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  • Q: Slot Port Length

    I'm doing an econo-stereo upgrade for my wife's Mini Cooper S. Space is a premium, and I'm planning a small ported sub-woofer to sit flat in the cargo area - using a Tang Band W8-1853. To get a decent port area, I need to use a slot port.

    Details aside, I'm curious on everyone's thoughts on "theoretical" port length. I'm basing port length on the center of the port (no brainer). But what about where the port terminates inside the box?



    Picture 1 - The port "length" ends at the end of the internal port wall.



    Picture 2 - The port "length" ends at a line drawn 45-degrees from the port wall.



    Or is it something else, since the port length is only defined by 1 of 4 walls.

    Really, for this build, it likely won't matter one bit.:p More than anything, I'm just curious what the "right" way is.

  • #2
    Re: Q: Slot Port Length

    I've always understood picture #2 would be the correct way to do it. From my experience, you can tune a bit higher in a vehicle than you normally would. Impedance sweeps I've taken in-car vs. out-of car almost always show real reduction in tuning once the enclosure is loaded into the vehicle - so if you're going to err on the side of tuning too high you should be fine.

    Comment


    • #3
      Re: Q: Slot Port Length

      I've always understood picture #2 is the best way to model a slot port. Its also understood that slot ports shouldn't be too far out-of-square, i.e. 1" tall and 8" wide as this sometimes throws the tuning off a bit too much (lower). It looks like your port has a H/W ratio of maybe 1.5:1 so that shouldn't be a problem. There is also the school of thought that slot ports will tune a little lower (with equal cross-sectional area) because they do not have the open interior end sitting in free space (more than 1 port diameter) away from any interior structures. Since they use up to 3 walls to make the port, there some additional port friction from the walls and hence lower the tune just a bit so SPLEclipse is probably right error on the high side by an Hz or 2 and things should sound just fine.

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      • #4
        Re: Q: Slot Port Length

        The 45* double-bend half-way along the port is a nice touch, but not really necessary.
        It would be more important to run a (router) "roundover" bit around the port's mouth (exit). 3/8"-1/2" would be great, but even a 1/4" should reduce potential "chuffing".

        Comment


        • #5
          Re: Q: Slot Port Length

          Originally posted by Chris Roemer View Post
          The 45* double-bend half-way along the port is a nice touch, but not really necessary.
          Not only is it unnecessary it also reduces the effective duct length compared to a 90 degree turn, and compensating for that robs box volume.
          There is also the school of thought that slot ports will tune a little lower (with equal cross-sectional area) because they do not have the open interior end sitting in free space (more than 1 port diameter) away from any interior structures.
          I've never found the duct input position to make any difference. But a very narrow duct does affect tuning. There is a high friction zone at the walls of the duct, which effectively makes that zone act more like a second duct wall, reducing the effective port area and in the process lowering Fb. With a very narrow duct, less than 1.5 inches or so, the high friction zone is a high enough percentage of the entire duct area to make the Fb shift significant.
          www.billfitzmaurice.com
          www.billfitzmaurice.info/forum

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          • #6
            Re: Q: Slot Port Length

            Thanks everyone - this is along the lines of what i understood, but it's helpful to get clarification.

            I actually threw in the 45's because I thought I would be chided for not having them..they're gone.:p

            I'm not going to get too fancy with port radii though - simply because I want to bang this project out in an afternoon. The Mini has a loud intake and exhaust, so port noise would have to be pretty bad to be audible. I'm not going to "over-engineer' this thing, it's just a quick-n-dirty project.

            Comment


            • #7
              Re: Q: Slot Port Length

              Originally posted by chad1376 View Post
              I actually threw in the 45's because I thought I would be chided for not having them..
              Reflectors at bends are only a necessity for transmission of frequencies much higher than those passed by a reflex port.
              www.billfitzmaurice.com
              www.billfitzmaurice.info/forum

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              • #8
                Re: Q: Slot Port Length

                I'll issue the dissenting remarks. This is a very good question and one isn't often addressed. I have personally performed some testing on this and arrived at some conclusions that are contrary to what is normally stated, but line up well with the mathematics of fluid dynamics.

                In normal Helmholtz theory the tuned port is assumed to be in the center of the chamber, or at least a considerable distance from any of the interior walls. Even at that ports have an "end correction" built into the length formula. This end correction is designed to make the length tuning more accurate due to an additional mass of air that is moving with the internal air of the port just outside the ends of the port due to friction between air molecules. Due to this, the effective length of the port is always longer than its actual physical length. So, the end correction allows for a shorter port to arrive at the correct tuning.

                Now, all of this changes with a tube port if you place it near the corner of the box. I found on one project with a 3 cu ft enclosure that the 3" port that was located on the bottom corner of the box needed to be shortened by an additional 2" to arrive at my desired 25 Hz tuning frequency. The reason was the additional friction along the cabinet internal walls that effectively made the port behave like it was longer than it actually was.

                A slot port will demonstrate the same thing since it is inherently using an internal wall of the cabinet as part of the port, and this side of the port will extend on past the end of the port. As a result, a slot port needs to be calculated with a different end correction factor than is typically used for a round port, but no one seems to adjust this end correction. If measurements are taken you will always find a slot port has tuned a cabinet a little lower than targeted. So, this long drawn out answer is to say that your third picture will actually be more accurate, but it may still be off a little.

                Jeff
                Click here for Jeff Bagby's Loudspeaker Design Software

                Comment


                • #9
                  Re: Q: Slot Port Length

                  Originally posted by Jeff B. View Post
                  Now, all of this changes with a tube port if you place it near the corner of the box.
                  Maybe, maybe not. I usually use a baffle mounted tube for prototyping, because if you use a sleeve mount you can easily swap tubes to see what happens to measured results. But I use corner ports for the finished design, to use them not only as ports but also as bracing for the panels they attach to. I haven't found the corner ports to give a significantly different result than the baffle mounted tube port of the same net area and length. This is with pro-sound designs with typical Fb in the 40-60Hz range. It may be different with lower tunings.
                  www.billfitzmaurice.com
                  www.billfitzmaurice.info/forum

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                  • #10
                    Re: Q: Slot Port Length

                    Take a look at this for your end correction values. Its in german, but its pretty easy to figure out.

                    Click image for larger version

Name:	PortCorrection.jpg
Views:	3
Size:	89.6 KB
ID:	1166271

                    I build a LOT of slot ported enclosures. Even when using the above end correction values my enclosures usually end up being tuned slightly lower than I intend.

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                    • #11
                      Re: Q: Slot Port Length

                      Nice, actual numbers to work with. So, I use WinISD. I take it that it already incorporates the K value for a centered port. To use a slot port, it appears I should apply a ratio of centered port K / slot port K to reduce the port length? This would be consistent with Jeff B's observations that a slot port needs to be shorter.

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                      • #12
                        Re: Q: Slot Port Length

                        Originally posted by the-jessman View Post
                        Take a look at this for your end correction values. Its in german, but its pretty easy to figure out.

                        [ATTACH=CONFIG]56994[/ATTACH]

                        I build a LOT of slot ported enclosures. Even when using the above end correction values my enclosures usually end up being tuned slightly lower than I intend.
                        Thanks, I had not seen that before, but it is exactly as I expected it would be.
                        Click here for Jeff Bagby's Loudspeaker Design Software

                        Comment


                        • #13
                          Re: Q: Slot Port Length

                          Originally posted by the-jessman View Post
                          Take a look at this for your end correction values. Its in german, but its pretty easy to figure out.

                          [ATTACH=CONFIG]56994[/ATTACH]

                          I build a LOT of slot ported enclosures. Even when using the above end correction values my enclosures usually end up being tuned slightly lower than I intend.
                          Ok, I didn't get the k values. How do we apply the corrections?

                          Comment


                          • #14
                            Re: Q: Slot Port Length

                            Originally posted by ani_101 View Post
                            Ok, I didn't get the k values. How do we apply the corrections?
                            They fit in the standard port length equation. The standardized equation uses English units so,

                            L = length in inches
                            Fb = Tuning frequency in Hertz
                            Vb = Cabinet net volume in cubic inches (1 cubic foot = 1728 cu. inches, 1 Liter = 61.06 cu. inches)
                            R = The radius of the port in inches (assuming a round tube port)

                            If you have a slot port, like one 2" x 8" then you need to convert this to an equivalent round port to get the equivalent radius:
                            2 x 8 = 16 in^2, 16/3.14159 = 5.093 , Sqrt(5.093) = R = 2.26" as an example

                            Now, the port length equation is :

                            L = ((1.463 * 10^7 * R^2) / (Fb^2 * Vb^2)) - (2*K*R)

                            So, you would need to substitute the value shown in the graphic above for K in this equation. The end correction (2*K*R) shortens the port to accommodate air moving just outside the port that is making it behave like it is longer than it physically is.

                            Jeff B.
                            Click here for Jeff Bagby's Loudspeaker Design Software

                            Comment


                            • #15
                              Re: Q: Slot Port Length

                              Jeff,

                              If I do Vb^2, the number is huge and results in odd numbers. I'll guess that it should not be squared (not squared matches your Woofer Box Model and Circuit Designer's results).

                              Now, using the 2.227 for a slot port makes a lot of difference, a [email protected] using 2 flush ends (k=0.732) gives 34.98"
                              With the 2.227 I get 28.27". 6.71" in difference.

                              Is that number in line with what you would expect to see?

                              Originally posted by Jeff B. View Post
                              They fit in the standard port length equation. The standardized equation uses English units so,

                              L = length in inches
                              Fb = Tuning frequency in Hertz
                              Vb = Cabinet net volume in cubic inches (1 cubic foot = 1728 cu. inches, 1 Liter = 61.06 cu. inches)
                              R = The radius of the port in inches (assuming a round tube port)

                              If you have a slot port, like one 2" x 8" then you need to convert this to an equivalent round port to get the equivalent radius:
                              2 x 8 = 16 in^2, 16/3.14159 = 5.093 , Sqrt(5.093) = R = 2.26" as an example

                              Now, the port length equation is :

                              L = ((1.463 * 10^7 * R^2) / (Fb^2 * Vb^2)) - (2*K*R)

                              So, you would need to substitute the value shown in the graphic above for K in this equation. The end correction (2*K*R) shortens the port to accommodate air moving just outside the port that is making it behave like it is longer than it physically is.

                              Jeff B.
                              My Statement monitors
                              My Piccolatas
                              My LM3886 amp

                              Comment

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