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How easily does air need to flow within the volume of the speaker cabinet?

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  • How easily does air need to flow within the volume of the speaker cabinet?

    This may be an odd question (and obvious to those who know the answer). I am trying to build a speaker with a tunnel for the midrange. I am using 6" PVC pipe but because of the thickness of the PVC walls I have left very little room (about 1/4" inch) between the outside edge of the tunnel and the inside edge of the speaker walls. As can be seen in the Sketchup picture, there is a decent amount of volume (approx. 0.25 cf) above the tunnel that would be nice to use for the woofer volume. Is there any problem with having such a narrow gap between the top portion and bottom portion of the cabinet volume? (I can just seal off the top and not use it if it is a problem.)

    Click image for larger version

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  • #2
    I don't see any problem with it.
    www.billfitzmaurice.com
    www.billfitzmaurice.info/forum

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    • #3
      Your design with the extra-large tunnel so close to the cabinet walls is kind of a rare instance, and I can see why you were wondering about it, but I can't see it being an issue either.
      Air is being 'pressurized' and 'rarefied' (vacuum) as the woofer moves in and out and interacts with the cabinet air volume, it's not really blowing around in there.

      I wonder at what point it would be an issue, or restrictive.. 1/16"? 1/32"? I don't know if that would even matter... but if Bill says your design looks like it would be okay than I'm sure it is.

      TomZ
      Zarbo Audio Projects Youtube Channel: https://www.youtube.com/channel/UCEZ...aFQSTl6NdOwgxQ * 320-641 Amp Review Youtube: https://youtu.be/ugjfcI5p6m0 *Veneering curves, seams, using heat-lock iron on method *Trimming veneer & tips *Curved Sides glue-up video
      *Part 2 *Gluing multiple curved laminations of HDF

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      • #4
        As already highlighted 'flow' probably isn't the right way to think of the air inside the box. Your woofer needs to 'see' the total volume of air however I do believe there is a limit to 'restriction'. You wouldn't expose the woofer to its total volume via a straw for instance.

        billfitzmaurice at what point might you effectively create some sort of bandpass? Or introduce some strange internal resonance? If it wasn't a tube in this case for instance, and a box instead, you would effectively have two small slot ports leading to a second volume? In more extreme example if the woofers port was in that second volume would that not create issues?

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        • #5
          You could be changing the "impedance" of the air and you could be causing points of turbulence causing "chuffing" We sometimes stuff to change the impedance. Chuffing us always bad
          The only fully accurate answer is PROTOTYPE and test. If it works, then, well it works. If I were to guess, like those guessing above, I would guess you are fine

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          • #6
            If you DATS'd it (when done) you'd see any anomalies compared to a WinISD model. Then you'd know.
            I'd also guess it's fine.

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            • #7
              I'd be interested in seeing test results, impedance and SPL. You might set it up so that you can remove the tube and seal the box for comparison. It might actually be a benefit in one way. The tube being a fairly large cross section at the one end may help to reduce the internal resonances that are bound to exist in the vertical axis.

              dlr
              WinPCD - Windows .NET Passive Crossover Designer

              Dave's Speaker Pages

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              • #8
                The designer could avoid the question by locating the midrange above the tweeter.
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                • JRT
                  JRT commented
                  Editing a comment
                  If the midrange is located above the tweeter, then the desired open back cavity can be created using a simple horizontal bulkhead/shelf using the same material used in making the rest of the cabinet instead of using the PVC tubing.

              • #9
                Originally posted by DeZZar View Post
                at what point might you effectively create some sort of bandpass?
                When the upper chamber is separately ported.
                www.billfitzmaurice.com
                www.billfitzmaurice.info/forum

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                • #10
                  If your car tire had a hole that big, how long would it stay inflated?

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                  • #11
                    Originally posted by dlr View Post
                    I'd be interested in seeing test results, impedance and SPL. You might set it up so that you can remove the tube and seal the box for comparison. It might actually be a benefit in one way. The tube being a fairly large cross section at the one end may help to reduce the internal resonances that are bound to exist in the vertical axis.
                    Originally posted by JRT View Post
                    The designer could avoid the question by locating the midrange above the tweeter.
                    Unfortunately I thought about the issue after I started gluing up the cabinet. If I had realized before hand I would have just made the cabinet 1/2" wider. I think I will follow Chris Roemer's advice and DATS it and compare to the simulation. I can still shim some 1/4" MDF into the gap when done to seal off the top if the impedance looks unfavorable.

                    My original concern was because I know that restricted airflow behind a woofer can cause "tunneling" and a chamfer can reduce that, and a port terminating near an interior wall can create problems. But in this case I think the air above the tunnel is just adding mass to support the driver but doesn't need to "flow" that much (as DeZZar pointed out), so hopefully it will be fine.

                    Thanks everyone for the input and ideas!


                    Click image for larger version  Name:	Tunnel.jpg Views:	0 Size:	116.8 KB ID:	1463124

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                    • #12
                      If you really want/need that upper volume, you could insure access (to the woofer) by heating up the PVC till warm enough to deform (at least) one side inward some.

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                      • #13
                        Originally posted by djg View Post
                        If your car tire had a hole that big, how long would it stay inflated?
                        +1. Too much is being made of the 1/4" dimension, and not enough of the cabinet depth. It would be a concern to have a port with that narrow a slot, but the behavior of the air mass in a port isn't going to be duplicated here.
                        www.billfitzmaurice.com
                        www.billfitzmaurice.info/forum

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                        • #14
                          I think we are talking about resonance here and not air flow, right?

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                          • #15
                            Originally posted by Steve Lee View Post
                            I think we are talking about resonance here and not air flow, right?
                            No, my concern was air flow. If you look at the pic in post #11, once I put the other side and back on there will only be a small space between the PVC pipe and wall separating the volume of air below the tunnel and the volume of air above the tunnel. I didn't know if this would restrict the air in the cabinet and create some negative effect.

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