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Author Topic: Anyone used a sub on stage to null house PA subs for a musician?  (Read 2873 times)

Denny Jagard

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Anyone have experience or advice about using a sub on stage to reduce house low frequency spill?  An upcoming tour has various house PAs, with a fixed position musician that is sensitive to low frequency.  If I place a sub near his position, feed it with the same signal I send the house subs, and  manipulate phase/time/tonality (all pass filter, delay, EQ),  I expect I should be able to achieve a substantial reduction in LF energy (just at that position).  Iím basically trying to do the Bose noise cancelling headphone trick with a subwoofer (but with static settings, since I wonít be able to monitor from his position during the concertÖ or maybe I could set up a Smaart mic, hmmm).  I donít know how much change to expect once an audience enters; I presume the main cancellation would be based on the direct measurements from the rear of the subs to the musician's area, but perhaps the audience soaking up lots of reflections, changing the temp and humidity and effectively changing the room dimensions, would affect more than I'm expecting. Thanks in advance for opinions and advice!

EDIT:
Additional notes for clarity and additional questions:

I should mention that the musician is seated.  And I'm only trying to bring down the impact of the house subs in his position a few notches (any reduction is better than having to turn down the house PA that amount). I don't expect the stage sub to be loud enough to have much impact at other musician locations.

For positioning, my understanding is the most effective stage sub position would be between the musician and the loudest source of LF.  Then the waves from stage sub would be travelling the same path as the house sub (similar to delay speakers, or a rear facing cardioid sub).

If the sub has to be behind the musician, crossfiring into the house LF source, I presume there would be a nulling sweet spot perhaps only a couple of feet (depending on wavelength) where the stage sub waves are effectively destructive to the house subs (and then similar nulls every cycle wavelength, but less destructive due to lower amplitude, and probably smeared by different relationships to multiple house subs).  Is my math accurate on that 2 foot window?  At about 90 hz, will the usable cancellation between crossfiring subs with inverted phase be at least a foot or two wide?  This won't work if he gets substantial low end boost every time he leans forward or backward...:)
« Last Edit: March 29, 2016, 11:48:50 am by Denny Jagard »
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Lee Buckalew

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Re: Anyone used a sub on stage to null house PA subs for a musician?
« Reply #1 on: March 28, 2016, 11:43:38 pm »

Anyone have experience or advice about using a sub on stage to reduce house low frequency spill?  An upcoming tour has various house PAs, with a fixed position musician that is sensitive to low frequency.  If I place a sub near his position, feed it with the same signal I send the house subs, and  manipulate phase/time/tonality (all pass filter, delay, EQ),  I expect I should be able to achieve a substantial reduction in LF energy (just at that position).  Iím basically trying to do the Bose noise cancelling headphone trick with a subwoofer (but with static settings, since I wonít be able to monitor from his position during the concertÖ or maybe I could set up a Smaart mic, hmmm).  I donít know how much change to expect once an audience enters; I presume the main cancellation would be based on the direct measurements from the rear of the subs to the musician's area, but perhaps the audience soaking up lots of reflections, changing the temp and humidity and effectively changing the room dimensions, would affect more than I'm expecting. Thanks in advance for opinions and advice!

I have not tried this myself but I don't believe that this would be as effective as you are hoping.

This will not only cancel (some frequency or frequencies) at the musicians location but also at other locations where the wavelength and time offset create the proper phase shift.  However, it may not achieve your desired results at the musician location depending on the starting distance from PA subs to your sub.  As distance between the PA sub(s) and the "cancellation" sub changes the bandwidth of the cancellation will change.
 
Also, if house subs are split, your sub placement will have to be precisely located equidistant from them or you will create dissimilar effects on one compared to the other.

Another possible issue with greater distance between PA subs and your proposed "cancelling" sub is the movement of the "sensitive" musician.  If he/she does move around the stage they would experience areas of not only cancellation but also partial cancelation, addition, partial addition, etc.  The changing bass may create more of an issue than the PA subs on their own (or not, to borrow two words from Ivan, it depends). 

If you had a way to create true directional sub arrays it would help but I think that what you are suggesting will create even more problems.

Now I'll be curious to hear if anyone has tried this (other than for cancellation of room modes in smaller rooms) and if they achieved a positive outcome.

Lee
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Keith Broughton

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Re: Anyone used a sub on stage to null house PA subs for a musician?
« Reply #2 on: March 29, 2016, 07:04:11 am »

I think it might just work.
I recently had a situation where I had a sub for the percussion player (yes,I know...) and simply could not get enough punch for him.
The sub for the drummer was killer!
Turned out , the prec sub was being cancelled out by the mains bottom end.
I didn't have any processing , or time,available so flipped polarity and moved the sub and finall got some bottom end.
So, yes, I think your idea will work:)
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Denny Jagard

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Re: Anyone used a sub on stage to null house PA subs for a musician?
« Reply #3 on: March 29, 2016, 11:46:29 am »

I edited the original post for clarity and additional questions.
« Last Edit: March 29, 2016, 11:50:12 am by Denny Jagard »
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Lee Buckalew

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Re: Anyone used a sub on stage to null house PA subs for a musician?
« Reply #4 on: March 29, 2016, 11:54:47 am »

I edited the original post for clarity and additional questions.

With a stationary position it may work well enough for your needs.  Calculations will help you to determine this but the variations in room layout (distance to hard walls, ceilings, etc.) may have a greater impact room to room than the musician moving a bit. 

Lee
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Art Welter

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Re: Anyone used a sub on stage to null house PA subs for a musician?
« Reply #5 on: March 29, 2016, 01:26:10 pm »

1)Anyone have experience or advice about using a sub on stage to reduce house low frequency spill? 

2)For positioning, my understanding is the most effective stage sub position would be between the musician and the loudest source of LF.  Then the waves from stage sub would be travelling the same path as the house sub (similar to delay speakers, or a rear facing cardioid sub).

3)If the sub has to be behind the musician, crossfiring into the house LF source, I presume there would be a nulling sweet spot perhaps only a couple of feet (depending on wavelength) where the stage sub waves are effectively destructive to the house subs (and then similar nulls every cycle wavelength, but less destructive due to lower amplitude, and probably smeared by different relationships to multiple house subs).  Is my math accurate on that 2 foot window?  At about 90 hz, will the usable cancellation between crossfiring subs with inverted phase be at least a foot or two wide?  This won't work if he gets substantial low end boost every time he leans forward or backward...:)
Denny,

1)I read Ultrasound at one point did this for Phil Lesch of the Grateful Dead back in the 1980s, though I could not find any details of it in a quick search. As they primarily were doing arenas, the set up was pretty repeatable, one hockey rink is pretty much the same as the next.

2)Low frequency is pretty well omnidirectional, so the location of the cancellation sub won't make much difference, other than the proximity to the bass player. The closer the cancellation sub to the player, the less level needed.

3)Realistically you probably won't achieve much more than an octave of cancellation due to the complex phase arrangements of the direct and reflected sound from multiple sub locations.
Assuming you could cancel for 1/4 wavelength at 90 Hz from a single point source sub, you could expect around a 3 foot area of cancellation:
1130/90=12.55 feet/4=3.1 feet

Good luck!

Art
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David Buckley

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Re: Anyone used a sub on stage to null house PA subs for a musician?
« Reply #6 on: March 29, 2016, 04:54:46 pm »

Given that we've all seen what happens when you get subs out of phase, I suspect this must have a chance of working a bit, but I suspect the difference between noise cancelling headphones and bunging a sub by the muso is the processing.  As Art noted, the acoustic properties will limit the effectiveness, but I have a theory that if you could get a soundweb or something similar similar, you could assemble a patch of bandpass filters and delays, and tune (smaart?) the delays to work well at spot frequencies over a reasonable frequency range.

A couple of decades back I read an article about using several KW of subs to counteract the low frequency rumble  that comes out of gas turbines at power stations, similar sort of idea.
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Re: Anyone used a sub on stage to null house PA subs for a musician?
¬ę Reply #6 on: March 29, 2016, 04:54:46 pm ¬Ľ


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