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What port size for an RSS210HO-8?

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  • What port size for an RSS210HO-8?

    Tuned the box to 25.15hz. With a 2"x2" square port with flared ends(3/4 roundover bit on the plywood), the length is reasonable at 10.68" considering the depth chosen is 14". I'm just worried about vent noise. On one hand it's only 8", but it does have 12mm of xmax.

  • #2
    Look at the port velocity at full power in your software model. You'd want it no higher than 20 meters/second.
    www.billfitzmaurice.com
    www.billfitzmaurice.info/forum

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    • #3
      I've seen 8" NON-subs (limited Xmax) chuff with 3"id ports (w/out a roundover at the exit).
      I'd go a minimum of 2"x 3.5" (3" round equivalent) - WITH your generous roundovers - AND aim the vent away from your listening position. This WILL nearly double the vent length and still have velocities higher than Bill's ROT. (Guessin' your box is around 1.4cf?)
      2 alternatives would be a slot vent, or a passive radiator (which'll most likely need to be 10" or larger).

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      • #4
        Chris, Bill, thanks so much. At RMS, port air speed was absolutely insane. I redesigned the box for a large slot as Chris suggested. I went with a 1.85" x 12.5" port, snaked 56" port through the box. Now I have 22-ish meters / s at peak power, 17ish at RMS power.

        Are there problems with ports that long? And am I measuring them correctly?

        Should I need to brace this? I'm using .7" pine ply that's I would say medium to high quality, for pine. The interior is roughly 16.3 x 16.3 x 12.5.

        Also the log-out timer on this site is set to like a minute, which is worse than my bank! Holy crap!




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        • #5
          Reality check: you'll never put peak power into it. You're not likely to even put the full 300 watts RMS into it. It's reasonable to assume that you'll run it with at least 3dB of headroom, so cut power to 150w. That should allow you to use a smaller port area, and a shorter port. Also don't forget that the box volume is net, less the volume taken up by the port, so there's a point where Fb is better served by not making the port volume too large.
          www.billfitzmaurice.com
          www.billfitzmaurice.info/forum

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          • #6
            What Bill said. Also, "pipe resonances" from the use of long vents can result in a narrowing of the subwoofer's useful passband. A software tool like Hornresp can be used to model the enclosure and determine what impact those resonances are going to have on its response.

            The issue of long vents and those resonances is why I tend to look at using offset TLs rather than simple vented enclosures for subwoofer duty now.
            Brian Steele
            www.diysubwoofers.org

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            • #7
              Thanks Brian. I DL'd hornresp a while back, but it looked pretty intimidating.

              Bill, thanks again. You said full power, so I didn't guess you'd mean half RMS. I have seen some recommendations for keeping vent velocity to below 10 m/s, so at 150 watts I'm still over 12. I may look into yet another redesign if you think pipe resonances might be a problem. Heh, I thought you'd give me hell letting the peak power vent velocity get above 20!

              The volume I'm entering in WinISD is based on the area in the red square:



              Also, any thoughts on the bracing? Should I make something more robust?

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              • #8
                By full power I should have been more specific as 1/2 RMS, to allow for some headroom. I'd say that's adequate bracing.
                www.billfitzmaurice.com
                www.billfitzmaurice.info/forum

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