BSC for 2.5 way

Discussion in 'Home Theater Projects' started by Jerry Parker, Mar 18, 2004.

  1. Jerry Parker

    Jerry Parker Stunt Coordinator

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    Ive got a 2way TWW with a Eminence psd2002 compression driver and 2 Eminence Delta 12LFA woofers per cabinet. I use a Behringer DCX2496 for crossover duties.

    My problem is that there seems to be a bit too much lobing and the soundstage seems to be "drawn" downward due to having both woofers playing up to ~1.5khz. This problem is heightened by how small my room is and how close I sit to the speakers. So my idea is to buy 2 15awg aircore inductors and run the system as a 2.5 way instead. I'll get full baffle step compensation (although honestly my room is quite small and I don't need much) and hopefully my lobing problems will go away.

    According to my calculations, I will want a 4mH inductor for a first order butterworth @ 300hz for the lower woofer. Does this sound like an appropriate frequency to everyone? My baffle width is 14 inches and the speakers are 49 inches tall.

    Thanks [​IMG]
     
  2. TimForman

    TimForman Supporting Actor

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    In open space a 14" wide baffle will have a 6db drop at 325 Hz.
     
  3. Dan Wesnor

    Dan Wesnor Second Unit

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    If you can rebuild the speaker, make it an MTM instead. 2.5-ways have inherent problems (due to the phase difference between the two woofers) which are difficult to solve without simulation software.

    If go with a 2.5-way, you will also need to put in a steep cut at the top of the baffle step (probably around 650Hz) to keep the lower woofer from having destructive interference with the other woofer and even the tweeter.
     
  4. Jerry Parker

    Jerry Parker Stunt Coordinator

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    Dan, I can't really modify them to be MTM's. They sound ok right now, but nothing special. I am using a active xover, so I really don't want to add a lot of passive components in the mix. Destructive interference won't be much of a problem with the tweeter because i am using 8th order LR crossovers on both the woofers and tweeter @ 1.5khz.

    Tim, are you talking about a dipole design that has a 14" wide baffle? Sorry if I wasn't clear, I'm using a 3cubic foot enclosure tuned to 35hz.
     
  5. TimForman

    TimForman Supporting Actor

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    That's funny, I don't see a post where I mentioned dipole. I get confused where I post stuff though. If you have one woofer firing to the rear in phase baffle width becomes much less of an issue and BSC is not needed. This would not work well if you have to put the cabinets very near the rear wall but they do poorly in that configuration anyway. It's just an option to think about. Mechanical solutions are always better than electrical ones IMHO.
     
  6. Jerry Parker

    Jerry Parker Stunt Coordinator

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    Tim, I thought you were talking about a dipole design because you said there would be a 6dB drop in freespace @ 300hz. All other calculations I have found are that the drop should be much less than that, more like 2-3dB at 300hz, and then decreasing to 6dB at the lowest frequencies. I am not following why there would be a 6dB drop at 325hz?

    Both woofers are on the front baffle firing forward, 2 inches from each other. They are electrically in phase. Here is a picture:

    [​IMG]

    My room is rather small, so the speakers are only about 2 feet from the rear wall and 1 foot from the side walls.
     
  7. TimForman

    TimForman Supporting Actor

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    I was using John Murphy's formula. This assumes free space. Things are much different in a room, especially a small one as you're describing. It's hard to tell from a picture but that looks like a lot of woofage for that cabinet. Maybe tapering one off would help.
    http://www.trueaudio.com/st_diff1.htm
     

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