Lengthened my ports 2 inches now my woofers bottom! Why?

Discussion in 'Archived Threads 2001-2004' started by Chris Sherman, Feb 5, 2002.

  1. Chris Sherman

    Chris Sherman Second Unit

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    I bought a couple of used sonosubs from the for sale area here. The seller was outstanding handling the sale and shipping BTW. The sub tubes are 12" diameter x 37.5 inches tall so I calculated the internal volume at roughly 2.45 cubic ft. I am using Dayton 12" DVC's for drivers . The subs came with 16" dual flared 4" diameter PNR aeroport vents. I swapped these out with madisound 4" X 18" length dual flared vents virtually identical vents only 2 inches longer . My problem is I played Toy story 2 tonight and the woofers bottomed out horribly, where they had not with the shorter vents. This is contradictory to what I had expected. I was trying to lengthen the ports to protect the drivers from unloadind below tuning. What did I do wrong? and should I put the shorter aeroports back in? Any insight is appreciated.

    Chris Sherman
     
  2. JamesFarina

    JamesFarina Agent

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    Chris, If you are trying to prevent the sub from bottoming out you went they wrong way with lengthening the ports. Buy leaving the 16" ports in the tube you increase the volume of the enclosure and allow more performance at higher frequencies. If your goal is to get more performance at lower frequencies you either: A - get a better designed subwoofer or B - build a bigger enclosure. I also know that a subsonic filter will prevent any bottoming out of the driver. Hope this helps.

    JIM

    "What if I was a bombadier? Bomb Bitty Bomb bomb bomb"
     
  3. Chris Sherman

    Chris Sherman Second Unit

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    Thanks James . I had not thought about the loss of internal volume . May very well explain things, I'll put the shorter ports back and see what happens.

    Thanks

    Chris
     
  4. Dustin B

    Dustin B Producer

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    Increasing two, four inch ports by two inches will subtract less than a 1L of volume from the enclosure. This volume change will have next to no effect. The longer ports will however decrease the tuning frequency.

    By my calculations the original tuning frequency was 35hz and the new tuning frequency would be 33.8hz.

    I'm not sure how you would bottom with one but not the other. This is way to high of a tuning for a Dayton 12" DVC.

    My recommendation would be to use one of the Madisound ports and plug the other hole. This would tune it to 25.5hz.

    Now if you are talking about 1 port in each sub and I've completely missunderstood you disreguard. In that case the shorter port would tune to 27hz and the longer to 26hz (higher because a whole ports volume will make a difference). Again I don't know how you would bottom with one but not the other. Maybe one of the more experienced DIYers here can explain that one.
     
  5. Chris Sherman

    Chris Sherman Second Unit

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    Dustin , Im talking about one port in each sub . I have two identical subs. I also thought the loss of volume would be small enough to be inconsequential. I've already changed the ports back to the shorter length ones and will see what happens.
     
  6. Jack Gilvey

    Jack Gilvey Producer

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    Chris, my numbers agree with Dustin's (I'm assuming that 2.45ft^3 is net Vb). I wouldn't think a ~1.5 Hz change in tuning or the tiny change in Vb would have any perceptible effect on performance, certainly not causing the change from "ok" to "bottoming horribly". The only thing I can think of is that the new ports might not have been seated correctly, allowing an air leak.

    You are correct that these fairly high tunings don't afford much overload driver protection from modern DVD's like Toy Story 2, but to be worthwhile you'd really have to try a tuning down around 21Hz or so. At ~28", it looks like it should fit in that tube giving clearance for the back of the driver. Might be worth a try.
     
  7. Patrick Sun

    Patrick Sun Moderator
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    You should also re-calibrate your sub to the rest of your speakers in your setup. You may be running too "hot", so back off on the Subwoofer output to the sub/amp to ge the best blend before bottoming.

    In my experience, Toy Story 2 should not be used to evaluate how susceptible to bottoming unless you have an infrasonic rumble filter in place (especially if you have not calibrated the sub to integrate it with the rest of the speakers) because Toy Story 2's opening scene is a sub-killer on high volume.
     
  8. Chris Sherman

    Chris Sherman Second Unit

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    Thanks for all the responses. I siliconed the ports in and let them cure , so I don't think they were leaking. I agree Toy Story 2 is a sub killer and I was running the sub 4 db higher than my mains, as I usually do. I listen below reference except when testing something. I put the shorter ports back in last night and tried them out this morning with no bottoming, so I guess I'll leave well enough alone. Strange though, just wasn't expecting the change I got.
     
  9. Patrick Sun

    Patrick Sun Moderator
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    FYI, I run my sub around -4dB to -6dB under my mains so I don't over-tax my sub, and rely on room gain to give me that bass boost.
     

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