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To port or not ports Cabs ??

Discussion in 'Amps and Cabs [BG]' started by Gerard Rizzardo, Oct 14, 2003.


  1. I was reading a Music Magazine, which had an article by a PA Techo, who talked about that having vent ports in small cabs in not always a good thing. It was mainly to do with cabs with say, 2 x 10" speakers. He indicated that by not porting for small speakers, the sealed cushioning effect of the air inside, stops over extension of the speaker cone/driver. Therefore, you can drive the speakers harder, than ported speakers.

    However, doesn't this restriction and responding time of the cones physical moment, effect the quality of sound, especially the low frequencies etc??

    Does anyone know what cabs manufactures use this non-ported design ??
     
  2. Cabinet design is all about trade-offs. Closed cabinets are easier to design, sound tighter, but roll off quickly in the bass range. Ported cabinets are difficult to design, sound less tight, have better bass response, but are susceptible to unloading the driver (at low frequencies), possibly causing damage. Especially with small drivers with a high resonance frequency. But this is also the case with closed cabs, be it less.

    To summarize: closed cabs give a snappy, direct, "fast" sound; ported cabs give a deep, lush, "slow" sound.

    99% of the bass guitar cabs are ported. I think Rivera makes closed cabs (not sure), and many old-day cabs are closed (Fender and Acoustic spring to mind).

    The question is always: what do you want to achieve? You can't have everything. You can optimize for deep bass at the expense of size and efficiency, you can optimize for loudness and power/mechanical handling at the expense of bass depth.

    Cabinet design is part engineering, part black magic ;)

    I hope this is of some help.
     
  3. Probably the most notable sealed bass cab is the Ampeg SVT. I actually like the sound of a sealed cab. Like Joris said, sealed won't go as low as a properly tuned ported cab, but the transient response of a sealed cab is "better" since you don't have the time delay from the sound coming out of the port near the low frequency cutoff.

    Like Joris said, it's all about tradeoffs. Don't just blindly believe the "it's got to be flat down to 20Hz or else it sucks" idea that seems to be prevalent today. Use your ears. A tight sounding rig that covers 80Hz - 4kHz actually sounds the best to my ears. Death to tweeters in bass cabs! :D

    Chris
     
  4. With the right driver, a properly executed SBB4 vented design comes fairly close to a D2 sealed design. The Kappa Pro 12 and JBL E155 are prime examples.

    Group delay (mud) is worst at the tuning frequency. A box correctly tuned well below the lowest operating note of your bass will offer very little delay in your operating range. The Adire Maelstrom 18" driver has 15 msec delay, but it is tuned at 20 Hz. This is below a 6-string bass' lowest note. When the delay is plotted, it decreases significantly by the time it gets into the operating range. Even then, 15 msec is well below the accepted 25 msec "sloppy" criteria.

    Answering the original question, the driver parameters determine if the cab is ported or not. A driver with an EBP of 50 or less is ideal for a sealed box. Greater than 100 indicates a vented box. If you are going to be haphazard, use a sealed box because they are far more forgiving than are vented boxes.
     
  5. K-Frog

    K-Frog

    Feb 6, 2002
    Camden, AR, USA
    I got pretty good results going sealed. I had a combo amp and put an Eminence Delta Pro 15 in it. I really like the sound. When I sold the combo I pulled the Delta back out and kept it. When I wanted a compact 15 to put under my 112, I figured I would just build a matching cab to go under the 112 while keeping the same interior volume that the combo provided, if not a hair larger. I figured " It should sound as good or better than it did in the combo". What do you know, It worked.

    bgavin, what's EBP? any idea what it is for my driver?
     
  6. Hey Bruce,
    Have you done any research into transmission line enclosures? What are your thoughts?
     
  7. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    Do you think this might be why Acme cabs sound tighter than most?

    Alex
     
  8. notanaggie

    notanaggie Guest

    Sep 30, 2003
    Really the worst problems with a ported cabinet are

    1) the possibility of a speaker excursion (movement) problem above the tuning frequency.

    2) the rapid "unloading" of the driver below the tuning frequency

    #1 causes speaker damage at high power, and possibly farting/snapping/muddy sound. And you won't think its gonna happen

    #2 causes your six string to fart out on lower strings and probably damage the speaker.

    if you just try a box and speaker, and do a port, its kinda hard to get it right. There is a bit of calculation, and there are design programs foalting around the internet that can help, IF you know (or can test) the speaker parameters.

    But, if you want the ultimate way low loud roar, you are pretty much gonna have to go with a big cabinet. Only a lot of cone area can put out power at the low end.
    That was one problem with the alembic ELF deal. Sounded fine, but the power was not really there, just tone at low levels.

    If you want tight sound that is perfectly usable, you might want to try reasonable sealed cabs AND putting some kind of low cut filter inline in the amp somewhere. Some biamps can be used that way, or you might have to make it yourself.

    And, with a ported cab, a low cut to keep you out of problem #2 above will really help in a lot of cases.

    its just amazing how a low cut can clean up sound. You would think you are losing tone, but it does not have to happen unless you go too high.
    Why bother wasting amp power on stuff your speaker can't put out without sounding like a donkey kicking a wet cardboard box?
     
  9. Greg R

    Greg R

    Mar 11, 2003
    Avonmore, ON
    My old 2x10 became increasingly flatulent and had to be set aside for basement duty only. I bought a pair of Delta 12A's on ebay and put them in a sealed 2x12 that was part of a 70's Traynor guitar rig. Sounds great. No farts, not even with the B string even at D flat.

    So what troubles should haphazard-sealed-cabinet builders like myself be on the lookout for? I see the difficulties in designing a good ported cab, any rules of thumb for sealed?
     
  10. notanaggie

    notanaggie Guest

    Sep 30, 2003
    You need to know the speaker parameters for the speakers you are using. there are rules of thumb, if you know some data.

    That info is out on the web, and some of it is actually pretty correct.



    If your speakers became "more flatulent", possibly they were suffering from being pushed too hard and cones moving too far so the suspensions got stretched. That usually looses them up, changes the resonance and compliance, and would change the optimum box.

    Some, like Celestion, always seem to loosen up after normal use, so they are continually changing for a while.
     
  11. It's all in my spread sheet. Everything you could possibly want to know about your driver, especially if an Eminence, JBL, B&C, PAS, or Selenium.

    I use the 3 most popular vented alignments, and the two sealed types (B2, D2). EBP is calculated by the spread sheet for your driver. Right-most columns.

    TL boxes require a ton of engineering and construction iterations to get it right. The EA engineer has a thread in archives here about his test results.

    EBP for the Delta Pro 15 is 95... indicative of performing best in a vented box of 6.2 to 8.4 cubic feet. Smaller boxes will cut the bottom off, give it the Eden hump and fast rolloff. Sealed boxes are 4 and 8 cubic feet for B2, D2 respectively.