Sub materials and such

Discussion in 'Speakers & Subwoofers' started by jamesIS, May 7, 2006.

  1. jamesIS

    jamesIS Auditioning

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    I am building a set of subs for my theater. I've done a couple of loudspeakers in the past, but this will be my first attempt at subs. I picked up some (probably not the best) 12 inch infinity drivers which I will use for the project. I figure I can always use better drivers later with minimal cabinet changes (added baffling, for instance) if they deserve it. I have four of these twelves but will probably just build two boxes for now.

    I plan to use sealed enclosures because I want the highest clarity and detail from the final product.

    I have a set of five Martin Logan speakers with which these subs will be paired.

    I have been reading a lot. Probably too much. My plan has been to build an internal cabinet using three-quarter inch MDF and clad the thing with an outer shell of half an inch of some hardwood (maple, zebra, whatever, something handsome) so that I can work the outside and corners, make it look nice. Then some heavy brass feet with spikes and all should be good.

    It will be a passive box. I will be using some very powerful (and clean) external amplification to drive them. (With all the do-dads available to isolate gear from vibrations and to remove the resonate vibrations created by electricity, why would anyone want to put an amp in the most vibratory locale in the home?)

    According to these folks:

    http://mapleshaderecords.com/index.php

    air dried maple has magical properties that give more musicallity when used in certain ways. For instance they make plinths to place your speakers or your amps upon. They go into detail on the site and it's quite interesting reading. (I'm thinking I'll use their carpet spikes for the boxes.) Why not maple for a cabinet?

    Anyway, what kind of materials should be considered for building the box?

    I know that high-end loudspeaker cabinets tend to be made from MDF (with a veneer). I figured that dense was better, but having done some research at the lumber yard and having done all this reading I'm less sure.

    Let's have some lively debate. I expect that their must be a dozen different oppinions on this one, so let's have some of that old life-experience come shining through. Surely plenty of folks out there have built from different materials and have 'performed the experiment'.

    Besides, my brain is tired.
     
  2. Jack Gilvey

    Jack Gilvey Producer

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    If you want your cabinet to resonate magically, use maple. Most prefer as little resonance as possible.
     
  3. John Garcia

    John Garcia Executive Producer

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    Use the MDF with a veneer, but make sure you include internal bracing as part of the design. You don't need extremely thick, just a few well placed braces.

    Do you have the specs for the drivers? How do you know these drivers will work well in a sealed enclosure? Car drivers usually are designed with cabin gain in mind, are frequently intended for vented designs, and don't always work well in a HT setup; though that will vary depening on the sub's design. How are you going to figure out cabinet size? Sealed is actually harder to design and requires MUCH more power to implement.
     
  4. Anthony_C

    Anthony_C Agent

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    I agree, MDF with a veneer. I do a lot of car installs and box building and I would think they would be similar. The one main thing to remember is that when you calculate the box demensions don't forget to allow for the internal volume that the speaker will take up. Like John said, sealed enclosures do require more power. For a HT though I would go with a ported enclosure. Ported enclosures "IMO" will hit a lot lower than a sealed enclosure.
     
  5. jamesIS

    jamesIS Auditioning

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    Thanks, everybody.

    Sounds like everyone here agrees that MDF is the best material for the structural aspects of the box. From a wood working standpoint, the denser hardwoods can be a real pain to glue (due to all the resins and waxes) so I'll have to be careful to balance density against gluibility. I don't recall the name of the wood that I thought was the best balance, but I'll kick that in here later.

    I won't be using a veneer. I will be using about a half inch thick piece of material over the 3/4 MDF (assuming I use the MDF). Why? Because I want to be able to work the finished product and veneers are too thin (maybe 1/8 inch) to work, especially at the corners and joints. (I'm not the best woodworker, but I know my skill set.)

    Everything I have read about non-sealed boxes suggests that the reasons for moving in that direction are for increased output and range at the sacrifice of clarity. My ML's already hit pretty low. There is no gap, per se, in the theater sound. I am building the subs as reinforcements. I don't need to worry about power as I have several Carver m1.5t's that I can use. These drivers are dual voice coil. If I hook up two of these subs (two matching boxes), one to each channel of that amp, I'll have peak power of 1200 watts (at 8 ohms) for them to share (continuous rates 350/channel). I could always give each sub one amp (350 continuous per voice coil and 2400 peak at 1200 each sub). Power is not an issue.

    As concerns resonance... I don't know. What I want is a sub that will sound like it belongs and which does not call attention to itself (unless that is visually).

    I plan to use, maybe maple, maybe inch square braces at the corners to make gluing easier and to add additional lateral strength to the finished box.

    I have used the specs from Infinity for the box dimensions. They use a basic formula which is included in the documentations (they include info on three kinds of enclosures). If y'all want to see the specs from Infinity, I can probably find a .pdf (or make one). Basically, this driver wants 1.25 cubic feet (if memory servers) for a sealed enclosure.
     
  6. John Garcia

    John Garcia Executive Producer

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    Sealed boxes are less efficient, not more efficient, so you will not be getting MORE output from one. With a sealed box, you're going to need every bit of that wattage if you're looking for it to go really low.
     
  7. Brent_S

    Brent_S Second Unit

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    I would suggest downloading WinISD and modelling your drivers. Infinity's recommendations are undoubtably for a car environment. Sub drivers for the car are usually dependent on large amounts of cabin gain...unlikely to happen in a room housing 5 M-Ls.

    I think everyone here's trying to politely say that most likely, you're probably going to be severely disappointed with your present design plan.

    A driver's power needs are still governed by the thermal limits of the voice coil and the excursion limits of the driver/cabinet combination. My NHT1259 drivers are rated 300 watts thermally, but in a sealed cabinet, they're excursion limited to about 140 watts below 30hz. You can't just arbitrarily get more output from a sealed or vented design by adding more power.

    wbs
     
  8. Ryan Schnacke

    Ryan Schnacke Supporting Actor

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    Well, I'm sure it will look good.

    This would seem to be a reasonable order of importance in designing a subwoofer:
    1) Woofer selection
    2) Enclosure type/size/tuning (alignment)
    3) Amplification
    4) Enclosure physical design/bracing
    5) Enclosure material selection
    6) Finishing (veneering/painting/etc.)

    You'll notice that this puts the critical acoustic elements first. Of course this assumes a certain amount of common sense (a cardboard box won't work no matter how well its braced), but you get the idea. In your project the order seems to be reversed - most of the effort going into the selection of woods and finishes. So we're concerned that the acoustic results will be dissapointing.

    Did you research the woofers at all? They are the devices that actually move the air and create the sound. There's probably a good reason that 4 out of the 6 responses (including me) are questioning the choice of Infinity car audio woofers in a home theater.
     
  9. ChrisBee

    ChrisBee Stunt Coordinator

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    You certainly don't need thick wood veneer to make nice rounded edges.

    You inset a ready-made hardwood quadrant. (quarter round) Or round-off a strip of wood with a square cross section after it is fixed in place.

    Trying to shape what is really a very thick veneer leaves you wide open to disaster if you make a single mistake or a slip.

    An edge strip can even be made of contrasting wood. Perhaps a wood with better resistance to accidental damage? A thick rounded-off veneer would expose the edges to potentially easy damage. You'd also have exposed end grain unless you finish the ends with wood strips.

    Remember that a sub is a big heavy box and not easily moved. Just getting it from the workshop to the HT offers a number of danger points for delicate corners and edges.

    I'm just trying to save you some potential grief. [​IMG]
     
  10. jamesIS

    jamesIS Auditioning

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    Thanks again.

    So, I don't like thin verneers, as a general rule. And because I haven't worked with them I'd rather not start now. I'd do the whole thing in wood if not for the density concern. Verneers will be more susceptible to damage at the corners than would real wood, and they will always look worse once damaged. None of the wood workers I have talked with about this have offered the slightest resistance to my sandwiching idea (.5 inch wood and .75 inch MDF). End grain or not, I know I can make the boxes look good like this and make them strong.

    Luckily for me, Chris, my woodshop and my theater are located in the basement mere feet from one another.

    I appreciate all of your concerns. I find them well intentioned. I will not be disappointed with this and I will explain why. The drivers:

    http://www.audio-warehouse.com/web/mdl/1242w/detail.asp

    (similar to these: http://infinitysystems.com/caraudio/...er=REF&Cat=SUB )

    cost me $230 for four drivers. I figure I can build the subs at a cost of about $100 per sub (including what I've already spent). I already own the amplification. If they are sub-par (hahaha) compared to the other components in my theater, I think I can probably sell the boxes and drivers for something near to what I will have into them. Even if not, I'm only out maybe $400. The time spent would then be time spent learning how not to build a sub box and will have values all its own. All this considered against the fact that the subs I liked when out shopping were in the $5k-8k range, if these turn out just ok I'm in the black like nobody's business.

    Of course, if they turn out well and I learn how to make a good box from this experience, I will then be in a good position to seek out some fine drivers and either replace these drivers within my boxes or build new (probably improved) boxes specifically for them.

    So, please, think of this as an experiment, and remember that experiments don't fail--they just give results.

    John, this is how I understand matters concerning box types. Sealed boxes provide greater clarity at the cost of power. The other box designs will offer increases in SPL and/or frequency response at the sacrifice of clarity. I have power (I could easily melt the voice coils in these drivers with one of my amps) and so sacrificing power in the name of clarity, especially since I am aiming toward clarity, is no sacrifice at all. Sorry for not being clear on that point.

    Brent, can you offer a link for downloading this WinISD?

    Thanks.
     
  11. ChrisBee

    ChrisBee Stunt Coordinator

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    If you are looking for clarity then you should be building a true infinite baffle subwoofer. [​IMG]

    http://ibsubwoofers.proboards51.com/index.cgi?

    You still get to play with your tools but your present drivers are unlikely to be suitable.
     
  12. Brent_S

    Brent_S Second Unit

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    Search engines are your friend. Get WinISD Pro from www.linearteam.org

    Since you posted a link to the driver's T/S parameters, I was curious and went ahead and ran some sims. Hope you like boomy subs...that 1.25 ft^3 sealed box shows a Q of .942. Your f3 is 40hz, f10 is 27hz. Wired as a 2ohm load (is your amp stable), you can get 103db from 300 watts @ 20hz, 97db @ 20hz.

    For the commonly recommended Q of .701, you actually need a 3.65 ft^3 box. Drops your f10 to 20hz, actually pretty respectable. Power handling drops quickly below 40hz in that size box. To achieve a sound that meshes well with your M-Ls, I'd imagine you'd want an even lower Q (better damped) response, so you need to go even bigger.

    Overall, they actually do model better than I expected. However, a Dayton DVC12 will yield a .701 Q in only 1.75 ft^3 with essentially equivalent SPL output from only 240 watts.

    A well designed ported sub can sound every bit as good a sealed...it's an urban legend that needs to die. Ask any SVS owner. Wilson Audio's $10K Watchdog is also ported, as are B&W's 800 series speakers. Obviously, there are plenty of well regarded sealed subs as well. The point is, it's the overall execution of the design, not whether it's ported or sealed that determines the sound quality. What commerical offerings did you like? Maybe we could suggest a DIY solution that resembles them.

    Think about something like the Datyon RS12-HiFi in 4ft^3 tuned to 20hz. F3 is 19hz. MaxSPL is 109db @ 30hz, 108.6db @ 20hz...or the equivalent of 3 Infinity drivers @ 20hz, and 2 @ 20hz. Further, this driver has been measured as possibly the lowest distortion driver currently available to the DIY market.

    Another option is the SoundSplinter Premium 15 in a sealed enclosure with a little EQ boost for the bottom end and gobs of power. You can read all about lots of DIY versions of this over at htguide's DIY area.

    -wbs
     
  13. Ryan Schnacke

    Ryan Schnacke Supporting Actor

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    When I saw the T/S params for this woofer I thought "hey, that's not half bad". But Brent is right ... the Qts is little high and a Q>0.9 enclosure is definitely intended for the car audio world.

    If you could live with 2 cu ft enclosures (per woofer) and stuff them real good with fill then you could get down to a Q=0.78 alignment. F3 is 38Hz so they won't naturally dig super deep and you wouldn't want to give them more than 200 watts per woofer. But with 4 of them you could use some EQ to boost the low end response a good bit.

    Vented enclosures for this woofer don't begin to look good until you go 4.5 cu ft or larger. With a box that big, they won't need much power to reach max excursion (150 watts).

    I think you can probably get decent results with these woofers, but its going to take larger boxes than you were planning. If you're absolutely set on going with small enclosures, you could try doing 2 isobaric clamshell subwoofers. This gets you half the enclosure size for a given alignment (Q) but it requires 2 woofers per box and only gives output equal to 1 woofer in a box thats twice the size (same alignment, but not isobaric).

    This is the standard compromise we run into - the goals are Loud, Small and Cheap but we can only choose 2 of those because the 3rd will be dictated to us. In this case, you can choose loud and cheap and live with the bigger boxes or can choose small and cheap and live with the lower output.
     

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