Does enclosure shape affect sound?

Discussion in 'Archived Threads 2001-2004' started by Chris Tsutsui, Apr 19, 2002.

  1. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    I just finished a custom subwoofer box and installed a 12" JBL driver powered by a 180 watt JBL amp.

    The manual said to make the subwoofer box 0.75 cubic feet so I designed the box flat in the shape of a wedge to conserve space. The volume of the box is exact, but the shape is that of a wedge.

    I lined the walls inside of the box with a thin layer of polyfill and used bondo all over the inside of the speaker terminal to make it air tight.

    When I fired up the sub for the first time today the driver seems to work very hard, and not much SPL cames out of it. The bass was clean and in phase, but when I turned up the volume the bass sounded raspy like it was bottoming out.

    I tried a different 200 watt amp and it sounded the same, even after adjusting gain and crossover settings.

    I am thinking of remaking the box in the shape of a square how most boxes are shaped and that my wedge type enclosure was flawed. I used high quality 3/4" MDF glued, nailed, and air tight caulked. This is my 3rd box and the bass is much weaker than my dual 10" and single 10" (boxy shaped).

    The JBL driver is a 10" 8ohm GTO rated up to 275 RMS.

    So My assumptions are telling me the box is crucial and if the box is shaped irregularly then it may not produce the right bass. BTW, from a side view of the enclosure, there are no 90 degree angles... (were talking a slanted trapezoid shaped box).
     
  2. Dan Wesnor

    Dan Wesnor Second Unit

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    What's the x-max of the driver?
     
  3. Dave Poehlman

    Dave Poehlman Producer

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    .75 cuft sounds a little small to me for a 12" driver. I assume it's a sealed enclosure? Do you have the Theille/Small parameters for that driver?

    However, to answer your original question: When dealing with a subwoofer, the shape of the enclosure generally isn't important... since the wavelengths you're dealing with are larger than the enclosure itself, there is little chance for box resonance or edge diffractions. (and actually, if these things were and issue, a trapezoid enclosure is a good shape to avoid these problems).

    I think your problem lies in the size of the enclosure. If an enclosure is tuned below a driver's Fs, it requires a lot of power to get any reasonable SPL out of it... and the driver has to also be able to handle the large amount of power (this is the theory behind the ELF subwoofers patented by BAG END).
     
  4. Dan Wesnor

    Dan Wesnor Second Unit

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    That's about what I'm thinking. The only way to get any sort of bass response (
     
  5. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    Sorry, I meant the driver is 10" as stated below in the post.
    Thanks for the replys.. I probably won't rebuild the box then.
    These are the technical stats on the driver:
    http://manuals.harman.com/JBL/CAR/Bo...ers/GT1081.pdf
    I know it's a car audio driver, I'm just testing it before I give it to a friend. It shouldn't matter if it's for a car or home anyways right?
    Argh, I just read that the internals of the box should be 1 inch thick lined with fiberglass or fiberfill. I only put in some polyfill. That shouldn't effect anything though right?
    At least this is all good practice before I start work on a real HT sub with a tempest. Thanks
     
  6. Seth_L

    Seth_L Screenwriter

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    Car enclosures are very different from home enclosures. Cars have a very favorable cabin gain that allows small sealed boxes to work great. The same box in a house will sound very poor.

    Stereodude
     
  7. Shawn Solar

    Shawn Solar Supporting Actor

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    The shape of the enclosure shouldn't effect sound. But there are different shapes that have qualities or benefits from a building aspect.

    Like a sonotube based design is stucturally sound in that there are are only 2 parallel sufaces to worry about vibrations. This is because it is a cylinder and the round shape enables it absorb pressure equally across the the hole tube so internal bracing is not needed to keep the walls from vibrating. This makes the design much easier to build and has less chance of screwing up(well sort of).

    Now a cube or a rectangle design has more that can go wrong if it is not properly built and braced. It has more edges to seal and more parallel surfaces to deal with. adaquete bracing is needed to prevent the walls from vibrating.

    Sonically though, both types of enclosures, if properly implemented should sound identical with the same parameters
     
  8. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    Thanks, just what I wanted to hear
     
  9. Dan Wesnor

    Dan Wesnor Second Unit

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    BTW, if there's not enough room for the driver to "breath", it can seriously affect the Qts.
     
  10. Seth_L

    Seth_L Screenwriter

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    [​IMG]
    Here's my theory as to why it doesn't sound very good. Look at the anechoic chamber response.
    Stereodude
     
  11. JonWB

    JonWB Agent

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    Have you tried playing with various amounts of stuffing in the box. In a previous life I had an installer make me a box (sealed) for the back of my car to hold 2-12" Rockford Fosgate's. When I first got it, it sounded pretty good, but didn't seem to go very deep. Then I stuffed it (basically) full of polyfill, and it sounded AWESOME! It can't hurt to try. The stuff (pardon the pun) is cheap and its easy to try. You may also want to check out this link regarding the effect filling a box with fiberfill has on its net apparent volume.
    Good Luck!
     
  12. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    I stuffed it with fiberglass insulation. It was $11 at home depot for a whole roll. It's RM-12 wood wall sound and heat insulation material.



    Will it suffice as a substitute for polyfill?



    I plan on making a tempest box enclosure with the vented EBS or adire alignment type wood enclosure. I've decided on special ordering maple laminate and having my best friend (professional contractor) build it at his shop. [I know i'm lucky] [​IMG]



    I was going to use the RM-12 fiberglass insulation in the tempest enclosure as well.. Should I order or find polyfill? Or does fiberfill do the same thing?



    btw, I gave the box to my friend and he loves it. However, I feel that the bass has little/no response in the upper HZ range. The amp only has a high or low pass filter so I think he'll just have to deal with no mid to high range bass. I ran some crossover tests and the car's entire system definitly lacks the upper part of bass but has plenty of 40-60hz range.



    The driver was $60 and the amp was $80 both on sale. His monster cables costed him up to $100 though. The bass he gets in his firebird is pretty good for cost I must say. This 8 ohm single ten really works hard with 0.5" xmax.
     

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