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Front AND Back ports. Why not both?

Discussion in 'Amps and Cabs [BG]' started by MuzikMan, Jul 8, 2004.

  1. Before I get the tomatoes thrown at me and the BOOOOOS please allow me to explain. I am in the process of modifying my cabs and part of that process involves retuning them. I am interested in putting 2 ports in the back and 1 in the front simply for cosmetic reasons. I have never seen this done and I was wondering why?

    I have done extensive research here and on the web. The issue of front or rear porting has been beaten to death for sure but why not both? Is there a phase issue? Has it simply not been tried or needed before?

    Please share your thoughts with a budding cabinet maker, whether it be speculative or factual. I enjoy reading all points of view.
  2. bill h

    bill h

    Aug 31, 2002
    small town MN
  3. You missed me! HA HA!
  4. I've been to this page before and don't see the link to the port discussion. :crying:
  5. Go to www.acmebass.com, click on the Products link, then near the bottom there's another link titled "An explanation of the (un)importance of port placement."

    I guess there's not a direct link you can just click on. weird.
  6. So the Acme B4 has the same port design. Good enough for me!
  7. Yep, and it really thumps.
  8. Petebass


    Dec 22, 2002
    QLD Australia
    Because the design of commercially made cabs is often cost driven. It's just easier, and therefore cheaper, to put all the ports on the one timber panel.

    Go for it. There's nothing wrong with your plan.
  9. sheepdip


    Apr 14, 2004
    Put them wherever but make sure they are the correct length and diameter for your speaker/s.
  10. dnburgess


    Jul 20, 2003
    Sydney, Australia
    Proprietor: Bass People, Green Square Music
    The B4 has ports on the front and back - the B2 on the back - the B1 on the side.
  11. GrapeBass


    Jun 10, 2004
    Graphic designer: Yorkville Sound
    ummm, actually port shape size and placement is significant. There is sound eminating from ports! If you want to minimize that contribution to the overall sound rear placement would be preferable but you have to be a bit more careful when placing your cab in the room. SVT 8x10s are actually 4 2x10 cabs sealed... The infinite baffle (sealed) is more dependant on cab size. Remember the motor of the speaker is affected by the fact the cab is sealed.

  12. ERRR...check the link to Acme's explanation of port placement in my other posts. Also, Ampeg would have to disagree that rear ports are preferable. The HLF's and HL4's have them all up front. Genz Benz too. 4 big ass ports at the bottom front of their cabs. The shelf ports on Eden and SWR cabs fire out the front.
  13. For the record I have the calculations for the proper port sizes. I also understand that there's a heated debate about the linear placement of ports.

    My question is about the non-linear placement of ports. Let's assume that the cabinet is sitting in an open air environment with no walls to affect bass response..
  14. GrapeBass


    Jun 10, 2004
    Graphic designer: Yorkville Sound
    I used to work in a Quality Control department for a manufacturer of instrument cabs. No, I won't say who (sorry). I worked for 11 years, from 87-98. I've tested so many speakers, acoustically and electronically (using LMS, an aquisition/test card for speaker design). The IS significance with the shape, size and placement of ports. If you make the port too small it's possible it will make a puffing noise at the resonant frequency (depending on the power). Small subs usually have this foible. Ports on the front contribute midrange, mostly because you are also hearing from the rear of the drivers bouncing around and eminating from the hole in the baffle. Damping helps but does not eliminate it. I personnaly prefer a sealed cab an I am NOT too concerned with the speakers SPL efficiency. Quality of sound!

    Read all you want, a tighter, smoother cab is sealed, but it ain't as loud.
  15. BruceWane


    Oct 31, 2002
    Houston, TX
    Port size is significant. As you said, too small and you'll get "chuffing". Shape doesn't matter, as long as the area is sufficient to prevent chuffing. Square, round, triangular, it doesn't matter, as long as it's not too small.

    If you're building a really large, high volume sub, you'll have to take measures to minimize turbulence within the port, i.e. round off any turns in long ports.

    Port placement isn't significant in a bass rig because there's no way you're going to have a rig big enough to be generating directional bass frequencies anyway. Producing directional sound waves in lower frequencies takes more gear than you're going to be able to fit on an average stage. Unless, of course, you happen to be in Van Halen. Since bass rigs are too small to be directional, they act as pressure generating devices, which means they radiate sound waves in a 360 degree pattern, much like the waves you see when you toss a rock in a pond.

    As far as low frequencies, it doesn't matter which way the speaker is pointing, for that matter. The higher frequencies, anything above a few hundred Hz, are directional, so if you're relying on your woofer for mids as well (most rigs do), you won't want to point your speaker in the wrong direction.

    BUT, speaker ports only really come into play around their tuning point, which is around the speakers resonant frequency. The closer you get to the tuning frequency of the port, the more it contributes to the overll SPL of the cab; at tuning frequency, nearly ALL the low end is coming from the port. For a bass guitar woofer, that tuning frequency is going to be somewhere below 70Hz for sure.

    The fact that a typical bass rig is non-directional is why you get a low end boost from placing your rig near a back wall or in a corner, versus out in the middle of a room. All that sound pressure that is headed off in the wrong direction gets reflected back in the right direction. If your rig was directional, all the soundwaves would already be headed in the right direction, so placing the rig in a corner would make no difference.

    Sealed cabs sound tighter because they generally have better group delay characteristics than most vented cabs. The trade off is they have less low end. If you're very picky about the drivers you use and your cabinets design, you can get a vented cab that sounds very tight and still has a lot more low end than a sealed cab, but it's a very delicate balance. Just about all commercial vented bass cabs are quite a bit smaller than they should be, which keeps them sounding tight, but it also sacrifices a lot of low end response.
  16. Well let's see what the cab gurus think about the whole kit and kaboodle.

    I'm changing a 115 into a 112 cab. I like the sound of the 2x12 deltalf cab I have but have noticed all the talk about companies making cabs too small for the drivers. This is great for portability but I am already comfortable hauling my 2x12 115 rig so I wont be adding anything.

    I downloaded WinISD and went to work. The driver is a Delta12lf 4ohm, The cab is 3.9 cubic feet, and I have currently settled on a tuning of 37.80 hz, which will be achieved with 2 3x4.5 in round ports.

    The specs look nice, F3=41hz, F10=29hz. The group delay is high at 32hz (around 21ms) but I play a four banger and the delay at 41hz is 14ms.

    The question of the port placement has more to do with the 2x12 cab I have. I have decided to try different tunings on that cab as well (which is also 3.9 cubic feet). My current plan calls for a tuning of 46 hz which will be achieved with 3 3x4.5 in round ports. I like the look of ports in the front but my 2x12 plus tweeter dont leave much room for them. I want to try to keep the ports at least 2.5 inches from the walls so there's only room for one in the front. Therein lies my question WHY NOT FRONT AND BACK PORTS?
  17. Petebass


    Dec 22, 2002
    QLD Australia
    How can this be? When I stand behind a rear ported cab and face the ports, I certainly don't hear any midrange. IMO this is one of the most widely believed wife's tales in audio.
  18. Don't let the ports face each other if you do it (holy cow, I can see through your amp!), I've heard it's a no-no. Forgot why though...
  19. Saetia


    Mar 27, 2003
    My Eden 410T has a huge port in the back, I'd say probably 5" in diameter (never actually measured it).
  20. Kelly Lee

    Kelly Lee Yeah, I'm a guy! Supporting Member

    Feb 17, 2004
    Marana, AZ, USA
    I agree sub frequencies are not directional. Thats fits for a dedicated sub in a bi-amped bass rig or in your car. However, I disagree with a full range rig not being directional. Does a bass guitar stay under 200hz or so? No. The typical bass rig is definately directional. Secondary harmonics and such are way above this range and are directional. What would be the purpose of having cabs with a frequency response of 4khz or with a tweeter of 20khz? Only the lowest part is non-directional.

    Here is a little test. Stand 10ft in front of your cab and play a line you love that covers a wide frequency range. Now, as you continue to play walk around your cab till your 10ft behind it and tell me if its directional or not? You will hear the difference.