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  • port pvc sizes

    I see a lot of designs that leave a lot of the volume specs a little vague, like it doesn't matter if you're not exactly five liters in size, five liters ish, they seem to be saying, and the same is true with the port tubes, too, where they'll just say 1.5 pvc, but off the shelf pvc isn't actually 1.5, and there is pvc out there that is exactly 1.5, but you have to work to find it, and they don't make clear which one we should be using. Right now I'm looking at Paul Carmody's Isetta, and it just says "The port is 1.5" in diameter, and 10.5" long." That's it. Based on that, what am I looking for, the Home Depot special, or schedule 80 furniture grade pvc?

    Thanks.

  • #2
    I would guess the dimensions given would be for inside size, since the formulas are based on volume.
    ( not much difference between 40 & 80 )
    https://pvcfittingsdirect.com/pvc-pipe-sizes-chart/

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    • #3
      Your're correct on the PVC sizing. See the chart below. The 1.5" and smaller schedule 40 PVC (the common stuff) inside diameter can be meaningfully different than the nominal size. The inside diameter of 1.5" Schedule 80 PVC is exactly 1.5", but less common (at least in my neck of the woods, not much available at HD, a little more from Lowes).

      Given the TB W5 sub and the Isetta box sizing, it's relatively easy to model (WinISD) the longer length needed for the schedule 40's inside diameter of 1.61" to achieve the same box tuning (~12.25 long).

      I'd go with schedule 40 and a longer port. It's cheap, readily available and has a wide array of elbows, connectors, etc. to support the build.

      Click image for larger version

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      • #4
        You do not need to use plastic...cardboard tubes work also.

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        • #5
          So true.
          (I even used a model rocket body tube on my piconeo.)

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          • #6
            Cardboard mailing tubes used to be the go-to.
            Francis

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            • #7
              Whatever kind of pipe you use though, flares on the ends are a good idea.
              Francis

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              • #8
                There's a lot of vagaries going on when putting together a speaker.

                Does 5l really matter compared to 5.1l? Does an 8" port length matter to 7.75"? Will either of them tune to what the calculator said it should? How about the sound absorption inside - foam densities vary, as does the amount of fibrefill one person thinks is enough vs. another may say it is too much OR too little. So many randoms.. Even the driver to driver variance.

                Don't get me wrong - I like to build as exactly to plan as I can, but I try to not get too bent on the fine details when, say I mess up a cut and lose a couple % of cabinet volume.

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                • #9
                  Originally posted by Dukk View Post
                  There's a lot of vagaries going on when putting together a speaker.

                  Does 5l really matter compared to 5.1l? Does an 8" port length matter to 7.75"? Will either of them tune to what the calculator said it should? How about the sound absorption inside - foam densities vary, as does the amount of fibrefill one person thinks is enough vs. another may say it is too much OR too little. So many randoms.. Even the driver to driver variance.

                  Don't get me wrong - I like to build as exactly to plan as I can, but I try to not get too bent on the fine details when, say I mess up a cut and lose a couple % of cabinet volume.
                  Well said
                  craigk

                  " Voicing is often the term used for band aids to cover for initial design/planning errors " - Pallas

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                  • #10
                    Originally posted by craigk View Post

                    Well said
                    I agree. I would think most anyone would be hard pressed to hear a 5 Hz difference in F3. Not to mention how much influence speaker placement and the room have.
                    Craig

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                    • #11
                      I've actually started to 3d print my ports. So I think there's much more flexibility for designers to not be limited by standard pvc sizes. On cabinet volume, I believe Paul recommends staying within 10% of spec. That's a fair bit of wiggle room. My understanding is that baffle width is the most important dimension to adhere to but the more knowledgeable here can correct me if I am mistaken.
                      Carbon13

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                      • #12
                        12.25" it is, thanks!

                        Quick question, when playing with WinISD, how do I know what frequency I should tune the box to? Paul Carmody says on his Isetta build page "However, it does let the sucker play way down low, with a -3 dB point of 35 Hz" so I assumed I should tune the box to 35 Hz, but when I do my port tube lengths are much longer than the design calls for, leaving me saying, welp, I don't know what I'm doing! Tuning it to 45Hz gets me much closer to the 10.5" in Paul's write up.

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                        • #13
                          Vent tuning can be challenging. What diameter, how many, what frequency.. To make things more interesting, the F3 is rarely the vent tuning frequency.

                          As with so many things, it depends on what you are trying to accomplish. First, I like to decide what I want out of it - do I want it to 'dig' or 'hammer' and how much gain I want out of it. Rudimentary criteria to be sure but it is the difference between tuning closer to 30 or closer to 40 (or higher for small drivers) and it is different if it is for a full range system or just a subwoofer. Fs of the driver has to be considered, as does the listener and their musical tastes.

                          Then, when in WinISD I juggle enclosure size (sometimes not much of a variable if there is limited space) and tuning while keeping an eye on both the response shape and cone excursion. Once I am happy with that I move to vent design and try to get as much vent area as I can while keeping lengths either under 24" OR what will actually fit in the enclosure. I keep an eye on air speed, keeping it as low as possible and trying to never exceed ~70fps, again with some caveats there if it is for a room or a car with or without a separate trunk space. There CAN be too much vent area but I don't run into that situation very often. Some software will actually suggest a tuning frequency based on the enclosure size to keep in with whatever box alignment was chosen but WinISD does not.

                          Fairly basic guidelines but getting too crazy into the details gets confusing and I find that over-analyzing it gets into diminishing returns pretty quickly. YMMV

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                          • #14
                            When you're shopping for PVC for ports don't forget to look in the electrical area -- PVC conduit has some interesting pieces. I used a PVC "sweep" ( a 90 degree elbow that has a long bend, something like a 2' radius) for the port in a subwoofer I built. It was a cylindrical 12" tube about 3' tall and I needed the sweep to fit the port length in the tube and go out the side of the tube. Link is to one example, there are other sizes.

                            https://www.lowesforpros.com/pd/CARL...ing/1000976036

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                            • #15
                              Here's a little spreadsheet that I put together a few years ago.
                              Attached Files

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