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Passive Radiator Design Question

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  • Passive Radiator Design Question

    I’m interested in building a very compact passive radiator enclosure. For simplicity, let’s say I’m using one Dayton ND91-4 driver. According to Dayton, a good match for this driver would be their ND90-PR passive radiator. That PR has double the displaced volume Vd of that driver, so it would seem that one PR would be enough.

    Most sources suggest that you should begin your PR system design by designing a vented enclosure. For that driver in a vented enclosure, Dayton recommends a 0.85 l cabinet tuned to 73 Hz. If we set that up in WinISD and then size the hypothetical port to have the same cross-sectional area as the Sd of the PR, we get a port that is 38.5” long with a volume of 180 cu.in. The air mass enclosed therein would be 3.8 g. But when we go back to the specs on the ND90-PR, we see that it has a mass Mms of 8.1 g.

    From the design guidelines I have seen, as a starting point you want the mass of the PR to be about the same as the air mass inside the equivalent port, and we’re not even close.

    What am I missing here? Dayton says this PR is a good match for this driver, so clearly there is something I am not understanding. Thanks in advance for any help.

  • #2
    You can't directly compare the air mass to the PR mass, the suspension compliance of the PR throws that out the window. You'll need to use a modeling program that can handle a PR with all the parameters.
    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
      I use the vented model as a baseline to then tweak the PR alignment to get it to match (overall response). Then in reviewing the PR model I can determine if the single PR is adequate or need doubles or model a different PR altogether.

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      • #4
        Thank you both, your answers are both helpful. More insights/tipsare also welcome.

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        • #5
          Yeah, Dayton's box recommendations (Bass Box Pro ?) sometimes aren't the greatest.
          AAMOF, WinISD ( linearteam.org ) only shows an F3 of around 100Hz w/that size box/tuning.
          WinISD "likes" (defaults to) a bit larger box (0.05cf, or about 1.4L) at approx. that same tuning (mid 70s). Then the F3 is also in the low 70s.
          Using a single ND90-PR (w/NO "added mass") tunes 1.4L into the high 70s, w/an F3 in the 80s.

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          • #6
            Originally posted by tstone View Post
            Thank you both, your answers are both helpful. More insights/tipsare also welcome.
            It's been a while since my brain went down this path. Good suggestions in here so far. If you're worried about achieving correct tuning then you can build your project then do an impendence sweep to determine tuning. The peak will be at the tuning frequency. Assuming the tuning is too high just add mass until it drops where you want it. The best part about this method is that you can verify a model while taking into account all variables like construction and manufacturing tolerances.

            Adding some fill to the enclosure will also drop the tuning some.

            Aaron
            Thanks,
            Aaron

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            • #7
              Your brain hasN'T gone there (here) in a while.

              Actually, a "tuned" box (vented, OR PR'd) has a double peak impedance profile. The tuning freq. (Fb) is actually the valley between the 2 peaks.
              WinISD ("Pro") will show you these peaks if you click on the "Driver Impedance" plot.

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              • #8
                I used WinIsd Pro to model the PR setup in my truck. It's not too compact though 😃 lol it's a 5cu ft box, with two 15" PR's, tuned to 32 Hz. Best sounding enclosure I've ever built... Out of more than 30 I've built.

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                • #9
                  Originally posted by Chris Roemer View Post
                  Actually, a "tuned" box (vented, OR PR'd) has a double peak impedance profile. The tuning freq. (Fb) is actually the valley between the 2 peaks.
                  WinISD ("Pro") will show you these peaks if you click on the "Driver Impedance" plot.
                  Ahhh, that sounds more familiar (I was picturing a raw driver impedance plot in my head). With some corrected info my point is that tuning can done without PR specs with the use of an impedance sweep. Model a vented box, build it for a PR and tune with measurements.
                  Thanks,
                  Aaron

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