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  • Driver time alignment

    I recently ran across this market speak from Mission speakers for their bookshelf models:

    Mission’s classic IDG (Inverted Driver Geometry) driver format positions the high frequency below the bass / midrange driver meaning the length of path is equalised so the sound waves coincide at the listener’s head height. A proprietary principle that improves ‘time alignment’ and thus transient performance in Mission speakers

    Is there any general benefit to positioning the tweeter below the woofer as Mission claims, or do most designers address time alignment in the xover?

    Geometrically, Mission's method should give similar results to a slanted front baffle that some designers have used.

    What do you think?

    Marv

  • #2
    Time Alignment is a huge, complicated subject. Here's some light reading to introduce you to it: Phase, Time and Distortion in Loudspeakers (sound-au.com)
    Francis

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    • #3
      To directly answer your question: most passive crossovers only address phase alignment. Active (DSP) crossovers can easily introduce digital delay to get true time alignment.
      Francis

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      • #4
        You can do the same thing with any 2-way bookshelf speaker, just turn it upside down. Besides, who's to say that having the tweeter on the bottom puts it at the listener's head height? That depends on the placement of both the speaker and the listener. Besides, time align and transient response are only loosely related. Add this to the pile of advertising piffle that abounds in audio.
        www.billfitzmaurice.com
        www.billfitzmaurice.info/forum

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        • #5
          Yeah manufacturers do a lot of hand waving about time alignment, and how it will be transient perfect. And yes, in that one little tiny listening spot, it will be. Well, in an anechoic chamber at any event
          Francis

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          • #6
            ‘twas a big deal with the old Dahlquist speakers. Some hi-fi nuts still swear by their DQ-10, a 5-way “phased-array” open baffle with sealed bass unit.
            “Hear angel trumpets and devil trombones - you are invited!”

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            • Billet
              Billet commented
              Editing a comment
              A friend of mine built a 5 way speaker in the general style of a DQ10. I developed a crossover for the whole mess, and fortunately it turned out sounding very nice. He was very careful to align the voice coils and also keep the driver centers spaced apart by some multiple of the wavelength at the crossover frequency. The best description for these speakers is a "wall of sound".

          • #7
            If you were to draw a an equilateral triangle between the acoustic center of a tweeter and the woofer, which direction would it point? In many cases, it would point downwards since the woofers center is often deeper than the tweeters center. Since the speaker is also likely to be lower than ear height in many cases, a wavefront pointing downward is really less than ideal.

            Putting the tweeter under the woofer may be a simple improvement. It is not necessarily a complete solution, but it could be if all other variables are designed accordingly. A sloped baffle will have a similar effect.

            This time alignment is far more important when using first order crossovers. A higher order crossover can add delays to help align the woofer and tweeter wave fronts. Many writings on Linkwitz-Riley crossovers will show diagrams of this concept called lobing error or peaking.

            Like so many things speaker, It is a complex topic with many variables. I tried to simplify it as best I could.

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            • #8
              Some marketing color. Most speakers have the drivers time aligned on the "design axis". Inverting the speaker is a nice way of offsetting the tweeter. The hope is that the offset adds enough delay where you can achieve good time alignment/ phase alignment where you can use a simple 1st or second order crossover.
              John H

              Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower

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              • #9
                It's good marketing, yes, but there is a benefit to the inverted layout. That said, it's really only helpful during design IMO. I have a 2-way with an 8" ceramic midwoofer. Using one meter measurements on the tweeter axis I could not get a reasonable summed response. There was just too much acoustic offset...at one meter. So I measured on the midwoofer axis to design. Came up with a really good sounding design, reasonably flat on-axis...the woofer axis. I leave it inverted in use, bug when seated about eight feet away, I can't tell that it's inverted. I've listened both ways. But I leave it inverted since that's how it was designed.

                The result was, I have no doubt, a better power response. If you have much acoustic offset to start, then add more "offset" due to having to measure at one meter, then "correct" for this extra offset, the result will be a different power response. It's similar to the problem of trying to get a perfectly flat on-axis response when there's significant diffraction that alters the on-axis. Correcting for this can and likely will adversely affect the off-axis (power) response.

                Measuring other than on the tweeter axis may result in some slight droop in the measured tweeter response. But then most designers will make small changes to a design to achieve a preferred perceived response. In my case, I have added a 1db drop from 10-20k in a 3/4" tweeter. I've bumped up that area in larger tweeters. So designing on an axis other than the tweeter ultimately can be moot when it comes down to listening.

                It does make good ad copy, but for typical listening distances, I have not found it to matter. During design, yes, listening, no.

                dlr

                p.s. This was for an LR4 crossover, so that issue doesn't only affect low order.
                Last edited by dlr; 07-10-2021, 09:59 AM.
                WinPCD - Windows .NET Passive Crossover Designer

                Dave's Speaker Pages

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                • #10
                  Usually having the tweet under the woof, the tweet wouldn't be far enough behind on a 6db crossover.

                  i think a cap slightly moves a tweet forward (in time), but i could be wrong on that.

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                  • #11
                    Passive components add delay. If looking at a graph they add rotation which can look look a positive change in time, but that's against some physics law...
                    John H

                    Synergy Horn, SLS-85, BMR-3L, Mini-TL, BR-2, Titan OB, B452, Udique, Vultus, Latus1, Seriatim, Aperivox,Pencil Tower

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                    • #12
                      Thanks to all for your input and keep it up.

                      If there is a simple mechanical advantage to mounting the tweet below the woofer, then why has nearly everyone been doing it wrong for so long?!

                      To experiment, I flipped my Modula MTs. I need to do more critical listening before drawing a real preference. While testing with pink noise, I do notice a slight loss of highs directly on axis with the center of the woofer compared to lowering my ears to the tweeter height. Overall, from the listening position 9 feet away, it is negligible.

                      Maybe more of you can experiment and report your findings. That may be difficult for John to do with his Mini LXs LOL.

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                      • #13
                        Originally posted by jhollander View Post
                        Passive components add delay. If looking at a graph they add rotation which can look look a positive change in time, but that's against some physics law...
                        Right, it changes the group delay.
                        WinPCD - Windows .NET Passive Crossover Designer

                        Dave's Speaker Pages

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                        • #14
                          I did this with my Icthus 3-ways awhile back. Was able to use 2nd order elec xovers where it would have been different if flipped. Summation, IIRC, was LR4.

                          Besides the unconventional approach, they sounded quite good. Even though the tweeter was firing at my knees.

                          Marv, it will matter in terms of lobing, since your design axis was the tweeter.

                          Later,
                          Wolf
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                          • marvin
                            marvin commented
                            Editing a comment
                            Tweeter design axis comment understood. I think my observations confirm that.

                        • #15
                          Possible reasons to not put the tweeter under the woofer:

                          1. Probably not so good for a floor standing bookshelf speaker.
                          2. Could cause tweeter diffraction if mounted on a shelf or other flat surface.
                          3. Higher center of gravity, might tip easier.
                          4. Doesn't look "right"

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