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  • jhollander
    started a topic Butterworth 3 in PCD?

    Butterworth 3 in PCD?

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  • ReissM
    replied
    Re: Butterworth 3 in PCD?

    Originally posted by Jeff B. View Post
    ...With an even order crossover the main lobe is on the design axis, so this could be the axis with the highest summation, therefore you should never see this negative (destructive) summing. However, this is an odd order crossover. This means that the main lobe is tilted downward from the design axis, and on the design axis you are just beginning to see the development of the crossover null...
    Ahhhh, gotcha. That's the piece I was missing. Thanks for clarifying Jeff.

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  • Jeff B.
    replied
    Re: Butterworth 3 in PCD?

    Originally posted by Chris Roemer View Post
    I might be full of it, but I try to never have "negative summing" within an octave (or two) of the Fc. I'd think it'd really wreak havoc on any kind of imaging. If I'm doing a "budget" (limited parts) XO (and I'm not rolling off a woofer w/a strong break-up mode - steep enough) SOMEtimes I'll let some "minimal" negative tweeter summing go - but try not to.
    No offense intended here, but you are essentially doing the same thing that Reiss did - you're looking at it like you would an even order crossover and expecting it to act like one. With an even order crossover the main lobe is on the design axis, so this would be the axis with the highest summation, therefore you should never see this negative (destructive) summing. However, this is an odd order crossover. This means that the main lobe is tilted downward from the design axis, and on the design axis you are just beginning to see the development of the crossover null. You see this on the design axis as a small dip in the summation. If you plotted the vertical polar response you would see that this is just a snapshot on one axis and would see how the vertical summation really looks.
    Last edited by Jeff B.; 01-16-2016, 07:43 PM. Reason: fixed typo

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  • Chris Roemer
    replied
    Re: Butterworth 3 in PCD?

    I might be full of it, but I try to never have "negative summing" within an octave (or two) of the Fc. I'd think it'd really wreak havoc on any kind of imaging. If I'm doing a "budget" (limited parts) XO (and I'm not rolling off a woofer w/a strong break-up mode - steep enough) SOMEtimes I'll let some "minimal" negative tweeter summing go - but try not to.

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  • jhollander
    replied
    Re: Butterworth 3 in PCD?

    Here's another pic.

    I've never considered negative summing an issue, it's the result of the x-o slopes and polarity of the tweeter.

    In this case of BW3 I'm watching the power response and off axis summations and comparing that to LR4 off axis
    Attached Files

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  • ReissM
    replied
    Re: Butterworth 3 in PCD?

    Whoops, sorry about that. I screwed up. I think that I was too focused on the fact that there are some acoustic summing/cancelling issues going on and I missed his main goal.


    I do have a follow-up question: With his 3rd order Butterworth crossover... Is it typical to have the woofer/tweeter cancel from 500Hz to 1200Hz, then add from 1200 to 2000, then cancel from 2000 to 7000? Or phrased another way... Should the black line (acoustic sum of Woof & Tw) always appear above the woofer and tweeter curves? (Indicating that they are constructively adding, not cancelling.)

    The summing/cancelling near the crossover frequency is confusing me. Why do that? (Not trying to pick a fight... I truly want to understand.) -Thanks.

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  • Jeff B.
    replied
    Re: Butterworth 3 in PCD?

    Originally posted by ReissM View Post
    In another recent thread (Why does Phase matter?, Post #55) I showed a graph that had good phase tracking between the woofer and tweeter, but overall poor frequency response. You seem to have the opposite here...
    …good frequency response, but poor phase tracking. In your Graph #2, see how far apart the orange phase line is from the sky blue phase line? Try to get those to touch each other at the crossover frequency if you can. Currently your woofer phase at 1500Hz is approx -170 degrees. The tweeter phase at 1500Hz is approx -70 degrees. That about 100 degrees difference. So the acoustic sum is a bit chaotic. If you can make some small changes to the tweeter filter, you’ll see how the phase lines up better and the acoustic sum behaves nicer.

    What do I mean “behaves nicer”? Notice in your Graph #2 how the black line (acoustic sum of Woof+Tw) shows that the woofer and tweeter are not summing nicely… they’re actually cancelling from 500Hz up to 1200Hz. But then from 1200Hz to 2000Hz they sum. Then from 2000Hz to 7000Hz they slightly cancel again. If you were to get your phase better aligned with each other at the crossover frequency, the woofer and tweeter would probably sum together nicely, without the constructive/destructive interference that we see now.

    Hope this helps???

    P.S. - it's ok to use asymmetric crossover filters to achieve your goals. e.g. 2nd order LP with 3rd order HP, etc, etc...
    No Reiss, you are missing the point. He is targeting a 3rd order Butterworth acoustic crossover. This type of crossover does not have phase tracking between the drivers. Ideally, the phase between the drivers should be 90 degrees apart. What John is showing is very close to that, and right what it ought to be. He should not try to make the phase track, that would be a different crossover type. He has the correct phase response already. And his summation is pretty close, he just needs a little less overlap between drivers.

    In answer to your last question, yes, you usually always want asymmetrical crossover slopes if you have two drivers with an acoustic offset between them. This is the most practical way to get the desired results.

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  • ReissM
    replied
    Re: Butterworth 3 in PCD?

    Originally posted by jhollander View Post
    I’m building a MTM center channel and I would like to use a BW3 to minimize the off axis dip. Having recently learned that the BW3 is identified more by the phase than driver roll offs I created this x-o.
    In another recent thread (Why does Phase matter?, Post #55) I showed a graph that had good phase tracking between the woofer and tweeter, but overall poor frequency response. You seem to have the opposite here...
    …good frequency response, but poor phase tracking. In your Graph #2, see how far apart the orange phase line is from the sky blue phase line? Try to get those to touch each other at the crossover frequency if you can. Currently your woofer phase at 1500Hz is approx -170 degrees. The tweeter phase at 1500Hz is approx -70 degrees. That about 100 degrees difference. So the acoustic sum is a bit chaotic. If you can make some small changes to the tweeter filter, you’ll see how the phase lines up better and the acoustic sum behaves nicer.

    What do I mean “behaves nicer”? Notice in your Graph #2 how the black line (acoustic sum of Woof+Tw) shows that the woofer and tweeter are not summing nicely… they’re actually cancelling from 500Hz up to 1200Hz. But then from 1200Hz to 2000Hz they sum. Then from 2000Hz to 7000Hz they slightly cancel again. If you were to get your phase better aligned with each other at the crossover frequency, the woofer and tweeter would probably sum together nicely, without the constructive/destructive interference that we see now.

    Hope this helps???

    P.S. - it's ok to use asymmetric crossover filters to achieve your goals. e.g. 2nd order LP with 3rd order HP, etc, etc...
    Last edited by ReissM; 01-14-2016, 07:40 PM. Reason: additional comment added

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  • jhollander
    replied
    Re: Butterworth 3 in PCD?

    Chris, I can see why the BW3 may not be the the best after looking at the opposite axis. I did have fun playing with this in PCD as you can watch the dips develop.

    By saving the overlay and swapping CSP files I've been able to compare filter types. I need to spend more time with this to see if LR4 is the best compromise. I'm guessing it is.

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  • Chris Roemer
    replied
    Re: Butterworth 3 in PCD?

    I know that initially the "D'Appolito" MTM was designed as a BW3 filter type, but in some of his later designs he just went LR.

    Even JB's RS180 MTM is LR4 (II'mNM). I THINK he said that you'd still get most of the directionality characteristics of the "traditional" MTM (mostly due to driver layout - NOT having the normal TM lobe thrown a bit (15*?) up or down), even if the filters were not BW3.

    I MIGHT have made all that up, but it's what my old brain remembers.

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  • jhollander
    replied
    Re: Butterworth 3 in PCD?

    Originally posted by Jeff B. View Post
    Close, but not quite. While it's true that you need to look at the phase relationship and how the drivers sum to determine the crossover type, these are not exactly summing in the crossover region the way a BW3 crossover would. There's a bit too much overlap for BW3. Also, do you really mean to take the tweeter that low? That's certainly going to push it's performance limits. My recommendation would be to move the crossover point up a bit in frequency and decrease the overlap at the same time. You can probably do this by modifying only the tweeter circuit. This should dial it in a bit closer to BW3 behavior in the crossover region.
    OK got it, I should expect to see more dip at the x-o point. With respect to the tweeter x-o point this is really an exercise in if I could use PCD to design anything other than an even order x-o. I think I understand a bit more.

    To expand on rpb's comments the horizontal MTM with the BW3 stays surprisingly flat at the x-o point off axis, however vertically it's not great.

    As I'm managing dips, I may look at other x-o types

    Thanks

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  • Jeff B.
    replied
    Re: Butterworth 3 in PCD?

    Originally posted by jhollander View Post
    I’m building a MTM center channel and I would like to use a BW3 to minimize the off axis dip. Having recently learned that the BW3 is identified more by the phase than driver roll offs I created this x-o.

    In this sim it appears to me I have a 90 degree driver separation plus a 180 degree rotation for a total of 270 degrees.

    Electrically the filters are second order but the slopes are not close to BW3

    So is this a BW3?
    Close, but not quite. While it's true that you need to look at the phase relationship and how the drivers sum to determine the crossover type, these are not exactly summing in the crossover region the way a BW3 crossover would. There's a bit too much overlap for BW3. Also, do you really mean to take the tweeter that low? That's certainly going to push it's performance limits. My recommendation would be to move the crossover point up a bit in frequency and decrease the overlap at the same time. You can probably do this by modifying only the tweeter circuit. This should dial it in a bit closer to BW3 behavior in the crossover region.

    Leave a comment:


  • rpb
    replied
    Re: Butterworth 3 in PCD?

    So, you're trying to reduce the vertical nulls?

    Leave a comment:

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