Sound Quality in PORTED vs PASSIVE vented subs.

Discussion in 'Archived Threads 2001-2004' started by steve nn, Aug 10, 2002.

  1. steve nn

    steve nn Cinematographer

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    The four subs I have had thus far have all been ported. I am strongly considering a DIY sub project and I was wondering if any of you had any strong preferences in the sound a sub puts out with ether application for venting. I know one of the pros using passives for venting will enable me to reduce the dimensions of the enclosure but what effect "if any" do adding passives change the sound? This sub would be used for basically 100% HT. Any preferences out there? Are there any pros or cons in sound in ether application?
     
  2. Patrick Sun

    Patrick Sun Moderator
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    If it's for HT, you'll lose the low frequency extension into the low 20Hz range that people covet. At best (driver dependent and enclosure dependent, of course), with a sealed design, you'll need to rely on room gain for perhaps acceptable performance/output for the low 20Hz range because you'll be getting down only into the mid-30Hz range with a more gradual slope of decline in the output of the sub, otherwise. You can get by going with a sealed design, just be prepared for either having to double up the number of subwoofers, or sitting closer to it.

    With the ported design you can extend to output down in to the low 20Hz range, but then the output drops precipitously under the tuning frequency of the enclosure (provided that the proper drive/box size is selected/designed). This is one of the pluses of using a ported design for HT use, to get that low down rumble of DVD LFE tracks (and whatever else is routed through bass manageent) in acceptable output levels of LFE SPL to produce that quality of bass that brings the theater to the home.
     
  3. steve nn

    steve nn Cinematographer

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    Thanks Patrick. That answers that question. I definitely do not want to compromise SPL or extension and would rather not buy a 750 watt amp and beyond, plus two passives. I did not know with the use of passives a sub is still considered sealed. PORTED it shall be.
     
  4. Mark Fitzsimmons

    Mark Fitzsimmons Supporting Actor

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    Steve, wasn't your question concerning the use of Passive Radiators vs. Ports?
    Patrick's reply contrasted Ported subs vs. sealed designs.
     
  5. steve nn

    steve nn Cinematographer

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    Yes it was Mark. I was wondering to. I read it over a couple of times and came to the conclusion "well I guess a passive configuration is considered sealed? That's new to me.[​IMG] I guess he did misunderstood my question and I misunderstood his answer.
     
  6. Patrick Sun

    Patrick Sun Moderator
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    Oh, oops on my part!

    Passive Radiators and ported designs will sound similar in extension, but some prefer one over the other, but for me, it comes down to cost vs size of enclosure. PR designs will cost more due to the PR is usually another $50-$100 each (ports are cheap, under $20 easily), but with PR, you can usually get away with a smaller enclosure (provided that you use a driver that's suited for a smaller enclosure). Ported designs are cheaper, but can become larger in enclosure size (if you tune low, the length requirement of the port can get quite long, and sometimes too long - long enough for the port to not function as a slug of air anymore and introduce resonance into the mix).

    With PRs you eliminate the possibility of port chuffing noise (noise from the port at high SPLs) occurring. Again, it comes down to money vs. enclosure size.
     
  7. steve nn

    steve nn Cinematographer

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    Thanks Patrick and I do presume a guy "can not" go with both in the same unit? Does a passive vented unit have basically the same kind of bass in HT application?
     
  8. Dustin B

    Dustin B Producer

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    steve, to prevent further confusion I think we need to get some terms straight for you.

    Bass Reflex - this can refer to ported/vented or passive radiator enclosures.

    ported or vented - are used to refer to a bass reflex enclosure that use a port

    passive radaitor - (we often shorten to just PR) refers to a bass reflex enclosure that uses passive radiators.

    Passive vented isn't the correct term for what you are trying to say as a port is a passive vent.
     
  9. steve nn

    steve nn Cinematographer

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    MMMM-- Well corrected again. Us rookies must be a chore. Bottom line--I would rather make these mistakes and be corrected than not to have learned at all."My stupidity on this subject is causing me some pain". Port-vented the same-and PR for passive. Thanks Dustin.[​IMG] [​IMG]
     
  10. Dustin B

    Dustin B Producer

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    That is what the forum is for. I had a lot of terms screwed up a year and half ago too.
     
  11. Scott Simonian

    Scott Simonian Screenwriter

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    Do you think that a PR will move more air than the air coming from a port? I always had that assumtion. You think with the extra Sd from the 1,2,3 PRs you would get more air displacement.

    I dont know though. What do you think?
     
  12. Dustin B

    Dustin B Producer

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    A passive radiator will behave like a port that has the same cross sectional area as the Sd of the PR. So if you have three 15" PRs then it will be like having three 15" ports. They are tuned by changing how much they weigh. To figure out how much they need to weight you model a box with ports that have the same cross sectional area as the Sd of your PR (if you are using 3 PRs, use 3 ports in the model). Then using how long the equations tell you the ports need to be you figure out how much the air contained in that port would weigh. Then you make the PR weigh that much. So just like ports can end up too long. You can end up needing to put more weight on the PR than they can take.

    A passive cone flapping back and forth 20 times a second with a mass of 1-2kg can generate some serious forces. This is why it's also nice to keep them in pairs mounted opposite each other.

    With a port your concern is the air speed. Gets to high and you have compression and noise.

    With a PR, the concern is Vp (the volume the PRs can displace, and just like drivers it's the Sd multiplied by the Xmax). The rule of thumb is you want the total Vp of all your PRs to be at least twice the Vd of the active drivers. Preferably more.
     
  13. Scott Simonian

    Scott Simonian Screenwriter

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    So the PRs will create the effect of more air being pushed, eh?


    Hmmmm...
     
  14. Scott Simonian

    Scott Simonian Screenwriter

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    Exactly. Which do you think would be the least complicated way? Id have to say the PR way.
     
  15. Scott Simonian

    Scott Simonian Screenwriter

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    What I meant was that the PRs would be a less complicated way to create that same feel I was talking about, instead of a port of "epic proportions". :d
     

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