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Seeking advice for down firing port length/tuning in a subwoofer enclosure

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  • #16
    Originally posted by Chris Roemer View Post
    The Frequency of resonance (Fs, Radio Shack used to call it Fo <-this being a zero), measured in "free air" (meaning no box, not even a baffle) does not change with (a "reasonable amount of) drive voltage. Many modern "systems" (like DATS, or the WooferTesters) use something below 2.83v (I THINK?). Maybe more like 1 volt? They run a 20-20kHz sweep and it's not very loud.
    That's good to know, thanks, Chris. I'm currently still in prep phases so it will be some time before I have an enclosure to actually test with but when I get there I will make sure to keep the volume low. I can't imagine a 20-20kHz sweep being very pleasing to the ear when blasted haha.

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    • #17
      I recalled the other day why I originally decided against a slot port. The reason being that I was wanting to flare any port that I used so if I went with a slot port I'd add a round-over to the port opening. Since I'm planning to cover the enclosure with a walnut veneer the round-over didn't seem to be so compatible with that type of finish. After thinking about it a little more I don't think this would pose as big of an issue as I originally thought. I could even make the round-over deep enough to where the bit could cut a nice 90 degree edge at the start of the round-over which would make for a good transition for the veneer.

      I originally ordered a 4" Precision Port kit and was planning to use that but the the thing is massive and the air port velocity I'm getting in WinISD is a little higher than I'd like (max at just over 24 m/s) while the slot port provides better results (right at 16 m/s). Is the port velocity of the 4" Precision Port a concern at all?

      Click image for larger version

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      • #18
        Your 1.5 x 12 port will probably give a lower Fb and higher velocity than predicted. There's a zone of high friction that extend perhaps 1/4 inch from the walls, effectively removing that zone from the port area. That's not a problem when the smallest dimension is 3 inches or more, but it makes that 1.5 inches act more like 1 inch. This, BTW, is the reason why all ports tend to give a lower than predicted Fb. As for the precision port, whatever chuffing that it may create will be filtered by the down firing configuration.
        www.billfitzmaurice.com
        www.billfitzmaurice.info/forum

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        • #19
          Originally posted by billfitzmaurice View Post
          Your 1.5 x 12 port will probably give a lower Fb and higher velocity than predicted. There's a zone of high friction that extend perhaps 1/4 inch from the walls, effectively removing that zone from the port area. That's not a problem when the smallest dimension is 3 inches or more, but it makes that 1.5 inches act more like 1 inch. This, BTW, is the reason why all ports tend to give a lower than predicted Fb. As for the precision port, whatever chuffing that it may create will be filtered by the down firing configuration.
          Thanks for the confirmation, Bill! I will go forward with the 4" Precision Port then. That was my hope since returning the darn thing would have cost about the same, if not more, than what I paid for it.

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          • #20
            This slot isn't down-firing, but no reason why one can't be (with clearance).



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            • #21
              Originally posted by Chris Roemer View Post
              This slot isn't down-firing, but no reason why one can't be (with clearance).


              What was the expected tuning and the slot height you used? Looks like maybe 1" to 1.5"?

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              • #22
                10 y.o. build, already! HiVi SP-10. SA250 amp (w/NO boost).
                12"w x 17-1/2"h x 18-1/4" long (deep). Driver chamber ended up at 0.88cf i THINK (after amp & GIANT MAGNET). LOOKs like about a 2 cu.ft. sub (size-wise).
                Slot is 10-1/2" wide by 1" high, x 52" long. Target Fb was 24Hz. If it didn't turn out exact, it wasn't off by more than 1 Hz or so.

                The slot "height" ends up to be a critical dimension, as a slight variation in that short dim. (times the slot's width) can change the tuning quite a bit.
                For the same tuning at only 7/8" high, the slot length would have to change by about 8".
                I used 1" spacers between the 3/4" slot "fins" when doing the glue-up. I THINK the white material might have been a pre-finished shelving board (but I did not purposely sand/paint it - except maybe the end "turns")? It IS pretty smooth, though. Exit is rounded-over.

                This (used - door prize from meniscus) driver did not match HiVi's T/S specs. The Qts was pretty close (0.56), the Fs not much higher (40Hz), but the Vas was quite small (0.26 cu.ft.). The surround on this thing was like an over-inflated bicycle tube. You could lay the (heavy) driver face-down on the surround and it would barely budge it. These specs/box/tuning look pretty funky in WinISD - until you throw a 50Hz low pass on it. It's only used for the bottom octave (with 8" mains). Output peaks right at the tuning freq.

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                • #23
                  I could see how a slight different in the height could make a big difference for a slot port, especially at 52" long. I believe the modeled port length I got was somewhere around 25" and I would certainly use some kind of spacer to ensure correct slot height. Though I think I've convinced myself to go with the Precision Port. It will take up a little less space in the cabinet and will be easier to terminate alongside the veneer.

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