Mixing Cabs and Phase Cancellation

[Raf Seibert]

An additional concern I had with the original post was the reference to the amp having both 4 ohm and 8 ohm outputs. You don’t want to use both output jacks at the same time. Pick one or the other, but not both. If you plug into both, 4 + 8 = 2.6 ohms, usually too low for many amps.

Raf

 

[micguy]

A polarity reversal (or broad band 180 degree phase shift between sources) is a special case as it causes energy cancellation across most of the audio range (below say 500Hz) whereas the phase deviations from different speakers typically affect only a smaller range of the spectrum and the cancellations are nowhere near as deep or broad band. The equations governing these scenarios are quite different (with the polarity reversal being the simplest by far).

We're pretty much in agreement, I used the flipped polarity case to illustrate the concept of destructive interference, which is a big part of the issue here. I wasn't claiming it gave the same total result as mixing drivers, just using it as a mental exercise to illustrate what can happen.

With mixed cabinets, as I was at least trying to get across (maybe I didn't), you'll get dips in narrower bands - not all the bass will go away. But some of it might. And yes, in general, if you have a couple of different ported enclosures, you'll probably get dips that aren't too wide or deep - with tunings at different frequencies, the phase shifts will occur at different frequencies, so there will be a band where they're not that close to in phase.

However, if you have one sealed cabinet and one ported, the ported one at low frequencies has an additional phase shift that (when you sum the outputs) can make a pretty deep and wide cancellation region. Yes, it'll be at the extreme low end of things, but that's where the problem can be most problematic, as it's where excursions are already being pushed, and if you add eq to that mix, you're in trouble.

I've probably spent too much time recently staring at curves of boxes I've built and would like to build in WIN ISD - designing for this very issue with very different drivers is a bit of fun....if only I could get driver manufacturers to me me the drivers I wanted, it'd be a whole lot easier....

 

[tubedude]

Watts don't come into phase until too many watts cause distortion.

***??
Must be the language barrier of a common tongue.

 

[HaphAsSard]

Huh?
Could you expand further? I'm not following what you mean.
Thanks.

***??
Must be the language barrier of a common tongue.

Reread the OP:

I know mixing a 1x15 and a 4x10 or 4x8 is bad in most situations and that matching speaker sizes will produce in a better result.

Is part of that a wattage issue? Say if you have a 300w 1x15 and a 400w 4x8 or 500w 4x10 and you have a 200w head that has multiple 4ohm and 8ohm outs, would that still cause phase cancellation and it not generally sounding as good as matched speaker sizes?

Potential phase issues are usually cited as the reason not to go for mixed cabinet setups, with power handling mismatch as an additional drawback to specific combinations such as the common 410+115 stack, or comparable ones.
However, the OP enquired about differences in power handling as a factor in phase cancellation. To which DUW replied

Watts don't come into phase until too many watts cause distortion

which I take to mean input wattage of any given level is not a factor in the combined phase response of a mixed cab stack, until the less robust cabinet runs out of clean watts, drivers go into overexcursion and cone breakup changes the tone, asymmetrically in the stack: then all bets are off. Only, they weren't really on, even before that point.
In practice, "too many watts cause distortion" at the speaker level, not at the amp, or so I gathered - I may be wrong.

 

[Downunderwonder]

Yup.

 

[ThisBass]

You always have to complicate things with complex math.

IME way better than smplifying complex things with way to simple math

 

[ThisBass]

Potential phase issues are usually cited as the reason not to go for mixed cabinet setups,

Unfortunatelly only the bold word is true.
And it does not mean that phase "issue" was the most reasonable reason.
It only means nothing but phase "issue" is the best "understandable" reason for Joe Average just to explain "issues" that he can't explain with complex math beyond his skills.

So Joe Average will always attempt to make use on "bitchy" phase to make her responsible for any issues

 

[Old Garage-Bander]

Reread the OP:
However, the OP enquired about differences in power handling as a factor in phase cancellation. To which DUW replied

which I take to mean input wattage of any given level is not a factor in the combined phase response of a mixed cab stack, until the less robust cabinet runs out of clean watts, drivers go into overexcursion and cone breakup changes the tone, asymmetrically in the stack: then all bets are off. Only, they weren't really on, even before that point.
In practice, "too many watts cause distortion" at the speaker level, not at the amp, or so I gathered - I may be wrong.

Is another way to rephrase DUW's comment:
Power is only coherent when speakers are driven too hard?

I still don't understand how power comes into phase when a cab is overdriven.
Someone please dumb it down for me.
Thanks.

 

[HaphAsSard]

Unfortunatelly only the bold word is true.

That's why I put it in: when I'm parroting, make sure it doesn't sound like my own words...
I'm a(n amateur) translator, not an engineer, as good ol' Doc McCoy would say in my stead.

Someone please dumb it down for me.

Er, I'd need you to dumb this down for me first:

Power is only coherent when speakers are driven too hard?

 

[Old Garage-Bander]

That's why I put it in: when I'm parroting, make sure it doesn't sound like my own words...
I'm a(n amateur) translator, not an engineer, as good ol' Doc McCoy would say in my stead.

Er, I'd need you to dumb this down for me first:

Power (wattage) is only coherent (only in phase) when speakers are driven too hard (distorted).
Your turn.

I kind of have a thing about people saying wattage when they mean power.
(Or Ohmage, when they mean impedance.)
It may be correct grammatically, but in a technical discussion, not so much.
But you made a pretty good attempt at making sense of this all.
I'm still trying though. So you're ahead of me in this game.

To continue a theme.
"Cap'n, that's all the wattage I can give ya."

 

[HaphAsSard]

Power (wattage) is only coherent (only in phase) when speakers are driven too hard (distorted).
Your turn.

'Aight. Let's see. No. As far as I understand his point is that two dissimilar cabinets may or may not be phase-coherent below the overexcursion point of one of them, until which point it doesn't matter (as far as phase coherence, or lack thereof, is concerned) how many watts you put into them. When the less fartout-resistant does start breaking up, its tone changes and so does phase interaction between the two cabs: at that moment you could say that power is being a factor in the system's phase behaviour (in comparison with what the latter was a couple knob notches ago).

I kind of have a thing about people saying wattage when they mean power.
(Or Ohmage, when they mean impedance.)
It may be correct grammatically, but in a technical discussion, not so much.

Noted. I didn't give it much thought, what with there being a direct equivalent in my language, having seen "wattage" used casually here and elsewhere and knowing it is the dictionary (as is "ohmage", to the use of which I have seen a fair amount of...resistance), and, above all, barely knowing what I'm talking about if at all. In other instances I may occasionally use existing or made-up forms with full humorous or nonsensical intent (such as "enough cabbage" to mean what folks here usually, and simplistically, phrase as "enough speaker area for the gig"), all the time knowing full well that it takes nothing to go from witty to cringeworthy, and that to predict the reaction of a native speaker is largely above my paygrade.
By the way, what's your stance on "voltage"? Do you consistently say "tension" or "potential difference" instead?

To continue a theme.
"Cap'n, that's all the wattage I can give ya."

Fascinating...

 

[Old Garage-Bander]

'Aight. Let's see. No. As far as I understand his point is that two dissimilar cabinets may or may not be phase-coherent below the overexcursion point of one of them, until which point it doesn't matter (as far as phase coherence, or lack thereof, is concerned) how many watts you put into them. When the less fartout-resistant does start breaking up, its tone changes and so does phase interaction between the two cabs: at that moment you could say that power is being a factor in the system's phase behaviour (in comparison with what the latter was a couple knob notches ago).

Noted. I didn't give it much thought, what with there being a direct equivalent in my language, having seen "wattage" used casually here and elsewhere and knowing it is the dictionary (as is ohmage, to the use of which I have seen a fair amount of...resistance), and, above all, barely knowing what I'm talking about if at all. In other instances I may occasionally use existing or made-up forms with full humorous or nonsensical intent (such as "enough cabbage" to mean what folks here usually, and simplistically, phrase as "enough speaker area for the gig"), all the time knowing full well that it takes nothing to go from witty to cringeworthy, and that to predict the reaction of a native speaker is largely above my paygrade.
By the way, what's your stance on "voltage"? Do you consistently say "tension" or "potential difference" instead?
Fascinating...

Voltage, mostly. Potential difference on occasion. Tension rarely.

Well I didn't even realize you were ESL (English as a second language).
That's how well you've mastered our tongue.

I notice the wattage/power, Ohmage/resistance/impedance thing here all the time. Just sounds strange to me.
The unit of measure for both resistance and impedance being the Ohm, really confuses things for a lot of folks too.

Cabbage! I love that! If you don't mind, I may use that some time when I'm feeling especially rambunctious.

Part of the original comment was that it referred to power being in phase. Power, which is comprised of voltage and current, is no longer a factor as it never leaves the cab. Sound pressure, frequencies, distance between speakers, room acoustics and the locations where those things are observed are among some of the things in play now. Clean or distorted, makes no difference. The audio phase relationships don't change just because a signal is clipped by the voice coil reaching max excursion.

Think about a clean signal from a speaker cabinet, then clip the peaks off that signal by maxing out the voice coil travel in a second cab. The signals during both excursions when not hitting the limits are essentially in phase when leaving the speaker. I don't see how the time relationship between two signals changes just because the top of one of the signals is lopped off. The two signals would both go through zero crossing at the same time. Which is the definition of being in phase. The difference in perspective of the observed signals is the variable. That is what accounts for phase changes as you move away from the cabs. Being clipped has no bearing on that.

Of course this all assumes that the distorting voice coil recovers from max instantaneously. I don't have a good feel for how well that happens, but if it is not instantaneous, it likely also is not consistent between drivers. Then if it does happen, is the time "stuck" at max significant in relation to the audio it is reproducing? Perhaps that was the intended meaning of "Yup"?

Sorry for the stream of consciousness, I just got on a roll and started drifting around.
Standing by to beam up.

 

[HaphAsSard]

@Old Garage-Bander: I read ya, and that does seem reasonable to me.
I dunno. What if cab B - the hairy-at-volume one - has a dip somewhere between 500 and 1000Hz unless/until it starts breaking up, at which point its added content - say, harmonic #3 of the harmonic #3 - combs off the harmonic #9 coming from cab A at a given (moderately off-axis) listening position, which it couldn't before? Does what I just wrote even make sense, let alone be possible? I really only have a vague, intuitive grasp of this stuff myself. Language is where I'm at my less dull, and that's why I could finally - too late at this point - zero in on the exact source of misunderstanding: "[w]atts don't come into phase until" never meant "watts don't become in-phase (electrically) until", but instead "watts don't become of significance as regards (audio) phase (of a mixed cabinet stack) until". Duh, me.

And by all means go ahead with "cabbage", I may have ripped it off someone else myself.

Bridge to ensign @Downunderwonder.
(Ears must be burning enough without the need for a communicator.)

 

[Downunderwonder]

Beam me up too.

Haphazard told it better than I could.

The phase response of cabs will be independent of the power while not overdriven.

I only suspect that pushing one or both cabs too hard into distortion would screw with their phase response along with the generation of distortion tones.

 

[agedhorse]

You guys are trying way too hard!

 

[Old Garage-Bander]

You guys are trying way too hard!

It made my brain hurt in a good way.

Haphazard told it better than I could.

But he said he was parroting you!

 

[HaphAsSard]

It made my brain hurt in a good way.

Ayuh, better than crosswords!

But he said he was parroting you!

Nope, I was interpreting him; the parroting part was "folks say most mixed cabs setups are bad because phase, man", which in fairness hasn't happened this thread around.

 

[Old Garage-Bander]

Ayuh, better than crosswords!

Nope, I was interpreting him; the parroting part was "folks say most mixed cabs setups are bad because phase, man", which in fairness hasn't happened this thread around.

Sorry! I got that wrong. My apology. Thanks for correcting me.

"Cause of the phase man." "Cause of the phase."

Been an interesting discussion, not sure if we got anywhere with it, or if there was really much of anywhere to go, but it was interesting.
Fun chattin' with ya. All the best.

 

[HaphAsSard]

Been an interesting discussion, not sure if we got anywhere with it, or if there was really much of anywhere to go, but it was interesting.
Fun chattin' with ya. All the best.

Likewise, sir.

 

[JN8642]

Haha I think I might need to re-read the posts in this thread now that I know the technical terms