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Author Topic: Why not Aux Controlled Subs?  (Read 31024 times)

Stephen Gregory

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Re: Why not Aux Controlled Subs?
« Reply #30 on: January 15, 2014, 09:54:40 pm »

One of the reasons against sub on aux might be the fact that the acoustical x-over point changes. Usually the phase response of a sub -> low driver is only matched at 1 point. As the frequency rises or drops the phase won't match.

When both speakers are driven with equal level, the acoustical x-over doesn't change and therefore the summation at x-over stays good (+6 dB when using LR) to keep a flat response.

Example: Raising the sub level without adjusting the phase response to match up might lead to cancellations and thus a reduction in perceived level / sound quality.

I guess I'm still not understanding.  How does altering the level of the sub affect the amount of cancellation between the sub and tops? I would have thought any phase cancellation frequencies are going to be there no matter what levels you run.  I guess the cancellation might become more obvious if you boost the sub, but surely it was still there before you boosted. Levels don't affect timing/phase/cancellation do they?

I totally take Tom's point about aux-fed subs as an "effect".  That is what I'm doing, and I quite like it.  It's probably not a classy way to handle the low end, but it can work to a first approximation (our tops go down to 52Hz and the subs to 48 - not much in it).
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Sean Chen

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Re: Why not Aux Controlled Subs?
« Reply #31 on: January 15, 2014, 10:58:34 pm »

I guess I'm still not understanding.  How does altering the level of the sub affect the amount of cancellation between the sub and tops? I would have thought any phase cancellation frequencies are going to be there no matter what levels you run.  I guess the cancellation might become more obvious if you boost the sub, but surely it was still there before you boosted. Levels don't affect timing/phase/cancellation do they?

I totally take Tom's point about aux-fed subs as an "effect".  That is what I'm doing, and I quite like it.  It's probably not a classy way to handle the low end, but it can work to a first approximation (our tops go down to 52Hz and the subs to 48 - not much in it).

I think an improperly setup of aux fed or bus fed sub would introduce phase cancellation above and below the Xover frequency, if the slopes on both sides are not steep enough, and delay is not set right. Having said that, using a very steep LPF on subs would pretty much annihilate this issue.

On the other hand, instead of using bus fed sub or aux fed sub, if an older analog 2nd order crossover is used to separate LR mix to tops and subs, microphone signals will still show up in subs around Xover point. This is much less noticeable if the system crossover is 4th order or steeper.

For me, since my gears do not have steep crossover, (have not quite made the digital leap), I would rather aux/bus feed the subs, and live with a little phase incoherence around 80 Hz for the bass and kick channels, which my ears do not find that disturbing. That sure beats having my subs leaking semi-attenuated 160 Hz fundamental content from vocal mics.
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Jeff Bankston

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Re: Why not Aux Controlled Subs?
« Reply #32 on: January 15, 2014, 11:36:13 pm »

I think an improperly setup of aux fed or bus fed sub would introduce phase cancellation above and below the Xover frequency, if the slopes on both sides are not steep enough, and delay is not set right. Having said that, using a very steep LPF on subs would pretty much annihilate this issue.

On the other hand, instead of using bus fed sub or aux fed sub, if an older analog 2nd order crossover is used to separate LR mix to tops and subs, microphone signals will still show up in subs around Xover point. This is much less noticeable if the system crossover is 4th order or steeper.

For me, since my gears do not have steep crossover, (have not quite made the digital leap), I would rather aux/bus feed the subs, and live with a little phase incoherence around 80 Hz for the bass and kick channels, which my ears do not find that disturbing. That sure beats having my subs leaking semi-attenuated 160 Hz fundamental content from vocal mics.
i use the Ashly XR###1 crossovers with the 24db/oct LR crossover network. i can turn a woofer amp on and never hear a vocal and i cross at 130. i can turn a mid and high on and sound is crystal clear and verfy detailed and sounds like its the instrument and the singer without a pa. you gotta have a steep slope like the 24db.
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Mac Kerr

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Re: Why not Aux Controlled Subs?
« Reply #33 on: January 16, 2014, 04:15:54 am »

One of the reasons against sub on aux might be the fact that the acoustical x-over point changes. Usually the phase response of a sub -> low driver is only matched at 1 point. As the frequency rises or drops the phase won't match.

When both speakers are driven with equal level, the acoustical x-over doesn't change and therefore the summation at x-over stays good (+6 dB when using LR) to keep a flat response.

Example: Raising the sub level without adjusting the phase response to match up might lead to cancellations and thus a reduction in perceived level / sound quality.

It does change slightly, but unless the subs and the mains are located close together there is no way to make the phase correct at everywhere because of the geometry, plus, indoors the room response will be so complex with a grid of peaks and nulls due to reflections that it is not the same everywhere anyway.

If you get a better sound with aux fed subs, use them. Even with fairly steep HP filters on the input channels, the sound of vocals in the subs is far more degrading to the sound quality than a small shift in phase response between subs and mains to me.

Mac
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Bob Leonard

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Re: Why not Aux Controlled Subs?
« Reply #34 on: January 16, 2014, 07:33:52 am »

It does change slightly, but unless the subs and the mains are located close together there is no way to make the phase correct at everywhere because of the geometry, plus, indoors the room response will be so complex with a grid of peaks and nulls due to reflections that it is not the same everywhere anyway.

If you get a better sound with aux fed subs, use them. Even with fairly steep HP filters on the input channels, the sound of vocals in the subs is far more degrading to the sound quality than a small shift in phase response between subs and mains to me.

Mac

I agree 1000%.
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Jonathan Betts

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Re: Why not Aux Controlled Subs?
« Reply #35 on: January 16, 2014, 08:21:27 am »

I normally run subs left output, tops right output. Hard pan inputs you don't want in the subs, and center pan for bass, kick etc. During sound check,  simply center pan all inputs and see if it makes a difference. Depending on what board you are using there may be a +3db or so increase on the hard panned channels.
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Brad Weber

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Re: Why not Aux Controlled Subs?
« Reply #36 on: January 16, 2014, 09:43:32 am »

I guess I'm still not understanding.  How does altering the level of the sub affect the amount of cancellation between the sub and tops? I would have thought any phase cancellation frequencies are going to be there no matter what levels you run.  I guess the cancellation might become more obvious if you boost the sub, but surely it was still there before you boosted. Levels don't affect timing/phase/cancellation do they?
They levels, or at least the relative levels, do matter as far as the level or degree of cancellation and/or summation resulting.  For example, you only get full cancellation or summation if the interfering sources have the same relative levels, any difference in relative level will result in something between those extremes and the greater the difference in realtive level, the less the effects of the interference.
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Scott Carneval

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Re: Why not Aux Controlled Subs?
« Reply #37 on: January 16, 2014, 09:55:02 am »

I guess I'm still not understanding.  How does altering the level of the sub affect the amount of cancellation between the sub and tops? I would have thought any phase cancellation frequencies are going to be there no matter what levels you run.  I guess the cancellation might become more obvious if you boost the sub, but surely it was still there before you boosted. Levels don't affect timing/phase/cancellation do they?

Levels don't directly affect phase, but when you increase the level of the subs WITHOUT increasing the level of the mains you are shifting the acoustical crossover point.  The biggest thing to understand here is the difference between the ELECTRICAL crossover point and the ACOUSTICAL crossover point.  The electrical point is the value that you've entered into your DSP or chosen with the dial on your analog crossover.  But the acoustical point is the point at which the levels of the subs and mains cross, which is where the two speakers will acoustically 'sum' (or cancel). 

In the screenshots below I have chosen a 91hz L-R 24db electrical crossover.  As you can see, when the gain of both outputs is set to 0db the graphs do cross at 91hz.  But as I increase the level of the subs, the frequency at which the subs match the tops also increases.  At +3db it's barely noticeable, maybe 5hz or so.  At +6db, it's gotten worse, it looks like about 10hz (there is a mathematical way to calculate this given the 24db/octave slope of the crossover, but for our purposes we're just going by the graph on the DSP).  Just to illustrate the point, I took the subs to +20db, and now the acoustical crossover has shifted by what looks to be about 30hz.  If the two speakers aren't phase aligned at this frequency, you will experience cancellation somewhere in the room (you will also experience summation somewhere ELSE in the room).  But the reality is, unless all of your drivers are mounted in the same space, you're going to experience SOME cancellation SOMEWHERE in the room. 

As far as how this relates to aux-fed subs, if you set your aux master to the same level as your L/R master, and use post-fader sends, it should never be an issue.  If you desire more db out of your subs you should increase their gain in the DSP, and align your system to account for this.  This probably means lowering the sub electrical crossover point to achieve the desired acoustical crossover point.   
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Samuel Rees

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Re: Why not Aux Controlled Subs?
« Reply #38 on: January 16, 2014, 11:53:07 am »


Levels don't directly affect phase, but when you increase the level of the subs WITHOUT increasing the level of the mains you are shifting the acoustical crossover point.  The biggest thing to understand here is the difference between the ELECTRICAL crossover point and the ACOUSTICAL crossover point.  The electrical point is the value that you've entered into your DSP or chosen with the dial on your analog crossover.  But the acoustical point is the point at which the levels of the subs and mains cross, which is where the two speakers will acoustically 'sum' (or cancel). 

In the screenshots below I have chosen a 91hz L-R 24db electrical crossover.  As you can see, when the gain of both outputs is set to 0db the graphs do cross at 91hz.  But as I increase the level of the subs, the frequency at which the subs match the tops also increases.  At +3db it's barely noticeable, maybe 5hz or so.  At +6db, it's gotten worse, it looks like about 10hz (there is a mathematical way to calculate this given the 24db/octave slope of the crossover, but for our purposes we're just going by the graph on the DSP).  Just to illustrate the point, I took the subs to +20db, and now the acoustical crossover has shifted by what looks to be about 30hz.  If the two speakers aren't phase aligned at this frequency, you will experience cancellation somewhere in the room (you will also experience summation somewhere ELSE in the room).  But the reality is, unless all of your drivers are mounted in the same space, you're going to experience SOME cancellation SOMEWHERE in the room. 

As far as how this relates to aux-fed subs, if you set your aux master to the same level as your L/R master, and use post-fader sends, it should never be an issue.  If you desire more db out of your subs you should increase their gain in the DSP, and align your system to account for this.  This probably means lowering the sub electrical crossover point to achieve the desired acoustical crossover point.

This is what I thought I was brining up!
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Dave Bednarski

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Re: Why not Aux Controlled Subs?
« Reply #39 on: January 16, 2014, 02:55:46 pm »

I've tried to follow this... now getting dizzy.

With a pair of KW181s and 153s, am I best to let the cabinets handle the cross over... or do I continue to aux/bus feed the subs from my X32?

I started aux feeding the subs, when I had KSubs and felt I got a tad extra thump out of them in my mix.  Never gone back since upgrading the system... that said, only at higher volumes am I ever really thrilled with the system sound/response.  The low end I am chasing around. 
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Re: Why not Aux Controlled Subs?
« Reply #39 on: January 16, 2014, 02:55:46 pm »


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