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  • front or rear port?

    I've been watching and looking at all these enclosure designs for vented systems - and building yet another set (actually two) right now. What I have not seen is any justification for a front port versus a rear port. Can someone elaborate? My systems are always 12 to 24 inches from a wall is that helps.

  • #2
    Re: front or rear port?

    The lower the vent tuning, the less it matters as the wavelengths are just so long at 30Hz.
    R = h/(2*pi*m*c) and don't you forget it! || Periodic Table as redrawn by Marshall Freerks and Ignatius Schumacher || King Crimson Radio
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    • #3
      Re: front or rear port?

      Originally posted by mtngoat View Post
      I've been watching and looking at all these enclosure designs for vented systems - and building yet another set (actually two) right now. What I have not seen is any justification for a front port versus a rear port. Can someone elaborate? My systems are always 12 to 24 inches from a wall is that helps.
      Do you have Hornresp or MJK's Mathcad worksheets?

      I was pretty surprised how much difference front vs rear ports make in simulations. I've never actually done any meaurement comparisons but a capable simulation program (like the ones I mentioned) show a fairly dramatic difference over a fairly wide bandwidth.

      If you don't have one of those programs and there is interest I can probably post a comparison picture or two later when I get some time.
      Don't even try
      to sort out the lies
      it's worse to try to understand.

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      • #4
        Re: front or rear port?

        Ok, I had a few minutes and made a few graphs using MJK's MLTL worksheet, which I believe still has the original inputs it came with (not my design).

        I wasn't sure how to present this properly (and I've never really looked into this in much detail before) so I ended up making a bunch of pics.



        The first graph in pic 1 is the phase 1 output , which means it does not including diffraction (w/baffle step), boundaries, defined port location on box exterior, etc.

        THe second graph in pic 1 shows the same thing as graph 1 but with a front port designation, but with diffraction and a single boundary (rear wall).

        The third graph in pic 1 shows the same box as graph 1 but with a rear port (w/ diffraction and rear wall).



        The first graph in pic 2 shows the same box as pic 1 graph 1 with a front port and with diffraction but with NO boundaries nearby.

        THe second graph in pic 2 shows the rear port (w/diffraction and no nearby boundaries).



        And pic 3 is probably most telling. The first graph is PORT OUTPUT ONLY from a rear firing port w/ no nearby boundary.

        The second graph is port output only from a front firing port w/no boundary nearby.

        The moral of this story is that there's a whole lot of mids escaping through the port causing all sorts of havoc. In this case it's mainly due to 1/4 wave resonances within the box but even if your box is small enough and your port is short enough that there are NO 1/4 wave resonances to worry about, there's still regular midrange port leakage concerns to cause issues (direct and reflected mid frequencies escaping through the port) which will have virtually the same effect.

        Pete is right, at 30 hz the effect if negligible. BUT if you have any midrange frequencies coming out the port (which is pretty much unavoidable) it does matter where you place the port, especially once you get the box in a room.
        Don't even try
        to sort out the lies
        it's worse to try to understand.

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        • #5
          Re: front or rear port?

          Does that modeling include any internal damping?
          R = h/(2*pi*m*c) and don't you forget it! || Periodic Table as redrawn by Marshall Freerks and Ignatius Schumacher || King Crimson Radio
          Byzantium Project & Build Thread || MiniByzy Build Thread || 3 x Peerless 850439 HDS 3-way || 8" 2-way - RS28A/B&C8BG51

          95% of Climate Models Agree: The Observations Must be Wrong
          "Gravitational systems are the ashes of prior electrical systems.". - Hannes Alfven, Nobel Laureate, Plasma physicist.

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          • #6
            Re: front or rear port?

            For fullrangers (bookshelves and the like) I think I've seen far more rear-mounted ports. I'd be curious why that is, if not just a convenience factor.

            In this discussion I would also wonder how the rule of "you shouldn't be able to see the back of the woofer if you look through the port" is to be known.

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            • #7
              Re: front or rear port?

              Does that modeling include any internal damping?
              Sure does, otherwise there would be some crazy undamped spikes in pic 1 graph 1 that would give it away immediately. But point taken.

              There's not much stuffing, only .25ft/lb through the first 3/4 or so line length. As you can see in graph 1 pic 1 it's not the best design in the world but it's not terrible. (More on this later) And personally, I like to use the least amount of stuffing that will get the job done, I don't want to overstuff just to tame port leakage.

              And for full disclosure, it isn't exactly the original design that came with the worksheet. Right before I stated that it was, I took a quick look and it appears to still have the lowther driver parameters it came with but the box is different. It's 42 inches high and obviously causing quite a bit of 1/4 wave action, so compared to the small boxes you usually see around here it's kind of a worst case scenario. I took a quick look at FR and since the driver parameters looked original and the output seemed reasonable I thought it was all the worksheet's original inputs.

              I'm not trying to be misleading here, just trying to show the results. It's true that if the box is very small, the port is very short, and the box (and maybe even the port) are well stuffed, then this isn't a big concern. At least I assume it's not, I don't simulate many small boxes.
              Don't even try
              to sort out the lies
              it's worse to try to understand.

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              • #8
                Re: front or rear port?

                Does that modeling include any internal damping?
                Exactly what I'm wondering as well. Unless I have truly crappy ears (which is a real possibility), it doesn't seem like there's midrange leakage from either of my ported systems.

                EDIT - I see you just answered the question. Thanks...and thanks for taking the time to run the simulations. Both of my vented systems are bookshelf, so it makes more sense now.
                Bryan K.

                Midwest Audio Club

                Speedster | Sub Attachť | The Wildeman | Sean's NLA Towers | CO‹GAR, COUGAR II and CO‹GAR JR | Triton | Lithium | J-Boom | Trym MLTL | Docere MLTL

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                • #9
                  Re: front or rear port?

                  Originally posted by diy speaker guy View Post
                  Ok, I had a few minutes and made a few graphs using MJK's MLTL worksheet, which I believe still has the original inputs it came with (not my design).
                  Something is wrong with either the program or the use of it. Bass reflex systems simply don't exhibit that type of behavior, as port output peaks at Fb and rolls off above and below that. As for front versus rear ports, all that matters is having the distance from the port exit to the driver cone less than 1/4 wavelength within the port passband, ie., 1 octave above Fb.
                  www.billfitzmaurice.com
                  www.billfitzmaurice.info/forum

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                  • #10
                    Re: front or rear port?

                    Originally posted by diy speaker guy View Post
                    It's true that if the box is very small, the port is very short, and the box (and maybe even the port) are well stuffed, then this isn't a big concern. At least I assume it's not, I don't simulate many small boxes.
                    I just took a VERY brief look and it appears I was wrong about this. Making the box MUCH smaller by reducing the height to 15 inches (by 9.5 x 11) still shows quite a bit of 1/4 wave resonance all the way up into the midrange frequencies coming out the port, so basically the same thing as a large box but not as bad.

                    Also, stuffing didn't seem to have too much effect either. I used this smalll box (15 inch height as described above) and increased the stuffing to 1 lb/ft and there's STILL a lot of 1/4 wave mid frequencies coming out the port.

                    I can add some pics later but I'm out for now...
                    Don't even try
                    to sort out the lies
                    it's worse to try to understand.

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                    • #11
                      Re: front or rear port?

                      Originally posted by billfitzmaurice View Post
                      Something is wrong with either the program or the use of it. Bass reflex systems simply don't exhibit that type of behavior, as port output peaks at Fb and rolls off above and below that. As for front versus rear ports, all that matters is having the distance from the port exit to the driver cone less than 1/4 wavelength within the port passband, ie., 1 octave above Fb.
                      That's what I thought the first time I saw the results, but I've never known MJK's results to be innaccurate before. And if I'm making a mistake I don't know what it is. As I said, I've never done any measurements of this phenomenon.

                      Anyone care to check my inputs and assertions with MJK's worksheets or compare against a similar capable program?

                      BTW, this isn't a bass reflex, it's an mltl. But like I said, even reducing the height to 15 inches shows strong internal resonances up into the mids in the port output even though this is small enough to ordinarily consider a simple bass reflex.

                      And for the record, the port is only 2 inches long, so it isn't causing any issues.
                      Don't even try
                      to sort out the lies
                      it's worse to try to understand.

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                      • #12
                        Re: front or rear port?

                        Originally posted by diy speaker guy View Post
                        The moral of this story is that there's a whole lot of mids escaping through the port causing all sorts of havoc. In this case it's mainly due to 1/4 wave resonances within the box but even if your box is small enough and your port is short enough that there are NO 1/4 wave resonances to worry about, there's still regular midrange port leakage concerns to cause issues (direct and reflected mid frequencies escaping through the port) which will have virtually the same effect.
                        Any port with a given length will have resonance at it fundamenta resonant frequency(Fo), then at 2Fo , then at 3Fo, etc.

                        That is why you are seeing multiple peaks eaching in to the midrange.

                        The sealed enclosure have much better transient response without
                        all these 'peaks' escaping from the vent. As in any engineering exercise there is no free lunch, you get extended lower bass response with trade off of transient response, port noise, port resonence, etc.

                        Andrew

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                        • #13
                          Re: front or rear port?

                          If there's a specific simulation you'd like to see, let me know, like for example a very small box (in all dimensions) with very heavy stuffing (like 1 ft/lb).
                          Don't even try
                          to sort out the lies
                          it's worse to try to understand.

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                          • #14
                            Re: front or rear port?

                            Originally posted by andrewkim View Post
                            Any port with a given length will have resonance at it fundamenta resonant frequency(Fo), then at 2Fo , then at 3Fo, etc.

                            That is why you are seeing multiple peaks eaching in to the midrange.

                            Andrew
                            No it's not. The port is only 2 inches long so it's not much of a factor in the bandwidth we're looking at here.

                            I agree with the first sentence though.
                            Don't even try
                            to sort out the lies
                            it's worse to try to understand.

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                            • #15
                              Re: front or rear port?

                              Ok, I'm having a hard time backing away from the keyboard, so 1 more pic.



                              That box is 15 inches high, 9.5 wide, 11 deep. That's more like the size of stuff you see around here. Stuffing is increased to 1 lb/ft - which I think is probably absurdly overstuffed for a small ported box.

                              The first graph is the phase 1 frequency response (not including diffraction, boundaries, and the drivers and port are not in a defined location on the box exterior.)

                              The second graph is the driver and port separate outputs that sum to the frequency response graph above.

                              For the record, I don't think MJK's software or my usage of it is wrong. Even though small and heavily stuffed there's still significant port output up into the mids. I think this is much more accurate than what WinISD shows, there's much more going on than WinISD shows, even in very small boxes with very short ports, it seems.

                              I still think it matters quite a bit where you put the port. It would be nice if MJK saw this thread and commented.
                              Don't even try
                              to sort out the lies
                              it's worse to try to understand.

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