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Port FR scaling - check my math

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  • Port FR scaling - check my math

    Apologies to anyone who finds this a stupidly basic question...just want to check!


    So when measuring bass reflex the process to combine the port response and woofer response is to scale the port response before merging.

    The degree to which the port response is scaled is expressed as:

    20 LOG(Port Diam / Woofer Diam)

    So as a practical example:
    20 x Log(50mm/150mm) = -9.542 (db)

    However, principally its just ratio of port to woofer....and I want to scale ports that are not circular. In these cases I had just been using the eyeball method of aligning the tails of the woofer and port response before merging.

    If we convert to using area instead of diameter I found that:

    20 x LOG(... != does not work...

    However:
    10 x Log(1963.4mm2 / 17671.46mm2) = -9.542 (db)....same result

    So am I correct in assuming that I can re-state the above as:

    10 LOG(Port Area / Woofer SD)
    Constructions: Dayton+SB 2-Way v1 | Dayton+SB 2-Way v2 | Fabios (SB Monitors)
    Refurbs: KLH 2 | Rega Ela Mk1

  • #2
    Originally posted by DeZZar View Post
    Apologies to anyone who finds this a stupidly basic question...just want to check!


    So when measuring bass reflex the process to combine the port response and woofer response is to scale the port response before merging.

    The degree to which the port response is scaled is expressed as:

    20 LOG(Port Diam / Woofer Diam)

    So as a practical example:
    20 x Log(50mm/150mm) = -9.542 (db)

    However, principally its just ratio of port to woofer....and I want to scale ports that are not circular. In these cases I had just been using the eyeball method of aligning the tails of the woofer and port response before merging.

    If we convert to using area instead of diameter I found that:

    20 x LOG(... != does not work...

    However:
    10 x Log(1963.4mm2 / 17671.46mm2) = -9.542 (db)....same result

    So am I correct in assuming that I can re-state the above as:

    10 LOG(Port Area / Woofer SD)
    Yes, makes sense since area is proportional to diameter squared.
    Francis

    Comment


    • #3
      Use speaker modeling software and it will do all the calculations for you.
      www.billfitzmaurice.com
      www.billfitzmaurice.info/forum

      Comment


      • #4
        Originally posted by billfitzmaurice View Post
        Use speaker modeling software and it will do all the calculations for you.
        Yeah? And which modelling software can you recommend that deals with the above measurement specifically?
        Constructions: Dayton+SB 2-Way v1 | Dayton+SB 2-Way v2 | Fabios (SB Monitors)
        Refurbs: KLH 2 | Rega Ela Mk1

        Comment


        • #5
          WinISD allows you to specify port shape and size.
          www.billfitzmaurice.com
          www.billfitzmaurice.info/forum

          Comment


          • #6
            Originally posted by billfitzmaurice View Post
            WinISD allows you to specify port shape and size.
            Yes it does. How does that help apply scaling to the measured result for combining port and woofer near field measurements?
            Constructions: Dayton+SB 2-Way v1 | Dayton+SB 2-Way v2 | Fabios (SB Monitors)
            Refurbs: KLH 2 | Rega Ela Mk1

            Comment


            • billfitzmaurice
              billfitzmaurice commented
              Editing a comment
              My mistake, I didn't realize that's what you were trying to do. I never bother with that myself. I do all measurements outdoors at a minimum of two meters. If I wanted to model port and cone output separately I'd use HornResp.

            • DeZZar
              DeZZar commented
              Editing a comment
              No worries. Outdoor measurements aren't an option for me so I have to follow the quasi anechoic process. This is ultimately the measurement to validate model, its just the only white paper that details this formula explicitly assumed round ports.

          • #7
            Originally posted by DeZZar View Post
            So am I correct in assuming that I can re-state the above as:

            10 LOG(Port Area / Woofer SD)

            Yes

            10 LOG (Area1 / Area2)
            If both are circles, then
            = 10 LOG ( pi * R1^2 / pi * R2^2)
            = 10 LOG (R1/R2)^2
            = 20 LOG (R1/R2)
            = 20 LOG (0.5*D1 / 0.5*D2)
            = 20 LOG (D1 / D2)

            Comment


            • DeZZar
              DeZZar commented
              Editing a comment
              Thanks mate! 👍

          • #8
            Originally posted by DeZZar View Post
            Yes it does. How does that help apply scaling to the measured result for combining port and woofer near field measurements?
            VirtuixCAD does have a tool for that. Imports FRD files and will calculate and adjust the close mic port and cone measurements. It'll also merge that to a far field measurement. I've only stumbled through it once though, so by no means an expert. Still a bit of a Virtuix noob.
            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
              Originally posted by wogg View Post
              VirtuixCAD does have a tool for that...
              Is that under the Merger tool?

              There's so much in Vituix I'm still finding things after using it for a couple of years!

              Constructions: Dayton+SB 2-Way v1 | Dayton+SB 2-Way v2 | Fabios (SB Monitors)
              Refurbs: KLH 2 | Rega Ela Mk1

              Comment


              • wogg
                wogg commented
                Editing a comment
                Yup, the merger tool!
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