks in place. 

JR
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Tom Young

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Re: Polarity reversal for live sound
« Reply #12 on: April 09, 2011, 09:03:18 am »

Brad; The venerable Grateful Dead used two mics at each vocal position (out of phase) while using the "Wall of Sound". The hundreds of drivers were actually above and behind the band so feedback was an early problem. Then their combined genius came up with the out of phase idea and it worked for many years at all types of venues. Sometimes I'd rather be lucky than smart...

First of all, the mic's were polarity inverted, not "out of phase". There is a big difference.

They were also spaced 2-3 inches to accomplish the best phase characteristics.

Secondly; this technique was (overall) not successfull in actual use. In order for this to work, the vocalist must stay directly on axis to one of the 2 mic's. Trying to get a bunch of free-spirited musicians who are also tripping their brains out to "obey the rules" was an exercise in futility.

(search "Grateful Dead noise cancelling microphone" on Google images)

and also see:
http://www.soundonsound.com/sos/jul10/articles/qa0710_4.htm

There are two cases that I know of where noise cancelling microphones do have some degree of success. One was developed by Lou Burroughs (EV) for jet pilots (so they could communicate with one another and the tower while in the midst of all the jet noise) and the 2 mic's are built into the helmet's chin strap, so the relative position of the primary mic to the mouth never changes. I believe such mic's are still in use for pilots of various aircraft but using headsets.

The other is the Crown Differoid mic which is no longer in production. See:
http://www.crownaudio.com/pdf/mics/136368.pdf
« Last Edit: April 09, 2011, 09:34:53 am by Tom Young »
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Mac Kerr

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Re: Polarity reversal for live sound
« Reply #13 on: April 09, 2011, 09:21:48 am »

The other is the Crown Differoid mic which is no longer in production. See:
http://www.crownaudio.com/pdf/mics/136368.pdf

It is hard to find, but the Crown CM-311 is still listed on the website.

Noise canceling mics are also used in other communications headsets from companies like David Clark and Peltor.

Mac
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Tom Young

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Re: Polarity reversal for live sound
« Reply #14 on: April 09, 2011, 03:06:48 pm »

Bzzt... not to quibble but the book is presenting an over simplified version of reality. ........... The meat sock standing at the mic alters the loop gain/frequency response of the sound being picked up by the mic from the speakers. For best results ring out the system with meat socks in place. 

JR

Agree 100%.  The Yamaha book remains a great entry-level book (talk about "legs"...... this book has been in print for, what, 25 years ?) but some of it is outdated and none of it is thorough. Not really a criticism, just a fact.

Ringing out mic's with no one standing in place is almost pointless. This is true for handheld mic's with rock/pop singers as much as it is for stationary and worn mic's. Both the mic's (proximity effect and presence peak excepted) and loudspeakers may look flat/fairly smooth when measured anechoically. Once in place they react with one another, room boundary surfaces, furniture (podium/pulpit, etc), the "meat socks" who use them *and* sundry adornment such as Raybans and hats with brims. There goes your smooth response.

No one said it would be easy.
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Dave Neale

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Re: Polarity reversal for live sound
« Reply #15 on: April 11, 2011, 01:27:55 am »

Bzzt... not to quibble but the book is presenting an over simplified version of reality. For the polarity reversal to create a cancellation of the reinforced signal from the direct they would need to be coincident in time. In the real world, the sound coming from the speakers must travel over several feet before reaching the mic, which delays the signal X mSec. The result is some frequencies will be in phase and some will be out of phase due to the speed of sound in air and the wavelength of the frequencies.  There will be multiple peaks and dips as the fixed delay creates different amounts of phase shift for different frequencies, and higher frequencies will rotate through being in phase several times.

Polarity inversion, shifts where these peaks and dips occur by one half wavelength. The former peaks become dips and vice versa. Feedback occurs where these peaks coincide with system loop gain greater than unity. For a perfectly flat system, you will get similar feedback in both positions just at different frequencies. For a system with narrow response peaks, this could help, or make things worse, depending on where the peaks end up. 

Yes position matters, wrt moving closer toward or away from a mic, in different ways. The meat sock standing at the mic alters the loop gain/frequency response of the sound being picked up by the mic from the speakers. For best results ring out the system with meat socks in place. 

JR


I got lucky with a box accordion that had a beta 98 stuck in it last night (not my mic choice),  and sometimes interesting things happen with acoustic guitars or pairs of electric guitars, bass, etc...

And of course very useful for making multiple mics  on the same source play nice, or keeping the kick drum fat in monitor world. (thank you Dave Rat)

But playing with polarity has never done a damn thing for vocals in wedges for me other than make things immediately weird.

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Hugh Brock

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Re: Polarity reversal for live sound
« Reply #16 on: April 11, 2011, 02:23:11 pm »

Thanks to all who replied.  My conclusion is that it's not worth worrying about polarity reversal as a way to improve GBF.

Cheers,
Hugh
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Tim Padrick

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Re: Polarity reversal for live sound
« Reply #17 on: April 24, 2011, 01:06:31 am »

With acoustic instruments that are fed into the monitors, reversing the polarity may reduce feedback, but it also reduces the perceived SPL level, owing to less summing with the acoustic sound of the instrument at the player's ears.  So, you have to turn the instrument up in the monitor, and you are back where you started feedback wise (and with reduced sound quality).
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Brad Weber

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Re: Polarity reversal for live sound
« Reply #18 on: April 24, 2011, 10:22:32 am »

With acoustic instruments that are fed into the monitors, reversing the polarity may reduce feedback, but it also reduces the perceived SPL level, owing to less summing with the acoustic sound of the instrument at the player's ears.
There is not less summing occurring, just different summing.  Inverting polarity does not change the levels, only the resulting frequency based interactions such that frequencies that were cancelled due to the relative phase of the signals now become reinforced and vice versa.
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Dave Dermont

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Re: Polarity reversal for live sound
« Reply #19 on: April 24, 2011, 10:40:24 pm »

With acoustic instruments that are fed into the monitors, reversing the polarity may reduce feedback, but it also reduces the perceived SPL level, owing to less summing with the acoustic sound of the instrument at the player's ears.
There is not less summing occurring, just different summing.  Inverting polarity does not change the levels, only the resulting frequency based interactions such that frequencies that were cancelled due to the relative phase of the signals now become reinforced and vice versa.

I have heard of running the monitor system's polarity opposite that of the mains, but that's something different than what's being talked about here.

So, do you really want to roll the dice and change the polarity of an input while the show is going on when you don't really know what's going to happen?

Of course, I am all for trying stuff during sound check.

I find that there are two distinct challenges when it comes to stage monitors and high volumes. One is when you need an acoustic instrument really loud, but still want it to sound like an acoustic instrument. The other is when you need to get a not-quite-so-powerful vocalist up over a raging Marshall stack.

The one similarity is that to do it correctly, the sound check needs to have all the same parts as the performance.
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Dave Dermont

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Re: Polarity reversal for live sound
« Reply #19 on: April 24, 2011, 10:40:24 pm »


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