Converting Closed-Back Cab to Open-Back: Advise Needed...

Discussion in 'Amps and Cabs' started by megatonic, Feb 4, 2012.

  1. megatonic

    megatonic Member

    Messages:
    2,475
    Joined:
    Aug 10, 2006
    Location:
    Northern CA
    I have a 2x12 closed back speaker cab that's ported on the bottom front. Here's a rough diagram:

    [​IMG]

    Anyway, I'm thinking of fabricating an open or partially open back for it, to open the sound up a bit.
    Considering the location of the port on the front, can someone with experience tell me what the best open/semi-open configuration would be for the back panel?
     
  2. sharpshooter

    sharpshooter Member

    Messages:
    3,338
    Joined:
    Jun 29, 2008
    Not a lot of info to go on,,,do you not like the "front port" sound?
    FWIW,,I picked an Eminence Cannabis Rex,since it's a popular/easy to obtain spealer,,has an effective cone area of ~86 sq. in., and is fair representation of many 12 in. speakers concerning its cone area.

    For a real "open back" type sound, the rear opening would be equal-to, or greater-than, the speakers cone area,,like a typical combo amp.
    As you decrease the area of the opening, relative to cone area, you start to move in the direction of "closed back",,but it's not linear,it's an exponential curve. It takes a lot of reduction, in order to start to provide some "loading" of the speaker.
    With your 2-12 cab, cone area is somewhere ~172 sq. in. To even start to provide some of the "punch" of a closed back, the port needs to be at least ~10>15% smaller than cone area. Once you get into the 40>50% range,,now we're getting into the realm of having a cab that will still provide an "airy" sound at lower volumes,,but when pushed with larger cone excursions will retain some punch and projectivity.
    The best shape for a port is an ellipse,,it has no parallel edges which can cause freq dependant edge diffraction, nor any "chuffing" from round ports.
    Hope some of this might help,,but only your ears can decide.
     
  3. megatonic

    megatonic Member

    Messages:
    2,475
    Joined:
    Aug 10, 2006
    Location:
    Northern CA
    Thanks. I've been playing open back combos, for the majority of my time, playing. The cab is a Tone Tools Fat Cat (production model) and I believe it's a "non-tuned" port/cab(?). It's a good sounding cabinet, but I did watch some comparisons of open vs closed back cabs and did notice a significant difference.
    Beside my preference of the open back sound, I think the open back actually lets my drummer hear me better on stage.

    I will definitely take your specs into consideration, but I guess what I was also wondering is if I placed the open part of the back toward the top of the cab so that it's almost directly behind the speakers, and then left the bottom 1/3 or so of the back solid, would it reflect a little more toward the bottom port? Kind of combine the benefits of open back and front ported?
     
  4. walterw

    walterw Gold Supporting Member

    Messages:
    33,532
    Joined:
    Oct 16, 2006
    i suspect you'd need to close off the front port for it to work right as an open-back cab.
     
  5. sharpshooter

    sharpshooter Member

    Messages:
    3,338
    Joined:
    Jun 29, 2008
    ^ Yeah, you need to close -off the front port,,. It's always a compromise between open/closed back,,the more open,,the more airy,,the less projection,,and "looser" at high volume. As we close down, we start increasing directivity,,and decreasing efficientcy,,

    But, we have a side benifit, as we increase the "loading" on the speaker we increase the apparent stiffness of the cone with no increase in mass. This leads to a better responce curve, with lower freqs, at higher spl, without flubbing,,as well as higher freqs with clarity.
    The typical open back cab becomes "lost" at high drive levels,but, then again, in todays "low volume" world,,,,,,,,,,
     
  6. megatonic

    megatonic Member

    Messages:
    2,475
    Joined:
    Aug 10, 2006
    Location:
    Northern CA
    So here's what I fabricated. In this pic it's on a drying rack in my garage.
    The little round hole is for the jack plate.
    after it dries and I spray another coat. I'll post pics of it mounted on the cab.

    [​IMG]
     
  7. Ed Reed

    Ed Reed Senior Member

    Messages:
    7,522
    Joined:
    Feb 21, 2004
    Location:
    Somewhere out on that horizon
    That picture is great.

    The back panel you made is classy too.
     
  8. megatonic

    megatonic Member

    Messages:
    2,475
    Joined:
    Aug 10, 2006
    Location:
    Northern CA
    Here's the finished product:

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]

    [​IMG]
     
  9. wrathfuldeity

    wrathfuldeity Member

    Messages:
    1,305
    Joined:
    Jun 17, 2010
    Location:
    state of delusion
    Looks nice, essentially an open back, maybe try another one but with an eliptical about the area of 12" hole. I did a diy 1x12 detuned and left the baffle detuned but opened the back to about 1/3 to get some room fill. Anyway sounds great in that it retained some of the push, open largeness and lower frequencies of the detuned though a little less low end; and traded the directional characteristic for room fill...so its big open 3-d and room fill.
     
  10. megatonic

    megatonic Member

    Messages:
    2,475
    Joined:
    Aug 10, 2006
    Location:
    Northern CA
    Thanks, I will probably try an eliptical opening sometime in the future to compare.
     
  11. walterw

    walterw Gold Supporting Member

    Messages:
    33,532
    Joined:
    Oct 16, 2006
    it would be worth trying a piece of wood to cover that front port; you'll probably get the low end back.
     
  12. megatonic

    megatonic Member

    Messages:
    2,475
    Joined:
    Aug 10, 2006
    Location:
    Northern CA
    I'm going to rig some kind of cover for the port that I can velcro in or something.
     
  13. blackba

    blackba Supporting Member

    Messages:
    9,295
    Joined:
    Dec 10, 2005
    Location:
    Michigan, USA
    +1 That is what I was thinking as well.
     
  14. wrathfuldeity

    wrathfuldeity Member

    Messages:
    1,305
    Joined:
    Jun 17, 2010
    Location:
    state of delusion
    Ime with my diy 1x12 of not having a front port...it tightened the low end but made it narrower...not as low, i.e., but lost the lower low end. Imo, I'd do some drilled out screw-in bolt inserts on the back side of the baffle (thus unseen from the front) and use some 1/2 ply for it; you need something solid that can be tightened down.
     
  15. megatonic

    megatonic Member

    Messages:
    2,475
    Joined:
    Aug 10, 2006
    Location:
    Northern CA
    I made a small panel out of my leftover plywood to cover the front port. I actually used velcro to attach it, but also used some dense 1/16" adhesive foam as a gasket around the edges, to prevent any vibration. I decided to use velcro because it's a quicker switch-out if I ant to change the back panel, and I didn't have to drill any holes.

    Man, I noticed a big increase in low end.

    Thanks you guys!
     

Share This Page