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Ports, front or back?

Discussion in 'Amps and Cabs [BG]' started by jmanyea, Jun 17, 2002.

  1. jmanyea


    Mar 20, 2002
    St. Louis
    Does anybody know what the advantages, if any, of porting on the front of a cabinet vs. on the rear?
    I've heard that a rear ported cab can get a boost by placement near a wall, does this mean that all things being equal, front ported cabs are louder or project better than rear ported cabs?
  2. Chuck M

    Chuck M Supporting Member

    May 2, 2000
    San Antonio, Texas
    You've got it right. Front ported cabinets project better than rear ported cabinets. My experience has been that rear ported cabinets tend to sound rounder and less articulate than front ported cabinets.
    The real drawback to rear ported cabinets is that they don't do too well on loud gigs with no wall behind them. I played a festival where the stage rig used a pair of the older SWR rear ported 410 cabs. I heard the mids and highs but weak lows. I'll bet it was really pounding behind the outdoor stage.

  3. Zirc


    May 13, 2001
    Los Angeles
    My ports are on the bottom :eek:

    heh, go figure, I like my sound
  4. gfab333


    Mar 22, 2000
    Honolulu, Hawaii
    I agree with Chuck M. I've noticed the same thing about the two types of cabs.

    I've played both, and I prefer front ports. (You might not always be able to set up near a wall)
  5. Si-bob


    Jun 30, 2001
    Hemel Hempstead, UK
    Focusrite / Novation
    i had a quick looky at my new ampeg setup, and i think the 210 is rear ported and the 115 is front ported, and it sounds great, so it seems its a good combination!
    i'v heard that with rear ported cabs, u gotta be careful about putting their backs to the walls?, is this right?

  6. 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
    I talked to a couple of QSC's speaker design engineers, and they agreed that it doesn't really matter where the port is, front or back, top or bottom, because at the very deep frequencies where the port comes into play, the cabinet radiates omnidirectionally. Unless it's a huge cabinet, like 10 to 20 feet across or larger; I think it's safe to assume that any cab we're talking about doesn't fit that description. ;)

    You wouldn't want a rear port (or any port) right up against the wall because it could interfere with the air moving in and out of it. You should set the cabinet away from the wall at least a distance equal to the diameter of the port.

    The job of the port is to extend the cabinet's bass response down to lower frequencies than it could produce if it were merely a sealed box. In a sealed, infinite baffle cabinet, at lower frequencies the air inside acts like a spring that resists the cone's motion and tries to move it back to its at-rest position. The smaller the volume of air in the cabinet in relation to the cone size and displacement, the greater this effect. The port adds a leak to the box to lessen the back pressure on the cone. That leak is the resonant slug of air in the port. At frequencies significantly higher than the resonance of the port and cabinet, the slug is almost a closed path, and the cabinet acts somewhat like a sealed enclosure. At low bass frequencies, it lets the cone move with less resistance, and at extremely low frequencies below the resonance, the port acts like a big leak that removes just about all the air loading from the cone. That's also why ported speaker systems are more susceptible to overexcursion at extremely low frequencies.

    The port emits sound at low frequencies, but in that range it, the driver, and the cabinet all amount to a single point source. Therefore, the port's position on the cabinet doesn't make a difference as long as it's not blocked.
  7. Captain Awesome

    Captain Awesome

    Apr 2, 2001
    Yeah, I think the main difference with front ported cabs is that they just tend to be bigger, especially the shelf ported ones.
  8. Thornton Davis

    Thornton Davis

    Dec 11, 1999
    I use two 410 rear ported cabinets. I've used them at indoor gigs with walls, outdoor gigs with no walls and to tell you the truth, there is no difference in the sound of my rig if there is a wall behind my rig or not. It always sounds outstanding.
    They sound great no matter where I use them, with or without a wall. But I have always noticed that they sound best when stacked on top of each other.
    I have considered many times changing my cabs to front ported ones, but those cabinets are bigger and heavier then my current ones and I don't notice a difference. So i'm just going to leave my cabs as they are and continue to enjoy. I'd rather spend the money on another bass anyway.
  9. geshel


    Oct 2, 2001
    I'm going to back Bob up on this one. I've read that people's experience has shown otherwise, they've found front-ported cabs louder or something to that effect. But I would chalk that up to other fundamental differences between the cabs. When it comes right down to it, you can put the port on the front, back, or side (Acme) and it don't matter. Just don't block it with any other gear or a wall.
  10. Brendan


    Jun 18, 2000
    Austin, TX
    I have front ported main cabs, and my practice cab is rear ported...and I agree that when placed against a wall, the rear ports can make it sound better...course, it's Crate, so better is mighty generous term to use...
  11. geshel


    Oct 2, 2001
    Putting any speaker near a wall or especially a corner will increase the room resonance in the bass. Try it with a front-ported speaker too.