Subwoofer internal bracing question

Discussion in 'Archived Threads 2001-2004' started by Jones_Rush, Feb 22, 2001.

  1. Jones_Rush

    Jones_Rush Stunt Coordinator

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    I'm building a 90 liter sealed subwoofer.
    I'm using 3/4" MDF.
    Is internal bracing imprtant ?
    If so, what is the easiest way of doing it?
    I've seen too many designs, what is the
    best bracing design ?
    Another question,
    My sub's height will be 22", where is the
    best place to mount the driver, closest
    to the floor ? or should it be at the top ?
    a friend of mine told me that the driver should
    be placed as far from the floor as possible since
    too close to the floor can create a more boomy bass
    which will rebound too much from the floor,
    on the other hand he told me that I will get less
    SPL's if choosing to mount at the top,
    is he right ? what should I do ?
     
  2. Greg Monfort

    Greg Monfort Supporting Actor

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    Yes.
    Build it out of 3/4" no-void plywood for starters. [​IMG]
    There is no 'best' per se. In your case, considering the modest size, I would just double up all the panels. IOW build the box, then use regular old wood glue to bond another layer of MDF to it.
    This will sufficiently stiffen it.
    GM
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    Loud is beautiful, if it's clean
     
  3. Greg Monfort

    Greg Monfort Supporting Actor

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    whoops! Hit send too quick. [​IMG]
    WL coming from a sub below the XO point are so long they don't 'bounce' off the floor, but spread out in a spherical wavefront that is contained by all the boundaries in all but the largest typical home rooms. For a floor 'bounce' at a particular frequency to occur, the distance from the driver to the floor must be 1/2WL, so for a driver sitting up 4ft would have a deep null at (1130/4)/2 = ~141Hz, or in the attenuated portion of the sub's BW. IOW, more of an issue for the mains/CC/surrounds than the sub.
    It is true that the closer the driver is, and perpendicular to, a boundary, the greater its reinforcement. Up to +3dB, so it can cause a 'hump' in the FR depending on where the sub is positioned in-room, and if a high enough Q, then it will indeed sound 'boomy'. I normally just put it on one of the sides, wherever it's most convenient.
    GM
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    Loud is beautiful, if it's clean
     
  4. Jones_Rush

    Jones_Rush Stunt Coordinator

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    "Build it out of 3/4" no-void plywood for starters."
    Why?
    I thought that MDF is the best material for a sub's
    enclosure...
     
  5. Greg_R

    Greg_R Screenwriter

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    Greg
    'No-void' plywood flexes significantly less than MDF (a good thing). However, you want to get good plywood (which is expensive). Plywood comes in many flavors and is made by layering various veneers. The more layers and the quality of layers is what differentiates their respective prices and qualities. Marine grade is usually made up of N, A, or B quality layers of hardwood (and is pricey). Baltic, Finnish, or 'cabinet grade' ply typically uses alternating layers of softwood and hardwood or lesser quality interior hardwood layers. Cheap (Home Depot) ply is usually made from B, C, & D layers. Below is the government's explanation of the letter ratings:
    Veneers for plywood are visually graded according to the size, number, and location of natural and processing defects that affect their strength and appearance. Knots, decay, splits, insect holes, surface roughness, number of surface repairs, and other defects are considered. More surface repairs, such as elliptical (boat-shaped) wood patches and bigger knots are allowed in the lower veneer grades. Veneers are graded as N, A, B, C, C-Plugged, and D. N-grade or natural finish veneers are virtually blemish-free, and they contain only a few minor surface repairs. A and B veneers have solid surfaces with neatly made repairs and small, tight knots. Knotholes up to 25 mm in diameter are allowed in C veneers, whereas D veneers may have knotholes as large as 51 mm (2 in.) across. Because their appearance is usually of secondary importance, panels meant for sheathing and other structural uses are made mostly from C and D veneers. The N, A, and B veneers are reserved for panels where appearance is the primary consideration in such uses as exterior trim and soffits, interior paneling, doors, and cabinets.
    Hope this helped...
    Greg
     
  6. Aaron Plaza

    Aaron Plaza Auditioning

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    Any speaker that i have built has been built with two laminated panels of 3/4" mdf. I cut 2 of each panel and then glue them together with wood glue. Using a drywall knife aids in spreading the glue.
    When building and bracing subs, I build the shell and then add bracing roughly in the center of each panel. Then using the knock test, listen for areas that need extra bracing. You can never over brace. Take a look at the inside of a pair of B&W speakers, they are braced almost every 2 inches.
     
  7. Greg Monfort

    Greg Monfort Supporting Actor

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    Good post GR! [​IMG] One clarification though, Baltic isn't made from any soft woods based on my info. Baltic Birch, Appleply, and marine are considered the best structural grade plys, and the only ones suitable for speaker building IMO.
    AP: I agree about bracing, but there's a point of diminishing returns WRT the intended application. Two layers of 3/4" MDF is quite a bit stiffer than the above plys, which pushes the Fs of a modest sized box like JR's well beyond a sub's BW, so bracing is wasted effort IMO.
    For wide BW use such as the B&W though, you're right, brace city! [​IMG]
    GM
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    Loud is beautiful, if it's clean
     
  8. Pete Mazz

    Pete Mazz Supporting Actor

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    Typical 3/4" ply has 7 laminations (plys). Baltic birch, apple ply have 13. Baltic birch usually comes in 60" x 60" panels while apple ply is typical 4' x 8'. Current pricing wholesale as of yesterday was $1.27/sq.ft. for BB and $1.67/sq.ft. for apple ply. Both are usually very high quality panels (void free).
    Pete
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