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Author Topic: Why not Aux Controlled Subs?  (Read 29770 times)

g'bye, Dick Rees

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Re: Why not Aux Controlled Subs?
« Reply #40 on: January 16, 2014, 03:04:22 pm »

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.

THIS is what I was waiting for.  I couldn't remember the threads where this has been brought up, but knew someone would chime in.  And thanks for the visuals.  That's the first time anyone has posted such a graphic representation and it's very helpful to be able to visualize what happens.

Thanks!!!
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Jerome Malsack

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Re: Why not Aux Controlled Subs?
« Reply #41 on: January 16, 2014, 03:14:14 pm »

With the aux fed subs and your statement that a tad extra thump out of them, are you comparing the Drive rack or speaker processing with a dsp with installed limiter and compressor?   With the dsp processing to protect your speakers and provide eq,  compression, crossover and limiting. it will effect the dynamics.   With Aux fed Sub you will not have the dynamics processing causing some changes to be audible.  So you will have more of the dynamics and attack getting out of the sub.
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Mac Kerr

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Re: Why not Aux Controlled Subs?
« Reply #42 on: January 16, 2014, 03:22:41 pm »

THIS is what I was waiting for.  I couldn't remember the threads where this has been brought up, but knew someone would chime in.  And thanks for the visuals.  That's the first time anyone has posted such a graphic representation and it's very helpful to be able to visualize what happens.

Thanks!!!

It's good to understand what is happening, but the question I have is, so what? Unless your subs and mains are located close to each other, and you are phase aligning them with an FFT analyzer, and you're in an environment with no reflections, the phase response in the room is going to be more complex than you can account for, and not in phase to begin with.

With the aux fed subs and your statement that a tad extra thump out of them, are you comparing the Drive rack or speaker processing with a dsp with installed limiter and compressor?   With the dsp processing to protect your speakers and provide eq,  compression, crossover and limiting. it will effect the dynamics.   With Aux fed Sub you will not have the dynamics processing causing some changes to be audible.  So you will have more of the dynamics and attack getting out of the sub.

Why wouldn't you have the same dynamics and eq chain as the mains? Just because the subs come off an aux doesn't mean you don't use the same kind of processing on them. They still need the same type of crossover, eq, and dynamics, it is just through a different channel of processing.

Use the method that makes the best sound. For me that is aux fed subs, YMMV.

Mac
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Rob Spence

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Re: Why not Aux Controlled Subs?
« Reply #43 on: January 16, 2014, 03:33:17 pm »

Why wouldn't you have the same dynamics and eq chain as the mains? Just because the subs come off an aux doesn't mean you don't use the same kind of processing on them. They still need the same type of crossover, eq, and dynamics, it is just through a different channel of processing.

Use the method that makes the best sound. For me that is aux fed subs, YMMV.

Mac

This is why my DSP has 3 (or more) inputs. L, R, Sub.  It doesn't care if the sub input is coming from a bus or an aux (or even low passing it out of a L+R sum).
 
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Scott Carneval

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Re: Why not Aux Controlled Subs?
« Reply #44 on: January 16, 2014, 03:53:10 pm »

It's good to understand what is happening, but the question I have is, so what? Unless your subs and mains are located close to each other, and you are phase aligning them with an FFT analyzer, and you're in an environment with no reflections, the phase response in the room is going to be more complex than you can account for, and not in phase to begin with.

Use the method that makes the best sound. For me that is aux fed subs, YMMV.

Mac

Right, that's sort of what I was getting at when I said 'unless your mains and subs occupy the same space…'.  I can't agree more with your last sentence.  If used PROPERLY, you'll usually get the best sound by using aux/bus/group (take your pick) fed subs.
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g'bye, Dick Rees

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Re: Why not Aux Controlled Subs?
« Reply #45 on: January 16, 2014, 04:25:01 pm »

It's good to understand what is happening, but the question I have is, so what? Unless your subs and mains are located close to each other, and you are phase aligning them with an FFT analyzer, and you're in an environment with no reflections, the phase response in the room is going to be more complex than you can account for, and not in phase to begin with.


Mac...

I should have highlighted the part that concerned me.  It was the potential shifting of the acoustic crossover point when boosting subs output.  The phase portion got glossed over as I went for what interested me most.  Seeing the sloped filter creep up as the sub level increased just brought things more into focus for me.  But thanks as always for the clarification.
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Greg_Cameron

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Re: Why not Aux Controlled Subs?
« Reply #46 on: January 16, 2014, 04:27:08 pm »

Aux fed subs all the way. Occasionally with bass guitars where the low notes have a lot more energy in the subs than wanted, I'll dial down the sub send from detent. Does it shift the relative acoustic crossover point down? Sure. But who cares? If it sounds good, it is good. And for the folks that think 24dB/octave crossovers are relative brick wall for cutting off audio signal, try running a tone sweep through a single band pass with the other drivers turned off some time. Tons of signal above and/or below the knee frequency gets through with standard IR filters. I was shocked years ago when I ran tone sweeps and heard how much audio gets through the LR24 filters in most crossovers above and below the crossover points. A low pass filter at 80Hz on your subs is still allowing quite a bit of energy above that frequency to get into the subs. And a lot of the nasty plosives from vocal mics and various other stage wash lows will get into the subs too. The 12 or 18dB per octave HPFs on your desk on "non-aux sub" systems do allow audible amounts of LF gack into the subs. Heck, even the 24dB per octave HPFs on the Avid venues can, though they're a lot more effective than most.

I'm definitely a fan of not having stuff you don't want in the subs not be able to get there in the first place. It's not hard to do and the benefits are audible.
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Scott Olewiler

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Re: Why not Aux Controlled Subs?
« Reply #47 on: January 16, 2014, 05:02:50 pm »

THIS is what I was waiting for.  I couldn't remember the threads where this has been brought up, but knew someone would chime in.  And thanks for the visuals.  That's the first time anyone has posted such a graphic representation and it's very helpful to be able to visualize what happens.

Thanks!!!

Of course you'd have the same issue if you reached up and turned up the subs at your DSP or turned the sub amp up during a show. Once the system is balanced you should just leave it alone. If you need more thump there's a channel fader labeled "kick drum" for that. 

Of course it's key to actually balance your system:

Here's a telling story from a show I did last month:  Even though I use the same tops and amp for every show, I'll switch out my subs depending on the room, and then in a small room dial back my tops and use maybe one powered sub, which might vary from room to room.   Of course I "garage" referenced various combinations ahead of time so I pretty much know where to set everything dependent on what I bring in. so I get to this show, set up with only one powered sub and turn the dials where I think they should go; play some reference music at low volume (mistake #1) and tune it by ear (I don't use any audio tools other than an RTA app on my Ipad).

Weather was horrible, band was late and we rushed a sound check, basically just making sure all the mics worked.  Mix was surprisingly good from the start except for this low "wooom" that would occasionally rear it's ugly head. So I'm rolling some low end off one of the guitar cab mics thinking it picking up the sub( and probably was), backing down the drummer's monitor and the problem never manifests long enough to get a good bead on where it's coming from. (before anyone asks, yes I have HPF on all mics) So all night I'm just not happy with the low end, and think the kick drum is causing my problem but don't want to turn it down and lose my "thump".

 About half way thru the last set my helper says he thinks the (powered) sub woofer is turned up too high, walks up on stage, turns it down some, comes back to the board and turns the kick drum up. Mix instantly goes from good to great. And I'm standing there feeling less than smart. We forget how much kick is in the tops. Sometimes you don't need more volume in the subs, you just need more kick drum.

So basically I was plagued all night by what I now believe to be a summing problem somewhere above my crossover point because I had my subs too loud to begin with. I will never forget this.
« Last Edit: January 16, 2014, 05:05:31 pm by Scott Olewiler »
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g'bye, Dick Rees

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Re: Why not Aux Controlled Subs?
« Reply #48 on: January 16, 2014, 05:18:34 pm »

Of course you'd have the same issue if you reached up and turned up the subs at your DSP or turned the sub amp up during a show.

It's not an "issue", it's just nice to have a visual representation of what happens when.
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Roland Clarke

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Re: Why not Aux Controlled Subs?
« Reply #49 on: January 16, 2014, 05:48:34 pm »

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.

I would have thought this was true if you "boosted" the level of the sub relative to the mains, however, if they were obstensively at unity, sure this would be no different?
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Re: Why not Aux Controlled Subs?
« Reply #49 on: January 16, 2014, 05:48:34 pm »


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