The Alcove Sub is Born

Discussion in 'Home Theater Projects' started by Felix_F, Jun 15, 2004.

  1. Felix_F

    Felix_F Stunt Coordinator

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    Here is how an alcove in our family room was made into a subwoofer using a pair of 18" Dayton drivers:

    [​IMG]


    Half the walls in the alcove are either cinderblock or concrete. The rest of the walls were lined with plywood for extra strength, and a false ceiling was built that also served as a support for the baffle. I lined the non-cinder surfaces with vibration damping material from PE in hopes of minimizing the enclosure's vibrations being transmitted to the upstairs. All the inside and outside seams were filled with Mortite and then sealed again with weather stripping tape. It took ~180' of rope calk to make the alcove air-tight.


    [​IMG]



    [​IMG]


    I built some cage-like structures out of chicken wire to hold the stuffing in accordance with MLTL design parameters:

    http://www.quarter-wave.com/index.html



    [​IMG]


    Here you can see the plywood part of the baffle braced by 2x8's with the drivers installed. Right under it lies the MDF part of the baffle which was glued and screwed into the plywood once the latter was installed in the alcove.


    [​IMG]


    Here she sits in all of her unfinished glory. A 4" deep port is fitted in the pic, but after a little listening, I popped in a 18" deep version. How does it sound? Well, keep in mind that it's virtually uncalibrated. I'll pay with some test tones and my SPL meter tomorrow, play some music, etc and get a better baseline. All that said, the performance is literally frightening. My wife jumped during the big battle scene in LOTR ROTK. I felt my intestines move. Really. We're hearing the house make noises we've never heard before.

    I wired each driver to its own set of binding posts. This way, they can either be configured to run in series or in parallel, or each woofer can be driven by a separate channel.


    [​IMG]


    Tomorrow, or the next day, I'm going to paint black the grill frame pictured above and upholster it with some nice, gray grill cloth at got at PE. It should look good in the room once everything gets back to normal.


    [​IMG]

    Other than the need to touch-uppaint the walls due to the damage sustained while fitting the baffle, the project is finished.


    [​IMG]

    I kinda like the way the sub looks without the grill. With small children and a couple of cats in the house, the grid over the port is mandatory.

    Felix
     
  2. Chris Brock

    Chris Brock Second Unit

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    WhOA!! that is very awesome. I am ancious to hear some of your spl readings. i would like to know what numbers you are hitting with that setup.
     
  3. Felix_F

    Felix_F Stunt Coordinator

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    Thanks for the kind words, Chris and Jack.


    I don't know if I dare take it to max SPL. Last night, during the giant "elephant" scene in ROTK it sounded like nails were about to start popping out of the walls :)
     
  4. Felix_F

    Felix_F Stunt Coordinator

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    Thanks for the kind words, Chris and Jack.


    I don't know if I dare take it to max SPL. Last night, during the giant "elephant" scene in ROTK it sounded like nails were about to start popping out of the walls :)
     
  5. Mike SJ

    Mike SJ Supporting Actor

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    have you tried to block the port to hear the 'sealed' response?
     
  6. Mike SJ

    Mike SJ Supporting Actor

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    have you tried to block the port to hear the 'sealed' response?
     
  7. Felix_F

    Felix_F Stunt Coordinator

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    I haven't, but I probably will. The port started out as a 4'x11.4" (internal diameter) sonotube. We cut a 4" and a 18" port, tuned to ~22Hz and ~16Hz respectively, which leaves 26" of tube waiting to be played with. I'll probably make a plug out of a ~2-3" section, although I'm not entirely sure how I'd go about it just yet. That's an awfully big area to make air-tight. Of course, there's always duct tape ;-)
     
  8. Felix_F

    Felix_F Stunt Coordinator

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    I haven't, but I probably will. The port started out as a 4'x11.4" (internal diameter) sonotube. We cut a 4" and a 18" port, tuned to ~22Hz and ~16Hz respectively, which leaves 26" of tube waiting to be played with. I'll probably make a plug out of a ~2-3" section, although I'm not entirely sure how I'd go about it just yet. That's an awfully big area to make air-tight. Of course, there's always duct tape ;-)
     
  9. Mike SJ

    Mike SJ Supporting Actor

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    ive heard of people using mini nerf footballs for smaller ports, maybe a regular nerf football will work for the 4", the 18"...hmm, duct tape wont 'hold the air back' and possibly perform like a bad, very bad passive radiator.

    cant weait to see the finished product though. kepp rocking.

    PS try getting the blue man group DVD [lots and lots of drums]
     
  10. Mike SJ

    Mike SJ Supporting Actor

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    ive heard of people using mini nerf footballs for smaller ports, maybe a regular nerf football will work for the 4", the 18"...hmm, duct tape wont 'hold the air back' and possibly perform like a bad, very bad passive radiator.

    cant weait to see the finished product though. kepp rocking.

    PS try getting the blue man group DVD [lots and lots of drums]
     
  11. Felix_F

    Felix_F Stunt Coordinator

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    I download some sine waves ranging from 10-80Hz. Here are the results with SPL meter correction coefficients already applied:

    10Hz - 89dB
    12Hz - 90dB
    16Hz - 93dB
    20Hz - 95dB
    25Hz - 95dB
    32Hz - 95dB
    40Hz - 96dB
    50Hz- 102dB
    63Hz - 95dB
    80Hz - 89dB

    By the looks of it, the sub's in-room -3dB point is at ~15Hz with the 4" deep, 11.4" diameter port, and the roll-off seems pretty smooth. The -10dB point is nowhere to be found, as the sub is only -6dB at 10Hz (!). It'll be interesting to see how the longer port length performs and how the sub does at higher SPL's. The two drivers are ~100dB sensitive, so it doesn't take much juice to get this thing going.

    Felix
     
  12. Felix_F

    Felix_F Stunt Coordinator

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    I download some sine waves ranging from 10-80Hz. Here are the results with SPL meter correction coefficients already applied:

    10Hz - 89dB
    12Hz - 90dB
    16Hz - 93dB
    20Hz - 95dB
    25Hz - 95dB
    32Hz - 95dB
    40Hz - 96dB
    50Hz- 102dB
    63Hz - 95dB
    80Hz - 89dB

    By the looks of it, the sub's in-room -3dB point is at ~15Hz with the 4" deep, 11.4" diameter port, and the roll-off seems pretty smooth. The -10dB point is nowhere to be found, as the sub is only -6dB at 10Hz (!). It'll be interesting to see how the longer port length performs and how the sub does at higher SPL's. The two drivers are ~100dB sensitive, so it doesn't take much juice to get this thing going.

    Felix
     
  13. Allen Ross

    Allen Ross Supporting Actor

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    AWESOME
     
  14. Allen Ross

    Allen Ross Supporting Actor

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    AWESOME
     
  15. ThomasW

    ThomasW Cinematographer

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    Those are low excursion drivers primarily designed for PA use. They have accordion surrounds (6 mm of Xmax). As a result DO NOT plug the ports. Doing that will double the excursion of the driver and possibly damage them!
     
  16. ThomasW

    ThomasW Cinematographer

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    Those are low excursion drivers primarily designed for PA use. They have accordion surrounds (6 mm of Xmax). As a result DO NOT plug the ports. Doing that will double the excursion of the driver and possibly damage them!
     
  17. Felix_F

    Felix_F Stunt Coordinator

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    Hi, Thomas. You may recall us discussing the options for the alcove sometime back, and as you can see the project is finally complete. At first, I was concerned about the woofers' relatively low x-max, even though the sims were predicting that they'd hit 100dB at 20Hz with 1mm of excursion, but the short time I've spent testing put most of my fears to rest. I still have a lot of tuning to do, but I haven't reached their limit yet. Hmmm....maybe I need a bigger amplifier...
     
  18. Felix_F

    Felix_F Stunt Coordinator

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    Hi, Thomas. You may recall us discussing the options for the alcove sometime back, and as you can see the project is finally complete. At first, I was concerned about the woofers' relatively low x-max, even though the sims were predicting that they'd hit 100dB at 20Hz with 1mm of excursion, but the short time I've spent testing put most of my fears to rest. I still have a lot of tuning to do, but I haven't reached their limit yet. Hmmm....maybe I need a bigger amplifier...
     
  19. ThomasW

    ThomasW Cinematographer

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    Hi,

    Sorry I don't recall any discussions about these drivers. Fundamentally they are high efficiency woofers designed for PA use in horn loaded boxes.

    Regardless of how they 'sim' know that new, the suspension is fairly stiff as they break-in, the suspension will soften and it will be much easier to bottom the VC's with excessive power.

    So whatever you do don't plug the ports. Doing that will double the excursion of the driver drastically increasing the chances of bottoming of the VC's.

    Also I'd wait on a bigger amp until the suspension has softened. I think you're going to find that once broken-in the drivers maybe quite easy to bottom, especially if you add any EQ
     

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