Shape of subwoofer - important?

Discussion in 'Archived Threads 2001-2004' started by Kimmo Jaskari, Jan 28, 2001.

  1. Kimmo Jaskari

    Kimmo Jaskari Screenwriter

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    Provided you get the proper internal volume to a DIY sub, how important is the shape of it? I'm talking about a traditional square sub from MDF.
    Can you make it long, wide and flat or should it be more square in design to get the best results out of it?
    Presumably a sub with larger/longer walls will need to be well braced internally to keep the walls from flexing, but are there other considerations as well?
    ------------------
    /Kimmo [​IMG]

    "Don't tell me; we're about to go over a huge waterfall?"
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  2. Greg Monfort

    Greg Monfort Supporting Actor

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    Yes there are, see http://www.teleport.com/~lynno/ME2txt.html#sw but since you're still talking about using MDF after I made a point about using no-void plywood for subs, which will have far more effect on it than shape, why bother?
    GM
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  3. Jack Gilvey

    Jack Gilvey Producer

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  4. Greg Monfort

    Greg Monfort Supporting Actor

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    >Keep pluggin' away, you are helping people. I listen to you. There, that should make your head stop hurting. Plywood's easier to work,too, and easier on the tools.
    Dr. Hsu and TV say cylinders are the best,btw. I paraphrase,of course.
    ====
    Thanks for the ATTABOY. [​IMG] Mostly I'm just frustrated because I can't build/repair anything right now, and I've got all sorts of projects on indefinite hold. [​IMG]
    I think plywood's easier too, but many have expressed MDF is. I hate working with it, and only use it for small Sats where I want the cab resonance < Fb. Of course when I get to the checkout, I don't like plywood's price. [​IMG]
    Yes, tubes are good, though you have organ pipe resonances to deal with. I've been using them since the late '50s with various materials, but it severely limits designs when used for subs. You want heavy, build a speaker out of 24" diameter concrete drain pipe.
    GM
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  5. Kimmo Jaskari

    Kimmo Jaskari Screenwriter

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    Ok, I just used "MDF" since that is the shortest way to describe a board of some type and I haven't ruled out using it yet... nor have I necessarily decided to use it.
    Why do you recommend plywood? What I have read elsewhere so far has all been pro-MDF for a speaker material.
    I'm neither a woodworker (my technique there would probably be classified as "crude but mostly effective") nor do I know much about acoustic theory, so that's why all these questions.
    ------------------
    /Kimmo [​IMG]

    "Don't tell me; we're about to go over a huge waterfall?"
    "Yep."
    "Sharp rocks at the bottom?"
    "Most likely."
    "Bring it on."

     
  6. Greg Monfort

    Greg Monfort Supporting Actor

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    For good mechanical efficiency, and therefore good electrical to acoustic conversion, a rigid cab is required. For a rigid cab to be acoustically inert WRT the driver's BW requires that its Fs must be either above/below it.
    MDF has a low Fs Vs no-void ply's much higher Fs. The MDF also has a low MOE (modulus of elasticity, a measure of 'stiffness') Vs an equal thickness no-void ply, so is much less rigid. Since MDF requires mucho bracing just to match an unbraced ply cab of equal thickness, it can never be as efficient as a braced ply cab without additional mass to 'help' stiffen it.
    Since stiffening increases the cab's Fs, it takes very little to raise a ply cab well beyond a sub's BW, ergo there's nothing to excite it. Acoustically, it's 'dead', even though it may ring like a bell if given the 'knuckle' test. You want an excellent sub cab, make it out of metal. So all you metal fabricators out there...... [​IMG]
    The MDF cab OTOH, winds up in a loop. Increase the stiffness to raise its Fs, and the added mass lowers it, so you add more stiffness, mass, etc...... Ultimately, you wind up with the equivalent of a concrete cab weight wise that's somewhat larger than the no-void ply to get the Fs below the sub's BW, if you want it acoustically 'dead'.
    Between being lazy and having a bad back, 3/4" no-void ply braced with a few 2x2s is a no-brainer for me. [​IMG]
    GM
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  7. Jack Gilvey

    Jack Gilvey Producer

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  8. Greg Monfort

    Greg Monfort Supporting Actor

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    >I'm assuming you're referring to constructing a standard box enclosure here.
    ====
    Correctomundo. [​IMG]
    ====
    > Would it work as well when used as round endcaps for a "sonosub"? Any bracing called for, or is doubling the thickness sufficient?
    ====
    ???? Tubes don't need to be braced unless there's the possibility/probability of denting them along their perimeter. Endcaps should be braced plywood for best performance, due to the strong standing waves of a long dimension. Rap a double thickness MDF endcap and you get a dull 'thunk' which most folks think means its 'dead', but in reality it's probably resonating right smack dab in the middle of the sub's BW. [​IMG]
    When I was building them, I made two piece caps, except the inner one was cut obliquely so that it presented a non-parallel surface, quelling the possibility of supporting pipe resonances. The gap between them was filled with whatever was laying around that would keep this small cavity from resonating.
    ====
    > Any particular type of "no-void" ply you prefer, or will that that descripton get me what I need?
    Thanks,
    Jack
    ====
    If you ask for no-void, you'll probably get shown the A-B veneered grade they stock since the voids are filled with putty. Real no-void ply only uses knot/split free lams and exterior grade glues (or supposed to anyway), and normally have to be special ordered, though their local supplier probably stocks it.
    AFAIK, there's only three no-void plys, Baltic Birch, Appleply, and marine grade. All relatively expensive, though marine is the bargain of the bunch, at least in my area, and historically the preferred material for PA/theater cabs. Baltic Birch has the highest MOE since it has more lams, and why it's used in ServoDrive subs/basshorns, and I assume other brand's ultra high output designs.
    I built my Contrabass kits from BB, but all my other ply construction has been with marine, once I 'heard' my first ply cab made from 'premium no-void' $$ Oak veneered ply.
    Oh to have had the Internet 40yrs ago! I think I've learned everything I know the hard way. [​IMG]
    GM
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  9. Greg Monfort

    Greg Monfort Supporting Actor

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    You're a Sick Puppy, Jack. [​IMG]
    I don't recall ever seeing any 1" marine grade. I always used 3/4". A 4ft x 8ft x 3/4" was $63 the last time I ordered it, about a year ago. If you don't do the oblique thing, then I recommend lamming two together, same as the baffle.
    I've never done this since I quit using tubes before the proliferation of high excursion drivers became available, but for max rigidity, I suggest running threaded rod from the endcap and extending through the baffle, to mount the driver, and extending down as required for legs. Now the driver/baffle is coupled to the entire cab assembly/floor.
    Before lamming the top finished cap on, install nuts on the rod in a countersunk hole, then do the same at the baffle, and use Liquid Nails or epoxy to keep the nuts from moving once they've been tensioned. Grind the leg ends to dull points. Of course if this presents a stability problem, then additional short legs will be required. If it sits on a nice finished floor, then rest the legs on pennies.
    GM
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    Loud is beautiful, if it's clean
     
  10. ThomasW

    ThomasW Cinematographer

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    Greg thanks for the link to Lynn's site.
    I'm sooo glad to see someone else had the courage to print info about polypropylene drivers. I'm astounded at the popularity of poly woofers/midwoofers for high-end systems.
    They're cheap and that may explain it [​IMG]
     
  11. Greg Monfort

    Greg Monfort Supporting Actor

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    Hey Tom,
    I thought everyone that had a clue dissed PP drivers. Then again, I don't waste my time with what's being used in consumer products.
    GM
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  12. ThomasW

    ThomasW Cinematographer

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    Greg
    I don't think it's common knowledge, or that it's believed.
    If it was/is then what explains the popularity of the multitude of systems that continue to sell PP woofer/midwoofers?
    I'm always amazed at the relative popularity of Dynaudio/Morel drivers. My impression is that these drivers in particular are quite bad sounding. But Seas and Vifa aren't far behind
    Probably shouldn't post this info on the Lambda PP vs Shiva sub thread, might get people upset [​IMG]
     
  13. Greg Monfort

    Greg Monfort Supporting Actor

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    >If it was/is then what explains the popularity of the multitude of systems that continue to sell PP woofer/midwoofers?
    >I'm always amazed at the relative popularity of Dynaudio/Morel drivers. My impression is that these drivers in particular are quite bad sounding. But Seas and Vifa aren't far behind
    ====
    Hi End audio preaches flat FR +/- 0dB is the be-all, end-all Holy Grail.
    Lynn remarks: ".......polypropylene flexes even at moderate levels of acceleration. This smooths the frequency response at the expense of increased distortion."
    I think he sums it up nicely. [​IMG]
    ====
    >Probably shouldn't post this info on the Lambda PP vs Shiva sub thread, might get people upset
    ====
    Well, those are some good composites for their intended BW, but I admit that I prefer doped paper for wide BW LF/mid drivers. This may be just because it's what I grew up with.
    GM
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  14. Dustin Haug

    Dustin Haug Agent

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    Greg, or whoever may be able to answer this, instead of using expensive Marine Grade or Baltic Birch plywood couldn't you use standard plywood (even if it is porous) and then buy a $4 can of rubber undercoating and completely coat the inside of your box with that. Granted if you have a small sub this may be a problem since manuvering through the small hole could be tough. But with a 12" or 15" sub it wouldn't be bad at all. You could still silicone the seams and such but to "seal" the sides the undercoating would be about perfect I'd imagine. And saving ~$40 a sheet on wood would be nice. What do you think? It could be a very simple, cheap way for us poor DIYers to get a good solid, airtight sub.
     
  15. Greg Monfort

    Greg Monfort Supporting Actor

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    Sadly, it doesn't work this way unless the undercoating has extraordinary damping properties, which it doesn't unless it's many inches thick since it's formulated to damp the HF in metal. By the time you 'mass it up' enough, I imagine that price wise you might as well have bought the better ply, and there's nothing quite so distracting as a sub that occasionally 'creaks' when Ray Brown is playing a solo. [​IMG]
    For HT, your solution may be an acceptable trade-off for you. You would have to build and audition it to find out though.
    In this case, IMO it's best to build it out of double wall MDF, or a tube, like most folks. If you still want to try it, applying cheap self stick floor tiles will work as well, or better, than spray undercoating, and is cheaper n' easier as it will take a lot more than one spray can to do a typical size sub to any functional thickness.
    GM
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    Loud is beautiful, if it's clean
     
  16. Kevin Kloet

    Kevin Kloet Agent

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    Hey Greg,
    All this talk about plywood stiffness and such is nice and all, but there's a acoustical reason why MDF is the better material to use.
    Resonance and reflection. MDF is one of the best materials out there (never mind the cost) in terms of absorbing reflection. This makes it THE ideal material for constructing a sub, as internal reflections and cancellations are deadly. A properly braced (does not require huge, heavy, volume depleteing braces) MDF box will outperform a plywood box.
    A good idea would be to get a bunch of 1/4 inch sheets of mdf and glue them together. This would give you stiffness rivaling plywood and keep the acoustical benefits of MDF.
    In terms of box shape, a cube is the WORST way to go (cancellation again). The "golden ratio" is 3x4x5. Anything close to that will work very nicely.
     
  17. Greg Monfort

    Greg Monfort Supporting Actor

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    >Hey Greg,
    All this talk about plywood stiffness and such is nice and all, but there's a acoustical reason why MDF is the better material to use.
    >Resonance and reflection. MDF is one of the best materials out there (never mind the cost) in terms of absorbing reflection.
    ====
    Actually, the cheapest, flakiest particleboard is far better if that's what the goal is, as it has a much higher % of flexible glue.
    ====
    >This makes it THE ideal material for constructing a sub, as internal reflections and cancellations are deadly.
    ====
    Please read all the info I've presented, which deals with them by raising them to a higher frequency through attenuation, so they may be quelled with less mass/effort, or if raised enough, pushed beyond the sub's BW, ergo there's nothing to excite them.
    I have sub cabs that ring ~like a bell when struck with a hammer, yet there's no audible sound being generated by it, or being reflected back through the cone in the form of distortion, when in use. It's so lacking in these that many folks consider them as sounding 'dry', and want to listen to them far louder than a typical MDF consumer sub.
    ====
    >A properly braced (does not require huge, heavy, volume depleteing braces) MDF box will outperform a plywood box.
    ====
    In what parameter(s)?
    Using your theory, any bracing will reduce the MDF's damping ability, so why brace it? A MDF sub cab can be made to equal a good plywood sub cab, but it will be larger/much heavier, as numerous folks have learned the hard way. As for the rest of your concerns, please (re)read Lynn's info at the link I previously provided, and here's a bit more:
    http://www.silcom.com/~aludwig/Loudspeaker_construction.html#Panel_vibration_damping_experiments
    ====
    >In terms of box shape, a cube is the WORST way to go (cancellation again). The "golden ratio" is 3x4x5. Anything close to that will work very nicely.
    ====
    I don't recall mentioning a cube in any of my posts, but if the WLs are long WRT the dimensions, there's nothing wrong with a cube, as the air in it has a uniform density.
    The golden mean 'g' is equal to (5^0.5 - 1)/2, so the golden ratio is ~0.618:1.0:1.618 (g:1:1/g).
    Yes, there are numerous acoustic ratios that work just as well, which 1:1.33:1.66 (3:4:5) is one of.
    GM
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