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Old 17th May 2011, 08:13 PM   #1
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Default Mix-and-match subwoofering

This whole topic is likely to set the teeth of engineer-purists on edge. And that is because the theory is inherently heteroskadastic.

There seems to be no or little hope of manipulating room responses feasibly and effectively using architectural means, at least in the domestic music-room context. Likewise, and somewhat inexplicably, experience with signal processing and filtering have not been favourable.

Which means that using multiple subwoofers is the method of choice to tame room influences - as taught by Toole's chapter and recently advocated by Geddes too.

Due to the necessities of research, tests of multiple subs are generally systematic and coherent. But the underlying message that I get is that the more heterogeneous ("mix-and-match") are the woofer locations, suspension types, resonances, and other factors, the better off you are in combating the pesky room influences.

I'm delighted by the results with my highly heterogeneous mixture of a corner horn and a midwall giant IB.

My impression is that you can add almost any woofer into the mix and through the magic of log-addition, even woofers with not-so-low cut-offs will improve the room.

BTW, as I have firmly believed for the last 40 years, it is largely a waste of effort to have more than one subwoofer signal. This belief is, of course, inherent in the multiple sub approach, because it is all but inconceivable to have any of the subs playing different signals.
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Old 18th May 2011, 08:34 AM   #2
AllenB is offline AllenB  Australia
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Quote:
Originally Posted by bentoronto View Post
because it is all but inconceivable to have any of the subs playing different signals.
Notwithstanding of course, frequency independent, continuously variable phase as you would no doubt have noticed. If I was doing this for a living I'd get on to a self contained product that had this feature as it would save so much time.
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Old 18th May 2011, 08:48 AM   #3
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Originally Posted by AllenB View Post
Notwithstanding of course, frequency independent, continuously variable phase as you would no doubt have noticed. If I was doing this for a living I'd get on to a self contained product that had this feature as it would save so much time.
Can't say as I understand any of that. No doubt the fault is mine.
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Old 18th May 2011, 09:50 AM   #4
AllenB is offline AllenB  Australia
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I'm referring to the times that a sub enhances the lower response but upsets the higher bass frequencies and when I invert the drive the upper bass pulls into line etc.

I might mention that I'm experimenting with helper woofers, which I'm liking, but they restrict the locations for some of the subs.
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Old 18th May 2011, 10:14 AM   #5
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Quote:
Originally Posted by AllenB View Post
I'm referring to the times that a sub enhances the lower response but upsets the higher bass frequencies and when I invert the drive the upper bass pulls into line etc.

I might mention that I'm experimenting with helper woofers, which I'm liking, but they restrict the locations for some of the subs.
Perhaps it is that "etc" that fogs my understanding of what you might possibly be talking about. Could your comment be related to what I - equally obscurely - referred to as "magic of log-addition"?

I am sure the excellent subtlety of your thought would be admired even if written so that simple minds could understand what you are saying as well.
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Old 18th May 2011, 10:38 AM   #6
AllenB is offline AllenB  Australia
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Sorry Ben, I'll come at it from another angle.

When I have set up subs, I've found it a little frustrating at times when placing a woofer that helps some frequencies but makes others worse. I find myself faced with the possibility of crossing over lower to avoid the problem region and maybe having to build another sub to make up for it.

The problem I'm talking about is phase. This may even show up when I change the low pass crossover only to find that the frequencies around the crossover no longer blend anymore. I assume that this is a factor in your theory on heterogenous woofers as I know that if you have two different subs that have otherwise been set up the same you can swap their location (to exactly where each other used to be) and the overall response will change.

By the way, I looked up 'heteroskadastic'. Great word

Last edited by AllenB; 18th May 2011 at 10:40 AM.
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Old 18th May 2011, 02:47 PM   #7
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Yeah, used the word just 'cause it sounded spiffy (and is roughly appropriate).

Now, "phase" is one of those concepts that can be neatly handled by quantitative analysis. So it has a prominence in acoustic textbooks.

I've been reading a great book, The Information, about Claude Shannon. Information transmission, in some contexts, is related to the statistical characteristics of a language or code. In the same sense, the influence of phase on music reproduction may well depend on the actual characteristics of music. And "music" now-a-days, maybe be divided into separate species of traditional mic-pickup music versus music that is synthesized and/or highly processed.

Is there enough phase-coherent information in mic-pickup music to make phase matter? Or, do even pedal organ notes wander all over the place, phase-wise, and make any model based on phase, such as your discussion, less relevant?

Obviously, if you test with sine waves, your music room will have hot and cold spots, as you point out arising from the familiar principles of acoustics. But with actual music, does it have any practical importance, given the "wandering about"?
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Old 18th May 2011, 03:56 PM   #8
AllenB is offline AllenB  Australia
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The way you are referring to phase, I agree it is of little consequence. I am talking about the relative timing of sound coming from more than one source. Similar to reversing the polarity of a tweeter and experiencing a hole around the crossover.

When adding a sub to the mix, reversing the polarity of the sub tends to produce the opposite effect. For example if adding the sub filled one hole and created another, when you switch the polarity it would then make a larger hole and create a peak (in that order )

Reversing polarity takes the phase to 180 degrees from 0 (360) degrees. Continuously variable phase as I am suggesting would allow anywhere in between as well as different amounts across the spectrum.
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Old 18th May 2011, 04:20 PM   #9
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Being a good boy, I used to check polarity of speakers. Yup, with conventional speaker set-up geometry makes a difference for woofers.

Let us talk about two heterogeneous subwoofers.

Even with sine waves, you get different results with different frequencies (and so does moving your head or mic a few inches). On music, beyond my ability to tell which polarity makes a difference with my subs.

So, in my view, phase can't matter much in practice with mic-recorded music.

And I am skeptical about lab studies where using trick stimuli, folks can sense phasing a wee bit. It proves that only with trick stimuli folks can....
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Old 18th May 2011, 04:35 PM   #10
AllenB is offline AllenB  Australia
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If you were to reverse the polarity of all of your speakers at once, it may not be noticeable (or if it is, it may not be due to the phase itself). If, on the other hand, you take measurements before and after you can make a good +/-5dB response blow out to e.g. +/-15dB by reversing only the sub polarity.
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