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rear ports in real world situations

Discussion in 'Amps and Cabs [BG]' started by basss, Jun 19, 2003.


  1. basss

    basss

    Aug 27, 2001
    NYC
    I know that the frequencies comming out of the ports in a vented speaker cab are supposed to be omnidirectional. I'm thinking maybe that this statement is true in an acoustically perfect environment but in real world situations port placement would make a difference. I'm especially curious to find out if anyone has compared front and rear ported cabs on an outdoor gig. What is your experience comparing different port placements?
     
  2. alexclaber

    alexclaber Commercial User

    Jun 19, 2001
    Brighton, UK
    Director - Barefaced Ltd
    From a theoretical standpoint, the obvious disadvantage of rear ports it that you can't place the cab hard up against a wall because it'll choke the port. In fact you should keep the cab a port diameter from the back wall, minimum for the port to work properly.

    Apart from that, the key difference is that rear ports are about 15"-19" (depending on the size of that cab) further towards the rear which may cause more booming or may be more beneficial when you need reinforcement off the back wall or want to use a corner as a horn.

    I've used my Acmes outdoors and had no problems with a lack of bass out front.

    Alex
     
  3. tornadobass

    tornadobass Supporting Member

    Nov 20, 2000
    Iowa City, Iowa
    Endorsing Artist: Black Diamond & SuperSensitive strings
    I'll be using a rear ported Goliath II outside today for the first time. It'll be on one of those stage trailer things, so there's some back wall, but it'll be several feet away.

    I wonder how this'll work out...I've used mainly front ported cabs in situations like this.
     
  4. Bob Lee (QSC)

    Bob Lee (QSC) In case you missed it, I work for QSC Audio! Gold Supporting Member Commercial User

    Jul 3, 2001
    Costa Mesa, Calif.
    Technical Communications Developer, QSC Audio
    That's a good question. I asked it of one of our loudspeaker design engineers here at QSC some time ago, and he said that at those very long wavelengths, front or back doesn't matter sonically.

    With any type of port, you should have no large-surface obstructions (wall, floor, et al) within a distance equal to the diameter of the port.
     
  5. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    I often see speakers where the fibreglass stuffing has come away from the wall of the cab and has fallen down to cover the port. The stuffing flaps around in the wind all night and I can't help but think that it can't be a good thing.

    Back on topic - I've heard that rear ported cabs have a tendancy to couple with the rear wall of the stage and create extra unwanted boominess. But these seem to be claims that aren't ever supported by owner temtimonials. I never hear people who own and use rear ported cabs complaining about it.

    Tornado, since you own one, you're now our official lab-rat. Lets put the facts straight once and for all!
     
  6. basss

    basss

    Aug 27, 2001
    NYC
    tornado
    let us know how it goes w/your Goliath.

    I asked the question after reading some posts about this issue. The tech guys all say it doesn't make a difference but several bassist have said that rear ported cabs lack bass response in outdoor situations or in places w/no wall behind the cab.
     
  7. tornadobass

    tornadobass Supporting Member

    Nov 20, 2000
    Iowa City, Iowa
    Endorsing Artist: Black Diamond & SuperSensitive strings
    Okay...back from the gig...the sound was fine. The trailer stage...every parks & rec dept. must have one...was actually deeper than I remembered.

    I was set up about 10 feet in front of the back wall, but still under the extended overhang. Rig was an SWR SM400S -- bridged into an 8 ohm Goliath II and an Eden D210T. Sitting on top of the Goliath, the Eden seems to add clarity rather than extra bass. My DI also went into the subs on the front of the stage, but it wasn't that big of a PA system.

    Amp was basically set flat, enhancer at about 1 o'clock and a slight bump in the mids at 500 hz. Master volume was at about noon, as was the input gain. I play an Azola Bugbass EUB, which has a ton of low end.

    I know it wasn't exactly outdoors without any wall behind it, but I wasn't close to the wall and it was outdoors.

    The verdict?

    Plenty of low end and plenty of clarity. Nice thick sound with good definition.

    But more testing is needed...especially in a purely outdoors with no back walls at all. That could be my next week's gig.

    Tornadobass

    www.tornadoesblues.com
     
  8. Run a low frequency sine wave signal through your ported cab at the tuning frequency (31 Hz, etc).
    Close your eyes and have a friend turn the cabinet around 180 degrees while playing the tone. You won't hear any difference.

    A sub crossed over at 125 Hz can be turned any direction. You can have your subs turned around backwards and it won't make any difference in the low end. An 18" subwoofer wave form is spherical up to approximately 225 Hz. A 10" driver is spherical to about 400 Hz. Ports are tuned much lower than this, so they are not directional.
     
  9. Matthias

    Matthias

    May 30, 2000
    Vienna, Austria
    This may be true, but any cab front- rear- or not ported at all, will sound thinner without a wall or corner behind it or when lifted off the floor.

    I've been using an Ampeg SVR 215 with rear ports AND a backfiring speaker (!) without any problems for years. (The backfiring speaker was crossed over at 200Hz or so, I don't remember.)

    Bottom line: I don't think that port placement makes a difference.

    Matthias
     
  10. Acoustic image and Flite sound both use woofers that fire down, and people rave about how good they sound. There again since the bass is non-directional, it shouldn't matter. And if you care, horn theory for bass says that you will extend your bass by two if your speaker is on the floor, by 4 on the floor with a wall behind and 8 if it's in the corner. Of course I have little practical experience with this. YMMV.

    BT
     
  11. tornadobass

    tornadobass Supporting Member

    Nov 20, 2000
    Iowa City, Iowa
    Endorsing Artist: Black Diamond & SuperSensitive strings
    Here's an experience closer to the original question.

    Last night, I played an outdoor party...not that loud a gig and I used my Goliath II rear ported cab again. This time, the band set up on a patio and played facing into the house. The back of the cab was out into the yard...no rear wall at all for reflecting sound from the port.

    I'm happy to report that the bass was as full and deep as in other setups...didn't lose any bottom that I noticed. EQ was almost flat, with just a bit of 500 hz added for clarity.

    Seems port placement didn't matter in this case.
     
  12. basss

    basss

    Aug 27, 2001
    NYC
    10-4
    Thanks!
     
  13. rok51

    rok51

    Sep 2, 2002
    Crawfordville, FL
    Having used Peavey 410TX (rear-ported) cabs for nearly a decade, indoors and out...I, nor any patrons...felt that the bottom was lacking.

    Kim
     
  14. Petebass

    Petebass

    Dec 22, 2002
    QLD Australia
    Well it's official! I'm convinced that rear porting and front porting are no different.

    Just shows to go ya - don't believe everything you read......