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what happens when you block ports on a vented cab

Discussion in 'Amps and Cabs [BG]' started by pfschim, Sep 25, 2013.

  1. pfschim

    pfschim Just a Skeleton with a Jazz bass

    Apr 26, 2006
    SF Bay Area
    I'd like to get some insight from those with experience with blocking the ports on cabs.

    I have a cab that is tuned to ~55hz. It is a 2x15 cab and has 4 ports (2 for each driver). I understand that if I block one port for each driver, I will drop the tuning of the box to ~40hz.

    Can anyone tell me what the effects will be of doing this. I am interested in both the possible technical effects ( ex. changes in response curves/frequescy plots, change in SPL, change in resonant frequency etc), as well as the possible practical effects (ex. it will sound deeper because it has a lower fundamental, it will have lower SPL because of "carrots" ... etc).

    I'm also interested to know what happens with this cab if I were to block all the ports, effectively making it a sealed cab (although one that was designed as a ported cab).

    anyone care to help educate me on this topic ?
  2. will33


    May 22, 2006
    It turns into a sealed cab....likely of more than optimal, or workable size.

    There are some drivers out there that can work either way, but in order to make them work to their best in one alignment, the box size will be different than the other alignment.

    There yet a few others that can sound good, and take gig level power in either alignment.

    Like everything to do with speakers, it's a juggling of compromises.
  3. will33


    May 22, 2006
    Ah....OK. I assume this is an EV box design?

    One thing....a ported cab "keeps control" of the cone motion for roughly 1/2 octave either side of the tuning frequency. Below that, it "loses control" and the cone goes to flapping uncontrollably.

    Also, oddly, the higher the tuning frequency, the more response increases towards the lower end of the drivers range.

    So, there is a compromise that must be reached.

    A higher tuning will increase lowend response, but if you get too high, can take power handling way down, due to losing control of the speaker below the tuning frequency.

    A lower tuning will have less bass response, but can take more bass being eq'd in because it keeps control of the speaker down there.

    A good compromise is usually somewhere in between those extremes.

    Your cabinet has 2 choices that are not at the extreme ends, but quite a ways away from the middle to choose from.

    So....try it both ways and see what sounds best.

    Hint: the right answer at lower power levels may not be the right answer at higher power levels.

    Various box tunings vs. speaker used also affect impedance of the whole system, "quality" of the lows as well as "quantity", and some other stuff too involved to write up on a phone.

    The answer is still to just listen, at various power levels and in various situations.
  4. taurus1


    Sep 13, 2006
    Vancouver B.C.
    there are speakers experts on this forum, I'm not one of them.
    I believe that closing the ports is a really really bad idea, you'll effect the damping factor in a negative way.
    cabs with ports are designed so the cone moves within the parameters of the design, closing the ports will effect the way the cone moves.
    some speakers are designed for sealed cabs, others for ported cabs, you could damage your amp and or speakers.
    so I'd leave it alone.
  5. Mudfuzz


    Apr 3, 2004
    They explode and kill everything within a 10' radius.
  6. will33


    May 22, 2006
    What I failed to mention up there was that reducing port area ( blocking a portion of it, like installing the port cover on the old EV cbs ) lowers the tuning frequency....hence the 55hz and 40hz tuning in your cab without, or with, the port cover.
  7. will33


    May 22, 2006
    We can only dream......

  8. SactoBass

    SactoBass A retired civil engineer who likes all-tube amps! Supporting Member


    I have heard that the defense department is now looking into this tactic.
  9. will33


    May 22, 2006
    I work with the War Department....Defense Department is a bunch of panty-waste daisy sniffers.
  10. basscooker

    basscooker Commercial User

    Apr 11, 2010
    cincy ky
    Owner, ChopShopAmps
    That only applies to 1x8's and it's actually 9.2 feet, with a second, possible kill-zone of approximately 1.1 feet. A 2x15 cab can turn a small bar into a cloud of pink mist on a slow night, and if the cab has a metal grill, or is under a larger tube amp, a packed house of 50 to 75 people, depending how close the stage is to the liquor.
  11. will33


    May 22, 2006
    Better than what I could come up with......
  12. two fingers

    two fingers Opinionated blowhard. But not mad about it. Gold Supporting Member

    Feb 7, 2005
    Eastern NC USA

    You should write a novel....... that I will never ever let my little girls read.

    OP, stuff a sock in 'em and see what happens. Report back after sock testing is complete.
  13. DukeLeJeune

    DukeLeJeune rational romantic mystic cynical idealist Supporting Member Commercial User

    Nov 24, 2008
    Princeton, Texas
    Owner & designer, AudioKinesis; Auth. mfg, Big E (Home Audio only)
    Will33's explanations are excellent, as usual.

    The shape of the low-end response curve changes when we change the tuning frequency; relative to the standard 55 Hz tuning, the 40 Hz tuning will start to roll off higher up, but the rolloff will be more gentle. Translation: The 55 Hz tuning will sound bigger, and the 40 Hz tuning will sound tighter.

    The 40 Hz tuning may increase your useable broadband SPL before fartout, but best to test it out to see.

    I do something similar in most of my cabs, allowing the user to choose between 50 Hz and 37 Hz tunings. Going all the way to a sealed box results in an anemic, overdamped low end with my cabs, and will probably tend in that direction with yours.
  14. pfschim

    pfschim Just a Skeleton with a Jazz bass

    Apr 26, 2006
    SF Bay Area
    cool. true to TB form, I get both enlightening answers and humor. totally great.

    Will33 - it's not an EV cab, it's a Faital (2 x 15PR400) loaded cab that I am having some possible resonance issues with (on C2/65hz).

    I say "possible" only because I cannot totally eliminate my room as a contributor to the issue.

    I have stuffed one port for each driver and did not get a hugely noticeable change. Stuffing the 2 ports, did momentarily stun my cat into total indifference..... wait ..... what ?;)

    I was told that closing the 2 ports would drop the tuning of the box to about 40hz.

    So, I am continuing to seek fixes to this vexing issue. The cab really LOOKS great, but it has to SOUND great too. :D

    to the rest of you military weapon jokers, keep 'em coming. :cool:
  15. will33


    May 22, 2006
    Listen to the cab in your backyard or somewhere outside, then do your port plugging experiments again.

    Likely room issues or just too much volume for the room happening there. Those results are beneficial too, do not discard them, you'll more than likely be playing that thing in funny rooms.

    Bringing it outside lets you take that much out of the equation temporarily to let you hear what your speakers are really doing.

    Then, by all means, bring it back inside and see how it does in various positions there. After all, most people will play most speakers indoors, most of the time in such a fashion such that most speakers have something to fight against most of the time.

    If the only place anybody ever listened to these things was in ideal environments, it would be easy. :)
  16. Ric5

    Ric5 Supporting Member Commercial User

    Jan 29, 2008
    I grow organic carrots and they are not for sale
    what happens when you block ports on a vented cab


  17. will33


    May 22, 2006

    Wow...I don't know what to say.....that'll give you a warm fuzzy.

    Y'all just listen to whatever this guy has to say....he's one of the best. He's one of those guys that can type one paragraph and it sets you off on a learning adventure.

    Anyway, OP, that's what your box tuning does. Listen and see what's best for you.

  18. will33


    May 22, 2006
    Of course, this is the desired result. The question is how to get there. :)
  19. I can't tell you how many times this has happened to me.
  20. pfschim

    pfschim Just a Skeleton with a Jazz bass

    Apr 26, 2006
    SF Bay Area
    Thanks to will33 and Duke (as well as some local friends), I got this cab situation sorted out finally. I also appreciate the non-related humor (the rest of you knuckleheads :D).

    plugged 2 ports (one for each driver) to drop the box tuning into the 40hz range (about 45hz I'd guess). As suggested, this tightened up the bottom end nicely. I then did some work with a Source Audio programmable EQ. Doing about a 8db cut around 62hz seemed to do the trick without gutting the low end response. Also turns out that the suspected resonance was probably 50% an issue with my room.

    Rig sounds great and I'm totally psyched to use it on its first gig this weekend. I'll get to use it in a decent sounding room I am very familiar with, so tweaking it (if needed) will be easier there.
    thanks again TB

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