Port plugging and SVS

Discussion in 'Speakers' started by Robert Hoffman, Mar 18, 2004.

  1. Robert Hoffman

    Robert Hoffman Stunt Coordinator

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    I've seen several people on here recommend not plugging the ports of various SVS subs. What is the justification? It was my general understanding that plugging a port would cut your output above 25hz by about 3db for each port plugged, but that the loss of output is really the only tradeoff for getting some deeper extention out of the sub. Is there something else I should know? I've got a 25-31pc+ in a pretty small room, and I always figured I'd just plug two ports since even then I only need the gain set a touch above 1/3 for proper calibration with the rest of my system. Thanks,

    Rob
     
  2. Edward J M

    Edward J M Cinematographer

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    Yes, there is a difference. For example, a 20-39PC+ has a larger enclosure and in its "native" tuning configuration of 20Hz will have three 3" ports open. In other words it's at the peak of its efficiency, with the least stress on its woofer in a configuration that is an inherent compromise for a 25-31PC+ that has been "tuned" to 20Hz.

    The latter sub would only have two 3" ports available for air flow without audible "chuffing" and of course it has less enclosure space for the woofer to act upon too.

    While at medium SPLs the two subs would be all but sonic clones, the overall headroom of a 20-39PC+ would be greater in a significant way over a 25-31PC+ in the 20Hz mode. To what degree this would be evident is going to vary greatly by room size, listener playback levels, the program material etc. Unless you are driven by size or cost, if you desire truly linear performance at high SPLs in the 15-20Hz range, then the 20-39PC+ is the superior product.

    With that said, many SVS owners are desirous of smaller size, lower cost, and also don't intend to drive a PC+ to its limits. If they have a small room then really the flexibility of a 25-31PC+ is perfect for them.
     
  3. Robert Hoffman

    Robert Hoffman Stunt Coordinator

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    Thanks Edward. I guess that's relatively consistent with what I thought. Given that my living room is really no more than 10x15, I think I'm going to tune my 25-31 to 16hz. The gain for the sub is still only a little over 1/3, and what I'm losing in efficency (and probably the response isn't quite as flat), hopefully I'll make up for in smiles as my pant legs shake when I'm watching movies.
     
  4. JohnSmith

    JohnSmith Supporting Actor

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    I wouldn't recommend plugging two ports- port noise & distortion increases. I've tried it in a small room (two plugged)- and more likely to bottom the driver.

    Recommend plugging one port only.

    I own a 20hz stock PC Plus.

    [edit] the gain on the sub has no relevance to whether or not you're close or far from the limitations of the sub. If you've calibrated the subwoofer hot (+12dB) then use bass boost, then block two ports you could be past the subs limit before you even use it)
     
  5. Dustin B

    Dustin B Producer

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  6. Robert Hoffman

    Robert Hoffman Stunt Coordinator

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    Very interesting, thanks for the description. Dustin, you make the comment at the end of you post that you haven't modelled the subsonic filters. What impact would that have on your graphs? Would a subsonic filter reduce the peak air-flow speed through the port? If so, to what magnitutde do you think?
     
  7. Dustin B

    Dustin B Producer

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    The subsonic filter would decrease the level at which the sub tries to play lower frequencies relative to the higher ones. So it would reduce the output and as a result the port air speed as well. How much depends on how steep the filter is and when the filter is applied.

    Personally I wouldn't plug any ports on the PB2 ISD. On the PB2+ I'd be willing to plug one if my room wasn't too big.
     

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