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Author Topic: Polarity reversal for live sound  (Read 14103 times)

Hugh Brock

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Polarity reversal for live sound
« on: April 08, 2011, 03:37:54 pm »

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
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Chris Davis

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Re: Polarity reversal for live sound
« Reply #1 on: April 08, 2011, 04:43:37 pm »

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

Some people just make inline XLR "adapters" for this.  It only needs to be a very short cable.
Most analog Allen & Heath boards have this feature.
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Ivan Beaver

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Re: Polarity reversal for live sound
« Reply #2 on: April 08, 2011, 05:58:03 pm »

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

Some people just make inline XLR "adapters" for this.  It only needs to be a very short cable.
Most analog Allen & Heath boards have this feature.
Or you can get barrel connectors with a male XLR on one end and a Fmale on the other and wire so pins 2 and 3 are swapped at one connector-NOT BOTH!
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Dave Dermont

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Re: Polarity reversal for live sound
« Reply #3 on: April 08, 2011, 06:06:22 pm »

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

A lot of analog mixers have polarity switches.

Exactly how does this address feedback issues?
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John Roberts {JR}

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Re: Polarity reversal for live sound
« Reply #4 on: April 08, 2011, 06:47:37 pm »

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

A lot of analog mixers have polarity switches.

Exactly how does this address feedback issues?
Feedback occurs when the system gain is more than unity, at one or more of a series of potential feedback nodes. The location and spacing of these potential feedback frequencies is a product of distances the physical sound wave must travel (and speed of sound in air). Flipping the polarity, shifts all the nodes up or down by one half the spacing between nodes.

In practice this "could' help if a system has a narrow peak near one of the potential feedback nodes, and flipping the polarity shifts it away. However, just as likely the feedback nodes could shift closer to a peak.

Note: this model is true for simple feedback dominated by one mic and one speaker... more complex systems with multiple sources, and/or multiple mics will have more potential feedback modes.

JR
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Brad Weber

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Re: Polarity reversal for live sound
« Reply #5 on: April 08, 2011, 07:14:25 pm »

There are many analog mixers with polarity invert switches for each input.  I would be a bit wary of putting something in line where you could not quickly and easily tell if an inout's polairty was inverted or not.
 
I am curious as the situations you are dealing with where inverting the polarity of some mics would be expected to provide that much improvement in gain before feedback.  I understand the concept that if you wire two mics out of polarity then in theory any noise or sound picked up by both is cancelled out in the resulting mix but there seem to be two aspects that would greatly limit the practical application and results.  One is that this would affect any signal common to both mics.  It would not discriminate between wanted and unwanted signal, to it two singers, one at each mic, hitting the same note and the sound from a monitor hitting both mics would be affected the same in terms of being cancelled in the resulting mix.  The other is that actual cancellation only occurs if the signals are exactly the same; the same relative phase, the same level, etc.  There would probably be very few cases where that would actually be the case and as JR said, it may tend more to change the problems than to eliminate them.  So what are the specifics of the application?
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Hugh Brock

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Re: Polarity reversal for live sound
« Reply #6 on: April 08, 2011, 09:24:41 pm »

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

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Dave Dermont

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Re: Polarity reversal for live sound
« Reply #7 on: April 08, 2011, 09:30:06 pm »



Thanks guys.

How is it that I have either never learned this, or if I did learn it, I forgot it?
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Ben Johnson

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Re: Polarity reversal for live sound
« Reply #8 on: April 08, 2011, 09:34:05 pm »

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.

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

It probably wouldn't be hard to have a tiny project box, or even a barrel, with a double pole double throw (DPDT) switch in it.  You'd get the polarity reversed as needed, although you'd still want to mute the channel momentarily, just while flipping the switch (particularly with phantom powered mics).  But you'd have to mute it for less time than it would take to plug in a barrel-style device.

But without a soundcheck, or knowing which channels will be problematic, you'd have to have one on each channel.  That's probably not worth it.

Ben
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thomas jones

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Re: Polarity reversal for live sound
« Reply #9 on: April 09, 2011, 12:27:11 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...
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Re: Polarity reversal for live sound
« Reply #9 on: April 09, 2011, 12:27:11 am »


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