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Offsets, Asymmetrical Slopes, and Mysticism - revisited *PICS*

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  • #16
    Re: Offsets, Asymmetrical Slopes, and Mysticism - revisited *PICS*

    Originally posted by Deward Hastings View Post
    It's a nice theory that explains everything.

    I'm looking for the practical application of it in a filter that address the cone breakup of, say, a RS150, and brings it's frequency response to flat through the breakup region (and its phase curve too). At the moment I'm stuck with notch filters that suppress the driving signal at the breakup frequency and that mess up phase relationships in the rest of the crossover . . . the ones jeff was talking about above).
    The phase must go along with the magnitude response.

    Just optimise the magnitude response and the phase will fall in line.

    If you optimised your crossover before addressing the breakup mode then when you address the breakup mode you will skew your phase alignment.

    You must address the breakup mode first then optimise your crossover.

    Or more accurately you must optimise them at once since different parts of your circuit will interact and changes in one part might require changes to others.
    Last edited by daryl; 06-10-2008, 10:41 AM.

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    • #17
      Re: Offsets, Asymmetrical Slopes, and Mysticism - revisited *PICS*

      Oh. My. GOD!!!

      I've been puzzling over phase since I started in this hobby, knowing vaguely that the phase needs to be right, but not having the slightest clue what "right" even looks like, or why.:eek:

      Jeff and Jay, between the two of you, you have shown me just what I've been missing, even though looking back, it's been there for me all along. Of course, this newfound knowledge will still take some time to set in my mind, and thus help me be able to design a decent XO, but WOW! The light bulb just went on!


      Many thanks to both of you!!!


      Mark
      You go your way, I'll go mine. I don't care if we get there on time.

      ~Pink Floyd

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      • #18
        Re: Offsets, Asymmetrical Slopes, and Mysticism - revisited *PICS*

        And Wolf, I know that you've been telling me what to do all along, but I just didn't get it. The fault lies with me, not you....

        Mark
        You go your way, I'll go mine. I don't care if we get there on time.

        ~Pink Floyd

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        • #19
          Re: Practical factors considered

          Originally posted by jkim View Post
          How about coining a new term "relaxed LR4?"
          How about because it's not a LR4 in any meaningful sense of that term?

          Once you're done what LR4 characteristics remain? It's like taking out the cord, the casing and the tread and calling what's left a "relaxed tire" because, well, because that's what you had before you started cutting.

          It's also an interesting exposition of how focusing on one thing leads to compromises in others . . . in this case trading stop band rejection, tweeter excursion, response ripple and vertical polar response for offset compensation (lobe steering) and "simplicity". The argument against is not that it doesn't "work", or even that, if one is impelled to compromises, it's a bad compromise . . . it's that the price seems high and the "relaxing" of other design goals diminishes too much else if all the compromises are *not* necessary. It does not appear to get the best they can deliver out of the drivers.
          "It suggests that there is something that is happening in the real system that is not quite captured in the models."

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          • #20
            Re: Offsets, Asymmetrical Slopes, and Mysticism - revisited *PICS*

            Originally posted by daryl View Post
            The phase must go along with the magnitude response.

            Just optimise the magnitude response and the phase will fall in line.
            Once again . . . I'd like to see the practical passive implementation that deals with the RS150 cone breakup and leaves a flat phase response. Part of the problem seems to be that the "magnitude response" associated with breakup is not a single, simple resonance like the suspension resonance. It may not reduce to a simple model . . .

            Originally posted by daryl View Post
            You must address the breakup mode first then optimise your crossover.
            Yes . . . let's start with that cone breakup . . . (or split the signal sufficiently that it does not matter).

            Originally posted by daryl View Post
            Or more accurately you must optimise them at once since different parts of your circuit will interact and changes in one part might require changes to others.
            Yes . . . and there's the rub. There are ways to isolate different parts of the filter so they don't interact . . . it's the unavoidable interaction in passive crossovers that makes for some of the difficulty in turning theory into practice. It is not alway *possible* to "optimise them at once" in a passive crossover . . . and usually something ends up far from "optimised".

            I'm liking the results better when I address driver response/phase as one issue and treat signal separation (crossover) as a separate and independent issue.
            "It suggests that there is something that is happening in the real system that is not quite captured in the models."

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            • #21
              Re: Practical factors considered

              These response plots follow the target LR4 responses quite closely through the XO region. They don't look much relaxed to me, yet we can all see the excellent on axis response as well as the tight phase alignment reflected in the 20dB reverse null.
              I know what you mean, Pete. It will be better for us to see "slight" relaxation on both drivers' rolloffs if you extend the SPL scale below 50 dB (e.g., the scale I used in my graphs). In addition, a 20 mm offset (versus 30 mm used in my sims) doesn't need much compensation.

              -jAy

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              • #22
                Re: Offsets, Asymmetrical Slopes, and Mysticism - revisited *PICS*

                Originally posted by Mark65 View Post
                And Wolf, I know that you've been telling me what to do all along, but I just didn't get it. The fault lies with me, not you....

                Mark
                Heh. I did try to explain it, but didn't know how technical to get before your mind exploded *yet again*. ;)

                I'm glad you got it!
                Wolf
                "Wolf, you shall now be known as "King of the Zip ties." -Pete00t
                "Wolf and speakers equivalent to Picasso and 'Blue'" -dantheman
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                • #23
                  Re: Offsets, Asymmetrical Slopes, and Mysticism - revisited *PICS*

                  There's only one other thing you can do and that is to treat the room.
                  I *think* you're getting my point here . . . baffle step is not a frequency response/phase issue (neither is cone breakup). It's a polar response and direct v. reverberant field issue, and *it cannot be "fixed" by messing with frequency response or phase*. You can get a "compromise" of sorts, but included in the compromise is . . . messed up phase. All "diffraction" issues are like that . . . they cannot be corrected in frequency response/phase/impulse response space *because any such corrections only work for one point in real space*. Putting diffraction rings around a tweeter or roundovers on the baffle edges changes the speaker's perceived frequency response (at some locations). Does it change the tweeter's "minimum phase"?
                  "It suggests that there is something that is happening in the real system that is not quite captured in the models."

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                  • #24
                    Re: Practical factors considered

                    Originally posted by jkim View Post
                    In addition, a 20 mm offset (versus 30 mm used in my sims) doesn't need much compensation.
                    I believe Jeff's design assumes a 25mm offset . . . and as Pete points out it does come much closer to actually *being* LR4 . . .
                    "It suggests that there is something that is happening in the real system that is not quite captured in the models."

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                    • #25
                      Re: Practical factors considered

                      Originally posted by Deward Hastings View Post
                      I believe Jeff's design assumes a 25mm offset . . . and as Pete points out it does come much closer to actually *being* LR4 . . .
                      Actually, that session included only 20mm offset, which would probably be about right for the RS28-RS150 combo. Jeff's discussion to start the thread did indeed model 25mm, but this version of the Dreydel, using the RS28 instead of the TDFC, only includes the 20mm.
                      R = h/(2*pi*m*c) and don't you forget it! || Periodic Table as redrawn by Marshall Freerks and Ignatius Schumacher || King Crimson Radio
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                      • #26
                        Re: Practical factors considered

                        Actually, that session included only 20mm offset, which would probably be about right for the RS28-RS150 combo. Jeff's discussion to start the thread did indeed model 25mm, but this version of the Dreydel, using the RS28 instead of the TDFC, only includes the 20mm.
                        Actually you're both right. The session I sent Pete sometime back used 20mm, and my most recent ones used 25mm, which is what I told Deward. As I also told Deward, this is a difficult parameter to measure. I use a method of taking a summed measurement and adjusting the simulation till it matches. You can look for the reverse null too, which is sometimes easier to see. What you are actually deriving when you do this is the relative acoustic offset between the two drivers and it will vary depending on how you mount your drivers too. So which is right? Hmmmmm....Deward came up with 24mm doing it the reverse null method and my latest sims used 25mm. In the one I sent Pete my sim used 20mm. But the difference between 20 and 25mm translates into 10 degrees of phase shift at 2kHz, and this won't alter the summed response much at all, so it is difficult to know for sure, and probably not consequential in the long run. I think though the actual value is closer to the 25 though, but it could be 23........
                        Click here for Jeff Bagby's Loudspeaker Design Software

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                        • #27
                          Re: Thanks Jeff!

                          Originally posted by biff View Post
                          That is a (relatively) concise overview and much appreciated. I feel that many times people with strong opinions who are largely in agreement are done a disservice by the lack of communication inherent in this nuance free one directional electronic exchange we generously refer to as communication ;) . Many conflicts would resolve themselves in the presence of both parties, good music, and a respectful klink of a couple of beverages.
                          You are no doubt correct, and perceptions are everything in this electronic medium. Like I said in Richmond - I seem much taller on the internet :D
                          Click here for Jeff Bagby's Loudspeaker Design Software

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                          • #28
                            Re: Practical factors considered

                            Now, check out an example whose target response is 2100Hz LR4, with a 20mm offset using response plots provided by Jeff B for the RS150 and RS28.

                            These response plots follow the target LR4 responses quite closely through the XO region. They don't look much relaxed to me, yet we can all see the excellent on axis response as well as the tight phase alignment reflected in the 20dB reverse null.
                            This example actually goes a bit to what Jay is saying. I am following the target with respect to the knee at the crossover point, but if you look at how the RS28 is rolling off you will see that the slope is less than LR4. This roll-off, even in this range, adjusts the phase in the crossver region too. You still need the right knee shape to sum correctly though.
                            Jeff
                            Click here for Jeff Bagby's Loudspeaker Design Software

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                            • #29
                              Re: Offsets, Asymmetrical Slopes, and Mysticism - revisited *PICS*

                              Originally posted by jkim View Post
                              Hi Brad,

                              The slightly lowered Fc in the sim of the second method is simply an artifact of this particular sim. If we want to, by adjusting the starting point of each rolloff, we can move it to the target Fc and still have the same effect.

                              -jAy
                              Yes, you are correct. And, thank you for your contribution to the thread.
                              Click here for Jeff Bagby's Loudspeaker Design Software

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                              • #30
                                Re: Offsets, Asymmetrical Slopes, and Mysticism - revisited *PICS*

                                Originally posted by Deward Hastings View Post
                                Once again . . . I'd like to see the practical passive implementation that deals with the RS150 cone breakup and leaves a flat phase response. Part of the problem seems to be that the "magnitude response" associated with breakup is not a single, simple resonance like the suspension resonance. It may not reduce to a simple model . . .
                                A real measurement won't be a simple model.

                                You would only use a simple model as an example to learn a concept.

                                You won't fix everything with a passive circuit even though it's possible it's not very practical.

                                Typicly you will have a two or three componet for a lowpass filter with a two or three componet notch (series or parallel resonant).

                                You are just going to tame the system not turn it into a perfect target response.

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