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Passively-assisted subwoofer (a worked example)

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  • Passively-assisted subwoofer (a worked example)

    a.k.a. "How to get to 30 Hz with a 12" subwoofer in a 1 cu.ft. box" ...

    The graph below shows the impact of adding 1000 uF of capacitance in series with a "cheap" high-inductance 12" car audio subwoofer driver in a small (~1 cu.ft.) sealed box, Fb ~40 Hz. The unassisted response is shown in blue. It's the classic "humpy" response of a high-inductance subwoofer driver loaded into a small box. The impact of 1000 uF of series capacitance is shown in red. The hump is basically eliminated, and the response @ 30 Hz is increased by 3dB. Not bad for $15 in caps, eh?

    If you want to try something like this, just model a subwoofer driver in a sealed box. Make sure that the model uses the driver's semi-inductance parameters for more accuracy. Then, export the FRD and ZMA files, import them into XSim, and just experiment with adding series capacitance until you get the best results.

    For this particular example, 850 uF of series capacitance theoretically yields the flattest passband and the most gain around 30 Hz, but I sacrificed a little of that to keep the response identical at 20 Hz.

    Click image for larger version

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    Brian Steele
    www.diysubwoofers.org

  • #2
    Excellent. I have to try this. Thanks for posting.

    Comment


    • #3
      Brian, that's a real difference. Pretty good for 1 cube. Thanks for detailing the process too.

      It would be interesting to see power handling figures for this, and a ported box with comparable FR.

      Which caps did you use, or would you recommend?

      TomZ
      Zarbo Audio Projects Youtube Channel: https://www.youtube.com/channel/UCEZ...aFQSTl6NdOwgxQ * 320-641 Amp Review Youtube: https://youtu.be/ugjfcI5p6m0 *Veneering curves, seams, using heat-lock iron on method *Trimming veneer & tips *Curved Sides glue-up video
      *Part 2 *Gluing multiple curved laminations of HDF

      Comment


      • #4
        It won't be possible to produce a portable box with a comparable FR with this driver. The box will need to be very big, and the FR will be even more peaky.

        I expect though that if you were able to put together a driver that would achieve the same F3 in a box this size, it will be a bit more sensitive, but have lower power handling below Fb, as it will be unloaded below that frequency, while a passively-assisted sealed alignment is not. You will likely also run into vent compression issues trying to keep up with the passively-assisted one because of the compromises made trying to squeeze a long vent into a box that size in order to tune it so low. Diaphragm displacement does increase at the same voltage level (which must happen to produce that extra output at low frequencies), but seeing that it's a 12" driver in a sealed box, I don't think that will be too much of an issue.

        I used the basic 500 uF NPE caps available from PE. The voltage rating is important too. These are 4 ohm drivers rated at 350 W, which works out to 37 V. The caps are rated at 100 V, so there's a decent margin there to work with. If these drivers are ever hit with 2.5kW, they'll give up before the caps do :-).


        Brian Steele
        www.diysubwoofers.org

        Comment


        • #5
          A first order high pass filter. Not exactly new, but you are right, many people don't think about it for a sub.
          But how does it perform in the real environment? Room/cabin gain? It may still be too boomy.
          Personally, I would use active eq, linear or digital. See "Linkwitz transform"

          Comment


          • #6
            Originally posted by tvrgeek View Post
            A first order high pass filter. Not exactly new, but you are right, many people don't think about it for a sub.
            But how does it perform in the real environment? Room/cabin gain? It may still be too boomy.
            Personally, I would use active eq, linear or digital. See "Linkwitz transform"
            It sounded fine for the brief period that I tested it.

            Try putting together an active EQ for $15
            Brian Steele
            www.diysubwoofers.org

            Comment


            • #7
              Just an op-amp and a few parts. Steal power from the plate amp. Easy. Sometimes you can even bugger the input buffer and don't actually add anything. For instance, what value cap would replace the input DC blocking cap to give the same result? A $2 film?

              Comment


              • #8
                An active first order prior to the amp wouldn't give the same transfer function. The cap prior to the woofer interacts with the impedance at resonance, in this case working really well with that particular woofer and box.

                I think this is a really cool solution. It can't be thrown around universally, but very cool nonetheless.
                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

                Comment


                • #9
                  Can I ask what model of sub you used in your example, and what were the Key T/S specs?

                  Comment


                  • #10
                    Originally posted by kevintomb View Post
                    Can I ask what model of sub you used in your example, and what were the Key T/S specs?
                    It's the Infinity 122.7W car audio 12" driver. I used to have them in my car, but replaced them years ago with Alpine Type Rs. They've been sitting in my garage cupboard ever since. Their specs didn't make them a great match for anything, until I considered a passively-assisted alignment.

                    Sd=531.00
                    Bl=15.24
                    Cms=2.65E-04
                    Rms=5.29
                    Mmd=225.58
                    Le=6.59
                    Re=4.00
                    Pmax=100
                    Xmax=12.0
                    Re'=4.55
                    Leb=1.20
                    Le=6.59
                    Ke=0.39
                    Rss=332.41
                    Brian Steele
                    www.diysubwoofers.org

                    Comment


                    • #11
                      It's not at all a first order filter. That would require a resistive load past the capacitor. Instead, there's effectively an LC circuit at the resonant peak.
                      Francis

                      Comment


                      • #12
                        Originally posted by Brian Steele View Post

                        It's the Infinity 122.7W car audio 12" driver. I used to have them in my car, but replaced them years ago with Alpine Type Rs. They've been sitting in my garage cupboard ever since. Their specs didn't make them a great match for anything, until I considered a passively-assisted alignment.

                        Sd=531.00
                        Bl=15.24
                        Cms=2.65E-04
                        Rms=5.29
                        Mmd=225.58
                        Le=6.59
                        Re=4.00
                        Pmax=100
                        Xmax=12.0
                        Re'=4.55
                        Leb=1.20
                        Le=6.59
                        Ke=0.39
                        Rss=332.41
                        Just found this....https://speakerboxlite.com/subwoofer...specifications

                        Comment


                        • #13
                          Originally posted by kevintomb View Post
                          Those are the parameters I measured using DATS. The semi-inductance parameters were derived from the measured impedance curve.
                          Brian Steele
                          www.diysubwoofers.org

                          Comment


                          • #14
                            Originally posted by tvrgeek View Post
                            A first order high pass filter. Not exactly new, but you are right, many people don't think about it for a sub.
                            But how does it perform in the real environment? Room/cabin gain? It may still be too boomy.
                            Personally, I would use active eq, linear or digital. See "Linkwitz transform"
                            You are thinking that it's just a first order high pass, well it's more than that. Look at the amount of capacitance used, how it interacts with the woofers impedance. it's a resonant circuit.

                            Comment


                            • #15
                              I've pretty much stayed away from this forum lately because it sucks. A member who has only been here about a month that craps on everyone else's posts is just another big reason. Good luck guys
                              Craig

                              I drive way too fast to worry about cholesterol.

                              Comment

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