A friend of mine, Jorge Omar, pointed out this page some years back -- I think the author has passed away. The info is too good to loose: a huge problem with the internet -- digital info wanes away, especially if no one is around to pay the bills....
Wood Properties and Cabinet Construction
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Wood types and usage
The primary method of building loudspeaker enclosures is sheets of 3/4" wood or wood composites. This isn't the best method, but definitely the most common Other materials that are suitable for enclosure construction include "RIM" board (Reaction Injection Molded) that typically has a very high strength-to-weight ratio and good damping but is very difficult to cut and assemble (and very expensive).; HDF, which is usually an expensive custom order with incremental improvements over MDF and no advantages over a composite layer construction; concrete, which has very high strength but is difficult to cast and finish, but has good mechanical properties (the typically very high resonant frequencies you can attain far outweigh the low damping); various other composite materials are available, most of them require specialized cutting equipment (diamond-tipped blades) and are nv[ worth the trouble for 99% of the applications. So what follows is a basic description of the primary wood types for conventional construction, and then a primer on construction for ease, finishing ability, strength and overall sonic performance.
Wood types: The five basic types of woods used for loudspeakers are particle board, OSB (chipboard), MDF (medium density fiberboard), plywood and various hardwoods. Prices are listed for a 4' x 8' x 3/4" board in the southwestern Michigan area. Prices may vary across the US and other countries
Particle board: Very cheap ($7), readily available, good damping properties and weight. Takes adhesives very well, but can be pulled apart pretty easily as well. Does *not* react well to moisture or water. Fairly low strength, very easy to split and chip off pieces, difficult to get a smooth finish if you plan on painting directly. Overall a poor choice of material for loudspeaker enclosures, except for prototyping.
OSB: Also known as chipboard in some areas, universally used as roofing decking (there's some over my head right now). Cheap ($10), readily available in lots of sizes, reacts fairly well to moisture (waterproof glues in the exterior grade), easy to cut, sand, decent damping properties and moderate to heavy weight. Also difficult to sand smooth, so don't bother trying to paint it unless you have a thick lacquer. Usually difficult to split with nails, takes practically any adhesive very well, does not pull apart. Used in a majority of MI products from EV, JBL, EA W, Meyer, etc for the low-end models of small band sound setups. Overall an excellent prototyping material, and not bad for building final products so long as you either protect the corners (for touring) or don't drop the speakers too often. The only disadvantage is that the stuff stinks. The chemicals aren't particularly hazardous to humans or to driver glues, but they definitely stink. My suggestion (which has worked well for my prototypes) is that you paint the inside of the cabinet after construction with a thin layer of any type of convenient spray paint. Just coat it lightly and it will seal in the stink. ..of course then you need to air out the paint... ;)
MDF: Medium density fiberboard is a very good material for loudspeaker cabinets. It is also available as "fibercore" veneered with oak, maple and various other hardwoods. Medium price ($20) and sometimes difficult to find, but has very good machining and finishing properties. Easy to split with nails, but takes adhesives very well, and doesn't pull apart like particleboard Very good damping characteristics, good strength/stiffness. Also suffers from a tendency to split when dropped..
Plywood: Available in various grades, from CCX exterior junk ($15-$20) to 13-ply 3/4" birch-
faced material that runs $40-80 a sheet. C-2 birch (cabinet grade 2 faced) is typically 9-ply 3/4" and pretty void-free but inspect the pieces. Excellent strength, takes adhesives well, a bit tougher to cut but has a bad habit of splitting, finishes a bit harder than MDF but is very consistent. If you're good, you can finish the C-2 birch without veneering... The better choice is Russian or Baltic birch, which can be found (sometimes) for $40-60 a sheet for 13-ply 3/4". It requires a bit more surface finishing, but is an order of magnitude stronger and a lot more void-free than the 9ply C-2 birch. This is the material of choice for professional audio cabinets, and the pretty much the only material for EV's pro-grade cabinets. The Finland birch is a better material overall, but will cost you $80-100 a sheet or more. The main advantage is a better surface veneer, as the Baltic birch will occasionally have "visual problems" with surface appearance defects (not structural, but visual). Overall a good but expensive choice, but it has one severe drawback: crummy internal damping. In many designs the strength means that the fundamental resonances are well above the areas that you would care about, so it might not be such a problem. The best material for subwoofers, period.
Hardwoods and softwoods: Quite frankly a poor choice for a single-layer construction, but usually very effective for a dual-layer. You can build the inner layer from MDF or OSB, and then laminate a nice piece of 0.5-1" oak around the outside. I haven't tested the glues long-term on oak and other natural woods, but they should work just fine, considering that the glues were all developed with exterior uses in mind. Advantages if you can cut well, a very nice exterior finish is possible, typically high strength. Disadvantages: very poor internal damping, consistency (flatness and grain) is poor to good depending on the piece you choose, cost is very high for a good grade of oak.
Basics of cabinetry: Okay...now that the materials are pretty well described, here are a few basics of cabinet-building This is just a list of a few tips, not a "how-to." If you don't know how to build a rectangular box, then ya might wanta learn that first' ;)
1) Use *lots* of glue. This is NOT a fine cabinetry and furniture shop, where the smallest amount of glue is the best choice. Here with speaker cabinets, the glue maintains a high-strength, airtight seal that is required for a tough, high-fundamental-resonance cabinet. On every joint you should see a bead on both the inside and outside when the joint is clamped/nailed/screwed/stapled together. Wipe off the outside bead, leave the inside alone.
2) Make very sure that every board is cut perfectly straight and square. No gaps greater than 1/32" should be accepted in a good loudspeaker, and those only for less than 10% of the length of the joint. Irregular cuts or chamfered edges will be low-strength joints that are tough to fill.
3) For prototypes, use OSB. For final systems, use MDF for single-layer construction, and C-2 or better yet Russian or Finland birch plywood laminated to MDF for double-layer construction. A primer on composite construction follows below. Don't bother with particle board. For the cost difference between it and OSB, it simply is not worth the hassle. I wouldn't recommend either if you might finish the speaker using the proto enclosures, since they are both pretty difficult to paint, and not astoundingly easy to veneer.
4) Brace the enclosures with "window pane" braces, on asymmetric locations. I.e. don't put braces in the exact center of any dimension, as the staggered location of braces will break up any fundamental resonances into two distinct, smaller and much less obvious ones. Symmetrically produced enclosures and systems (including horns to a *large* extent) are just plain bad news
acoustically. They make sense mathematically, but it causes a large number of standing-wave resonances that cannot be significantly reduced or eliminated, and are clearly obvious if you know what to listen for (a large pro sound company prides themselves on "mathematically correct horns", which is pure bunk in the real world).
5) For strength of joints, use dado construction if possible. It's tough with particleboard and MDF, but easy with OSB and plywood. Overall, the increase in strength is probably marginal over a well-built butt joint, but it is much easier to make tight joints with dado construction. Don't bother with miter joints unless you have access to a *really* good table saw and band clamps. Likewise with compound angles in enclosures (tapered sidewalls and a sloped fronL.), unless you have a large belt sander and a lot of patience.
Wood composite constructions:
The best overall choice for building cabinets is composite layer construction. The fundamental resonances are raised an average of 1-1.5 octaves and reduced by 10-15dB (based on my tests with an accelerometer), and the enclosures are *dead*, without any messy goop (that may be hazardous to driver adhesives) or roofing felt. The trick is the glue that constrains the two layers. I recommend you try this for yourself, but I spent a month and about $30 trying various types of adhesives, and found an excellent, cheap, high-damping, aggressive adhesive that is readily available...acrylic latex caulk plus silicone. Specifically, DAP ALEX Plus. The latex caulks don't have enough bond strength to the wood, and the pure silicones have no damping whatsoever. Rigid bonds, such as wood glue, are practically worthless in composite constructions, providing strength but no damping. Try it yourself.. .buy a tube of ALEX Plus and glue a I' square piece to another. Do the same with wood glue. Let them dry overnight, and then knock on them. The ALEX Plus is dead, but the wood glue still almost as resonant as the single thickness piece' (This is supported by accelerometer testing that I have done, and I may be able to have the data here for you to look at at some point.·..) I tried several other latex In' silicones from other manufacturers, but none of them had the right balance of adhesion, damping, curing time and viscosity (and a reasonable pH so that "acid evolution" won't destroy the bonds over a couple of years).
Methodology is simple.. .fasten the boards together with nails or staples only at the edges, and not in the middle. Trust me, it works better that way. I could start in on the shear stress and other related formulae for composites that were shoved at me in college, but the "why" is not so important for most people as the results. Spread the goop over one side to cover it with a fairly thin layer (1/32-1/16"), but thick enough that it will definitely make contact with both sides when you press it together. Fasten securely around the outside, and let it cure for an hour or so before cutting it or moving it around much. The curing time tends to vary a lot, so better safe than sorry. I suggest you experiment a bit with scrap until you are comfortable with how much to apply, and how long to let it sit. My preference is to just let the pieces cure overnight, but if you are building an inner-outer box, you can probably assemble the entire outer shell on the inner at one time, so long as you are extremely careful.
At any rate, the best construction that I have found is a high-grade plywood inside enclosure, with an MDF outside, ALEX Plus holding it together. This isn't the best way to make ultra-compact enclosures, but in my experience an ultra-compact enclosure usually gives ultra-poor response. Anyway, the plywood/MDF composite gives an extremely strong enclosure with good finishing ability, and a nice heft' The one thing to keep in mind is to stagger the seams, i.e. if the side overlaps the back on the inner layer, then the back should overlap the side on the outer layer. This gives maximum structural strength to the joints, which are typically the weakest part of the
cabinet. Other constructions that work well are OSB inside and MDF outside, and MDF over particle board. My personal preference is to build the proto enclosure from OSB, and then, if I'm happy with the results, laminate a layer ofMDF over the top. This hides all the blemishes of the previous layer, allows me to avoid building a whole new enclosure, and makes one *tough* box.
Assembly methodology: Cut the pieces, glue them together. Duh... ;)
All bad engineer humour aside, the best method for attaching panels together is an air-powered staplegun, with 3/8" or 1/4" crown staples, thermoplastic coated (very,very important), typically 0.5-0.75" penetration into the opposite layer (1.5" length for attaching 3/4" stock). It is the fastest, strongest method of attaching boards available. Outperforms most screws, nails, ringshank nails, screw nails, mending plates and pretty much everything else. Screws are still the best for holding power, but they are a serious pain to use. Staples work well if you can shoot them in straight and you can avoid splitting the wood (which is why I like OSB for protos). Otherwise, buy a bunch of ringshank drywall nails or smaller spiral deck nails. Straight nails are worthless, even thermoplastic coated "sinkers". Finishing ringshank nails aren't worth much either, as they don't hold well, and tend to split MDF and particleboard.
So there you have it, a partial description of my favorite materials and methods. Keep in mind, the best drivers in the world will sound like crap in a poorly-made, poorly-braced enclosure. You can't mess up a project faster than to make an enclosure that buzzes, rings or has undamped standing waves. Of course, it's tough to make a silk purse out of sow's ears... but a good enclosure can help I