# 12/24db crossover and phase



## capnxtreme (Feb 5, 2008)

So I've been messing w/ the settings on my JL 500/5 which I have tri-amping my 2-way front stage + sub, and thinking about the crossovers made me wanna brush up on my EE fundamentals.

12db/octave crossovers shift the phase 180 degrees, right (b/w my midrange and my tweeter, for example)? So... on an amp with selectable 12db/octave crossovers, is the amp already correcting this phase shift for me? Or am I effectively swapping phase when I switch from the 24 to 12db/octave crossover setting?


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## Candisa (Sep 15, 2007)

I don't think the amp will correct the electrical phase for you, but that really doesn't matter.

Electrical phase =/= acoustical phase. Most of the time, to get everything in acoustical phase, you have to f*ck up the electrical phases.

I use to have a 2-way + sub set-up in my Clio and I had to flip the phase of the sub and the right tweeter to get everything in phase, knowing that a 18dB/oct high pass filter (wich means a funky 270° electrical phase shift) on the tweeters worked better than a 12 or 24dB/oct slope (wich are both more easy to get stuff in perfect electrical phase)!

Just play around with it, try all possible combinations of phases, you'll probably notice the best blending combination makes no sense 

greetz,
Isabelle


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## capnxtreme (Feb 5, 2008)

Thanks for the reply.

I almost mentioned that, that I'm going to just try every combination regardless, and see which sounds best, so... yeah, it doesn't really matter. I just like trying to have an understanding of exactly what changes I'm making, when I'm making them. I can get probably get over it though.


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## Candisa (Sep 15, 2007)

I'm pretty sure the amp won't correct the phaseshift of the crossover 

greetz,
Isabelle


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## Foglght (Aug 2, 2007)

So I had a question regarding this. When I flip the phase of my mid, time alignment seems to stop working, as the image had shifted over but sounded better. What is going on?


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## tcguy85 (Oct 29, 2007)

Foglght said:


> So I had a question regarding this. When I flip the phase of my mid, time alignment seems to stop working, as the image had shifted over but sounded better. What is going on?


phase and t/a are not the same thing, but do still affect each other. so a switch in phase may require a change in t/a.


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## Foglght (Aug 2, 2007)

I never was assuming they were. I just don't understand why when I flip the phase and my stage moves over, I cannot use the time alignment or at least make sense of it.


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## capnxtreme (Feb 5, 2008)

Candisa,

Thanks again ma'am.

Regarding phase vs. T/A, I think I read someone on here sum it up pretty well:

Phase shift is effectively T/A, except it's a different amount of T/A across the frequency band.

It seems to me personally, that it would be the best approach to 1) get the phase as good as possible w/ no T/A, and then 2) mess w/ the T/A. Anyone have any thoughts on that? Is anyone with fancy processing capabilities still flipping the polarity of the driver's side mid?


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## Candisa (Sep 15, 2007)

Phase shift is indeed signal shift, flipping the phase 180° makes a difference of half the period of the frequency.

It's a kind of T/A, but indeed a varying one.
180° phase shift at:
500Hz = 1ms (period of 500Hz = 2ms, half of it is 1ms) time shift
2500Hz = 0.2ms time shift

To my opinion, time alignment is the very last step of the tuning process. First step is to angle the drivers and balancing (in groups and left-right seperate) them out
Next step is experimenting with crossoverpoints, slopes (if you have the possibility, left and right seperate tuning of x-overpoints and slopes) and phases (all possible combinations)
Next step is eq'ing (left-right seperate when possible)
And *then*, when you did the previous steps as good as possible, you can start getting it better with time-alignment.

greetz,
Isabelle


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## St. Dark (Mar 19, 2008)

Also, keep in mind, that a crossover affects phase only at/near the filter point; it does not change phase on the whole pass-band.


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## capnxtreme (Feb 5, 2008)

St. Dark said:


> Also, keep in mind, that a crossover affects phase only at/near the filter point; it does not change phase on the whole pass-band.


I believe that this is incorrect.

Why do 2-way home speakers with 12db passive crossovers connect the tweeter with inverted polarity?


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## Candisa (Sep 15, 2007)

capnxtreme said:


> I believe that this is incorrect.
> 
> Why do 2-way home speakers with 12db passive crossovers connect the tweeter with inverted polarity?


It is correct, a crossover only affects phase around the crossover point. They connect a 2nd order crossed driver out of phase because, at the crossoverpoint, the lower playing driver still plays a bit past the crossover frequency, so does the higher playing driver. You only hear matching/non-matching phases when you have a reference, so in multiple-way speakers, that's around the crossover point(s). It doesn't matter what phase does above the crossover frequency on the higher playing driver/below x-over freq on the lower playing one, because there's no reference there.

If you build a ported subwoofer enclosure, it'll have a very rising group delay the more you go lower in frequency. What is group delay? Acoustical phase turning... Does it matter? No, because it blends well at the point another driver (group) picks up...

greetz,
Isabelle


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## capnxtreme (Feb 5, 2008)

Thank you St. Dark, for spotting where my understanding was wrong.

And thank you again, Isabelle; very helpful.


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## owdi (Apr 4, 2008)

capnxtreme said:


> I believe that this is incorrect.
> 
> Why do 2-way home speakers with 12db passive crossovers connect the tweeter with inverted polarity?


With a second order crossover, one driver's phase is shifted +90 degrees at the crossover frequency, and the other is shifted -90 degrees. Since they are 180 degrees out of phase, the output from one perfectly cancels the output from the other. By reversing phase of one of the drivers, their output sums instead.

Changing the relative distance of the drivers also shifts phase by A LOT! If your tweeter to woofer crossover frequency is 3000hz, every inch of difference between the distance to the drivers will result in 90 degrees of phase shift. Given the typical car, it's not uncommon to see tweeters that are a foot closer to your ears than woofers (that's 1080 degrees of phase shift!) Since you cannot assume either in or out of phase is better without considering the install, it's best to choose how you wire by listening or measuring.

Dan


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## TREETOP (Feb 11, 2009)

Bumping this thread rather than starting a new one that's basically the same thing.

I'm trying to understand something.

Say I have my front stage actively crossed over at 12db/octave, midbass/midrange/tweeter. My drivers are all within a few inches of each other, with the backwaves of my midbass and midrange isolated from each other.

With a 12db/octave network, my midbass and midrange seem out of phase with each other at the crossover point (let's say it's 312hz). So, I reverse the polarity of my midbass drivers and the focus clears up around the crossover frequency.
Now the rest of the midbass frequency range and the rest of the midrange frequency range, outside of the surrounding crossover point area, are electrically out of phase, right?

Is my understanding correct then, that a 4th order (24db/octave) filter will keep them electrically in phase not only at the crossover point, but also in the surrounding frequencies? 
My understanding is that a 24db/octave filter shifts phase by 360 degrees (90 degrees per order), which technically would make put two drivers back in phase electrically. 
I do realize that electrical phase and acoustical phase are different animals but somewhat related, would a 24db/octave crossover help me with either, or both?


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## Candisa (Sep 15, 2007)

It doesn't really matter, phase only matters at the crossoverpoint, because at those points, there are 2 groups of different drivers playing the same frequency, so that has to be in acoustical phase otherwise they'll cancel eachother out.
It doesn't matter if the acoustical and/or electrical phase turns in the frequency range of 1 pair of drivers, because there isn't a second pair of drivers than can be out of phase and cancel the sound.


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## TREETOP (Feb 11, 2009)

Candisa said:


> It doesn't really matter, phase only matters at the crossoverpoint, because at those points, there are 2 groups of different drivers playing the same frequency, so that has to be in acoustical phase otherwise they'll cancel eachother out.
> It doesn't matter if the acoustical and/or electrical phase turns in the frequency range of 1 pair of drivers, because there isn't a second pair of drivers than can be out of phase and cancel the sound.


Isabelle, thank you for the explanation.
I'll admit I don't completely understand, because instinct tells me that a speaker with polarity reversed is bad, even though I know it's done often to get phase correct. 
I would assume then that if there's an overlap at the crossover point, the larger the overlap the more difficult to get the phase correct? 
My crossover is a Linkwitz-Riley type (FRX-456), so there shouldn't be much (if any) overlap, right? From what I remember, the Linkwitz-Riley 2nd order filter actually creates a gap at the crossover frequency to get the output closer to flat at the crossover point.

So other than the benefit of obviously steeper slopes, a 24db/octave crossover wouldn't help me any with phase? It seems like it would allow me to run my midbass drivers "polarity correct", but I guess that shouldn't necessarily be an advantage. 
With the 360 degree phase shift of a 24db/octave network, does that open another can of worms since the speaker is now back in phase but technically delayed one wavelength behind the adjacent driver? (I have the ability to time-correct but not between my midbass and midrange because of my front stage all going through my FRX-456)

Sorry if my questions seem misdirected, I always thought I had a good grasp on crossover types but never thoroughly thought about the WHY part of the science behind it.


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## DanWiggins (Jun 15, 2005)

> Also, keep in mind, that a crossover affects phase only at/near the filter point; it does not change phase on the whole pass-band.


Wanted to correct this... A crossover affects phase at the crossover point AND in the passband of the crossover. A high pass crossover, 2nd order (Butterworth), is typically 90 degrees shifted at the -3 dB (typical crossover) point. Above that point, it transitions to a full 180 degrees phase shift.

This is why you sometimes flip the tweeter polarity on a 2 way crossover - the tweeter's main passband is 180 degrees out of phase with the input signal.

You cannot only affect phase at the crossover unless you have considerable non-linear and non-causal signal processing equipment (where you can break the phase/frequency response relationship).


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## DanWiggins (Jun 15, 2005)

> I would assume then that if there's an overlap at the crossover point, the larger the overlap the more difficult to get the phase correct?


Not really; for every 2 orders you get a 180 degree ultimate shift. So for a 2nd order, you typically flip the polarity of a driver; for 4th, you leave them in polarity. 6th, flip. 8th, in. And so on.

Now, odd-order crossovers (1st, 3rd, 5th, etc) have a 90 degrees phase shift between the two sides, meaning you may or may not have to flip, depending upon the final implementation.



> My crossover is a Linkwitz-Riley type (FRX-456), so there shouldn't be much (if any) overlap, right? From what I remember, the Linkwitz-Riley 2nd order filter actually creates a gap at the crossover frequency to get the output closer to flat at the crossover point.


Linkwitz-Riley is essentially a Q=0.5 design, meaning the crossover points are 6 dB down. It attempts to remove the typical 3 dB bump at the crossover you have with a Butterworth (Q=0.7) design. However, the lower Q designs do roll off SLOWER than the higher Q designs when looking at the slope around the crossover. Thus you will actually have MORE overlap with the Linkwitz Riley than the Butterworth, but you end up with a flatter on-axis frequency response.



> So other than the benefit of obviously steeper slopes, a 24db/octave crossover wouldn't help me any with phase? It seems like it would allow me to run my midbass drivers "polarity correct", but I guess that shouldn't necessarily be an advantage.


Your second conclusion here is correct; there is no "polarity correct" for a driver; they are AC devices. You put a current through the voice coil in one direction, the driver moves forward; reverse the direction of the current and it moves backwards. It does not have a "correct polarity" other than for simple "hook this terminal to that terminal" verbiage.

Typically, I recommend running the highest order crossovers you can. Why? I don't want my woofers to tweet and my tweeters to woof. Cut the driver off as fast as you can, it will help with THD, IMD, dispersion, and thermal heating of the components (by virtue of limiting their frequency response).

And don't worry about it "hurting the sound quality"; the Event Opal was just rated the finest recording monitor by a major UK sound engineer's magazine, and it uses 8th order crossovers.



> With the 360 degree phase shift of a 24db/octave network, does that open another can of worms since the speaker is now back in phase but technically delayed one wavelength behind the adjacent driver? (I have the ability to time-correct but not between my midbass and midrange because of my front stage all going through my FRX-456)


Only if you listen in a non-reverberant field with specific test tones; otherwise the Haas and Franssen effects make it a non-issue with complex spectra like music.


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## capnxtreme (Feb 5, 2008)

Thanks Dan.


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## Candisa (Sep 15, 2007)

My advice: just try all combinations of possible slopes and phase-flips and use the combination that gives you the most coherent stage without strange dips, peaks or stage-shifts.
Forget the theory, a car is such a weird acoustical environment, you just have to test the try-and-error way to get it perfect. Theory is only a guide to get you in the right direction...


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## Mr Marv (Aug 19, 2005)

Candisa said:


> Forget the theory, a car is such a weird acoustical environment, you just have to test the try-and-error way to get it perfect. Theory is only a guide to get you in the right direction...


That could be the answer to a million different questions on these forums...


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## jsun_g (Jun 12, 2008)

Side question here...
does all this phase discussion also apply to active crossovers?


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## Oliver (Jun 25, 2007)

jsun_g said:


> Side question here...
> does all this phase discussion also apply to active crossovers?


OUCH...might want to try a different pastime 

Knit one..Pearl two ...


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## TREETOP (Feb 11, 2009)

jsun_g said:


> Side question here...
> does all this phase discussion also apply to active crossovers?


My questions above were about active crossovers specifically.


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## jsun_g (Jun 12, 2008)

Gotcha...caught that now after re-reading your posts and seeing FRX-456.


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## DanWiggins (Jun 15, 2005)

> Forget the theory, a car is such a weird acoustical environment, you just have to test the try-and-error way to get it perfect. Theory is only a guide to get you in the right direction...


Truer words are rarely spoken on the car audio boards!



> does all this phase discussion also apply to active crossovers?


Yes; in fact it also applies to every DSP based crossover I know of as well. I've only seen non-causal (non-linear-time based) filters in very specific, esoteric measurement applications (not speaker/audio reproduction based).


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## gabileung (Jun 22, 2009)

sorry for digging the few years old post
but it's a interesting info
luckily we have incar processor these days like bit1 3sixty etc
so that we could run them all active and T/A individual drivers solving a lot of image problem.
but the diffraction problem in car environment still a pita to cope with


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## jsun_g (Jun 12, 2008)

Taking this example a bit further, I have a couple questions.

Say I have a 200Hz xover point, which would be a larger delay resulting from the 180 degree phase shift (2.5ms if my head calculation is correct). That is a pretty substantial delay not even considering physical placement. Which driver (midbass or midrange in this case) is "ahead" by 2.5ms? Will flipping the wire polarity on one of the drivers cause both of them to be time-aligned?




Candisa said:


> Phase shift is indeed signal shift, flipping the phase 180° makes a difference of half the period of the frequency.
> 
> It's a kind of T/A, but indeed a varying one.
> 180° phase shift at:
> ...


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