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PCD Box Response for SB34NRXL75

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  • #16
    Originally posted by a4eaudio View Post
    You can click on the little wrench icon in WinISD and change the scale. I believe the rule of thumb is keep the port velocity below 14 or 17 ms. And then almost everyone breaks that rule of thumb because it's too demanding. Something in the mid to high 20s with a big roundover on the port is usually acceptable.
    If you see the single 4 inch port gives me 19ms. So that should be OK then right?

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    • #17
      That is fine in terms of just the port velocity.
      How long was that 4" port? If it was reasonable, then you should be okay as-is.
      If it was so long as to create problems you could try a smaller diameter. But, I don't know of any 3.5 tube options so you would have to drop down to a 3" port if you went smaller. This would give you a shorter port length but the port velocity may be unacceptable. But if it was 26ms at 200w then you could probably have a 1/2" or 34" roundover and be just fine since you will hardly ever (never?) be running it at 200 watts and if you were it might be so loud as to drown out any chuffing anyways.

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      • #18
        interesting. when I increase the number of ports from 1 to 2 to 3, the port length gets longer? Or do you divide the port length by the multiple?
        why would you use a short port for a singe unit and longer ports for multiples? I can go two 3 inch dia port as well

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        • #19
          Yes, more ports will be longer given the same diameter. But you would do that if you wanted to move down from a single 4 to multiple 3s, one 6" to two 4"s, etc.

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          • #20
            I did not model it, but suggest that you investigate using a pair of CSS APR12 twelve inch passive radiators (design includes easy external access to change moving mass) with each SB34NRXL75 twelve inch woofer. Mount one PR on each side panel so that the dynamic forces associated with the PR's heavy moving mass are in balance. PRs do not exhibit any port resonance or any port airflow chuffing. Include a little extra enclosed air volume to enable tuning that too, using some rigid foam insulation and hot melt to occupy some internal enclosed volume.
            "Our Nation’s interests are best served by fostering a peaceful global system comprised
            of interdependent networks of trade, finance, information, law, people and governance."
            - from the October 2007 U.S. Naval capstone doctrine
            A Cooperative Strategy for 21st Century Seapower
            (a lofty notion since removed in the March 2015 revision)

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            • #21
              Can WinISD do a sealed box?? I am trying to change from vented to sealed?

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              • #22
                OK Ive been trying to get the port velocity down so going with a 5 inch pvc works best but I may need to either have the port facing the floor (not looking forward to that) or having an 90 elbow on the port which is not good either right?

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                • #23
                  Originally posted by jimbones View Post
                  Can WinISD do a sealed box?? I am trying to change from vented to sealed?
                  Yes, it is one of the first steps when creating a new project - select driver, # of drivers, then CLOSED box

                  Originally posted by jimbones View Post
                  OK Ive been trying to get the port velocity down so going with a 5 inch pvc works best but I may need to either have the port facing the floor (not looking forward to that) or having an 90 elbow on the port which is not good either right?
                  I modeled the woofer in a much smaller box than you and DeZZar. 60 L tuned to 32Hz. At 200w power, with a 4" port 12.6" long I get port velocity that is reasonable.
                  If I use a 90 Liter box like you did then the 4" port is only 7.5" but port velocity is too high. 5" port is 12.5" long and port velocity is fine.
                  Downward firing port is fine, 90" elbow is fine. There are some PVC elbows less than 90 degrees that may work too.

                  Here is transfer function for 90 L vented (red), 60 L vented (green) and 38 L sealed (blue).


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                  Cone excursion at 200w

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                  Port velocity @ 200w. 60 L with 4" and 90 L with 5".

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                  • jimbones
                    jimbones commented
                    Editing a comment
                    a4eAudio, My sim was for 70L. If there is not a significant difference performance wise I prefer the smaller cabinet (60L) I dont see much difference performance wise...?

                  • a4eaudio
                    a4eaudio commented
                    Editing a comment
                    I would agree. There isn't MUCH difference between 90L and 60L so there would be very little difference between 60L and 70L. Both sims used 32Hz tuning to illustrate how close they are, so you can change the tuning a Hz or two and tweak the results to your liking.
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