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  • Isobaric loading

    Dan Neubecker's Scimitars use an isobaric loading configuration I believe, and got me thinking about it once again. I built an isobaric loudspeaker many years ago. What I don't remember is what the advantages and disadvantages are. It seems to me that at the time, the claim was of lower distortion. However afterwards, it was said that actually having two cones on the outside of the enclosure, which would halve the excursion needed, would actually produce lower distortion than the iso method. So what are the current thoughts on this loading technique?

  • #2
    I'm interested in the distortion piece myself. I've used ISO loading exclusively to trade a little efficiency while reducing box size, or extending response in the same box.
    Electronics engineer, woofer enthusiast, and musician.
    Wogg Music
    Published projects: PPA100 Bass Guitar Amp, ISO El-Cheapo Sub, Indy 8 2.1 powered sub, MicroSat, SuperNova Minimus

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    • #3
      http://www.enjoythemusic.com/diy/0512/kaboominator.htm

      The key to distortion minimization (even order only, and really, 2nd order primarily) is to have them clamshell mounted, and minimizing airspace between them (compliance of the air in the gap). Sometimes mechanical connection between the two surfaces is utilized for that reason. Some drivers have push/pull spiders (two spiders, "flipped") and push pull coils (JBL Differential Drive) to achieve much the same effect in a single driver.

      PPSL style builds (two drivers in a manifold, wired electrically out of phase, facing each other, with one magnet side and one cone side firing into the manifold) are more effective than isobaric for this, they're the opposite for box space though. They don't suffer from an additional compliant member in the gap between drivers, but they also are double the box size of the single woofer, compared to half, with isobaric (but 6dB more efficiency than isobaric, 3dB more than single-driver). PPSL style loading also gives force cancellation, helping improve box performance and is my preferred bass loading method (I've used both).

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      • #4
        Two drivers mounted isobaric results in halved Vas compared to one driver, but still with the maximum SPL of one driver. That was a benefit 30 or more years ago when a 20 cu ft or more Vas was common, not so much today. There's no reduction in THD, just overall cabinet size.
        www.billfitzmaurice.com
        www.billfitzmaurice.info/forum

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        • #5
          Originally posted by billfitzmaurice View Post
          Two drivers mounted isobaric results in halved Vas compared to one driver, but still with the maximum SPL of one driver. That was a benefit 30 or more years ago when a 20 cu ft or more Vas was common, not so much today. There's no reduction in THD, just overall cabinet size.
          Only clamshell style isobaric reduces distortion, as I mentioned, and that's limited by trapped air compliance (as I mentioned). If you're able to find inexpensive drivers with decent matching and performance, as in the kaboominator, you can have the same efficiency as the single driver, with twice the output, four times the thermal handling, force cancellation, all in the same box size. I'll take that setup over a single higher performance driver anyday.

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          • #6
            Have to check, but off the top of my head, it's due to having the two drivers out of phase and thereby you get cancellation of whatever motor and suspension nonlinear behavior there is. In other words, when one driver moves "out" the other driver moves "in". So, assuming the two drivers are similar, that kind of distortion would cancel out.

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            • #7
              That's funny... I've read that Kaboominator page before and appear to have not retained the distortion cause information. I get it now, motor assembly non-linearities will be averaged since one driver is going forward while the other is pulling in. Provided the nonlinearity is a one way tilt due to the coil position vs. the pole piece, that distortion should be reduced.

              The key text from that page:
              In subwoofers harmonic distortion is high. Low frequencies require more cone excursion than higher frequencies for a given SPL, and THD is directly linked to excursion levels in typical speaker designs. The reason for advanced motor types is to limit distortion and increase maximum output by extending and/or flattening the curves used to represent how a speaker changes performance with changing positions of the voice coils. Some examples are BL(x) and Le(x). This is the case in the XBL^2 motors used in the CSS Trio 12s. See this article.
              If you want more information on the curves representing loudspeaker performance, I suggest you review Klippel's site as they go extensively into discussion of most everything related to drivers. Here's a cheat sheet from their site at this link.
              If you look at these, you notice that often the curves are tilted- this creates even order (2nd, 4th and 6th) distortion components. When you do the "Equal but opposite" arrangement as used in A or C of the diagram, where the cone of one is moving away from the magnet and the other towards the magnet (but arranged and wired to pressurize the same space), the tilt is averaged out to a flatter overall line, giving lower even-order distortion.
              So your PPSL style builds are simply double woofer in single air space boxes, except one of the drivers is inverted (physically and electrically)?
              Electronics engineer, woofer enthusiast, and musician.
              Wogg Music
              Published projects: PPA100 Bass Guitar Amp, ISO El-Cheapo Sub, Indy 8 2.1 powered sub, MicroSat, SuperNova Minimus

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              • #8
                Wow, look what happens when I start an answer at work, and get interrupted for a bit! I would have been the third poster till the crowd showed up! ;-)

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                • #9
                  Originally posted by wogg View Post
                  That's funny... I've read that Kaboominator page before and appear to have not retained the distortion cause information. I get it now, motor assembly non-linearities will be averaged since one driver is going forward while the other is pulling in. Provided the nonlinearity is a one way tilt due to the coil position vs. the pole piece, that distortion should be reduced.

                  The key text from that page:


                  So your PPSL style builds are simply double woofer in single air space boxes, except one of the drivers is inverted (physically and electrically)?
                  They have to be mounted opposing each other in a manifold to get the force cancellation. The manifold also provides some additional damping/distortion reduction and Fs reduction if you keep the air volume minimized and magnet "nested" in the cone well (obviously leaving clearance up to xmech or thereabouts).

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                  • #10
                    Originally posted by jonpike View Post
                    Have to check, but off the top of my head, it's due to having the two drivers out of phase and thereby you get cancellation of whatever motor and suspension nonlinear behavior there is.
                    Isobaric doesn't have that effect, because only one cone is exposed to the air. This also means that the radiating Sd and Vd is that of a single driver. That's why the SPL and Max SPL is that of one driver.

                    www.billfitzmaurice.com
                    www.billfitzmaurice.info/forum

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                    • #11
                      Ok, I was thinking of a basic pair of drivers in push pull, not the isobaric configuration.

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                      • #12
                        Dan Neubecker's Scimitars are push pull in a vented cabinet.
                        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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                        • #13
                          May I suggest to read my analysis and look at the calculated examples here: http://www.musicanddesign.com/Isobaric.html
                          John k.... Music and Design NaO dsp Dipole Loudspeakers.

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                          • #14
                            Originally posted by billfitzmaurice View Post
                            Isobaric doesn't have that effect, because only one cone is exposed to the air. This also means that the radiating Sd and Vd is that of a single driver. That's why the SPL and Max SPL is that of one driver.

                            Which is WHY you have to use clamshell and the priority is in keeping the air volume between cones minimized so that both cones are coupled to the same acoustic load.

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                            • #15
                              I have no knowledge of what the kaboominator is or was, but in any isobaric mounting configuration for a pair of drivers, there cannot be any increase in output SPL since only one driver is exposed to the room. While the thermal handling will be doubled simply due to there being two driven drivers, power requirements are also doubled without any increase in SPL for a specific input voltage compared to a single driver. While distortion may be reduced with the drivers facing each other, the only real advantage is cutting Vas and, therefore, Vb in half.
                              Paul

                              Originally posted by badman View Post

                              Only clamshell style isobaric reduces distortion, as I mentioned, and that's limited by trapped air compliance (as I mentioned). If you're able to find inexpensive drivers with decent matching and performance, as in the kaboominator, you can have the same efficiency as the single driver, with twice the output, four times the thermal handling, force cancellation, all in the same box size. I'll take that setup over a single higher performance driver anyday.

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