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Thread: Ports in back on PA cabinet?

  1. #1
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    Ports in back on PA cabinet?

    Im in the process of designing some minimum size PA cabinets for the 15" 2226H along with the 2446J/2380A horn combo. I'm trying to make the cabinets as small as is physically possible, and don't need LF below 80 hz, but could still use any LF extension porting would give me.

    The problem is, there will be so little front baffle area left in these cabinets. I know numerous JBL home models use the ports in the back, but how would this play out in a PA situation? With home speakers, output from the rear ports is often reflected back off a wall, but this wouldn't be happening with a PA. Would the output from rear facing ports be detrimental in this case, (out of phase?), or do the ports add significant output to the LF driver itself, (easier movement?), to make it worthwhile? Or should I just go with sealed cabinets for something this small? No output at all below 80 hz is needed, but I still haven't calculated the cabinet volume to see how close I can get.

    Thanks for any insight.

    John

  2. #2
    Senior Member duaneage's Avatar
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    Could you use a slotted port with an internal shelf instead of round ports? Shelf ports have greater volume and while a little harder to tune they don't have as much a problem with pipe resonance tht tubes do. Using a port in the 80 hx range may bring on resonance that you don't prefer.

    As to ventin out the back, that would work in principle but may contribute to LF feedback in some cses. Probably not a big problem but it might happen. If your aiming sound out and away a port will contradict that. Ports do radiate significant energy at Fb, air in the port is at maximum pressure and minimum velocity while the cone barely moves (compared to other frequencies) providing bass reinforcement without cancelling out driver output.

    How about triangle shaped ports at the corner? You could use a single piece of stock inside to form a triangle port, or two.

    The 2226 uses 4 and 6 foot vented cabs for allignment, sealed would be a better choice if deep bass was not needed, just watch the xmax on the drivers.

  3. #3
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    The location of the ports makes no difference in a small box.

    Once the wavelength being reproduced by a transducer exceeds the baffle size the propagation of the wave is omni directional.

    The wave length at 80 Hz is 14.067 feet and the 1 to 2 foot extra path length to the back of the box is insignificant at this frequency.

    Above the resonance of the box the ports are “invisible” to the transducer and the box behaves as a sealed enclosure and there is no useful output from the vents.

    The main disadvantage of rear ports is the rear of the speaker must be clear.

    I.e. you can’t place it hard against a wall or something else.



    Note: The free space propagation is omni directional but as we use the speaker in a room our propagation is into 2π or 4π and we may ignore any phase change from radiating low frequencies from offset sources such as vents, the room reflections will have a much greater impact.

    The transition from directional to omni directional propagation is referred to baffle step refer to Olson 1957.

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