Solid hardwoods crack when constrained/glued at the cabinet corners. This is not because the wood is drying out. It is from the expansion/contraction caused by a prolonged difference in humidity/moisture on the outside vs. the inside. Waxes, or most any finish slows down the wood's response to any short-term humidity differential.
But no finish is impervious to H2O vapor, so solid wood cabinets run a real risk of cracking, as it moves the most %, especially after a few years have passed. It helps if that cabinet is ported to the outside atmosphere. And then it needs to have the same thickness of lacquer on the inside as the outside, to equalize the moisture levels between inner and outer surfaces.
One can sand solid woods so much deeper and finer than veneers, that the final appearance is far richer, with more grain. Unless you know what you are doing with veneer.
Is the consistency of resonance and stiffness a problem with solid hardwoods? No- not for 3/4" and thicker panels, not in my 30 years experience of design and manufacture.
What are those resonances?
- The first is the diaphragmatic motion; the pulsation of the cabinet walls like a balloon. Braces help control that motion, and so does the bending stiffness/thickness of the wood (Mr. Eckhardt said that the most concisely above). It is wrong here to use the word "strength"- as it means breaking point, not stiffness. E.g., concrete is strong under compression, weak under tension, and stiff either way in terms of % deflection/unit force.
Phenolic board, then birch plywoods and solid hardwoods are the stiffest for a given thickness, followed by HDF, then cabinet-grade particle board, and MDF last. Increased stiffness helps the low bass dynamics and extension. But so do braces if installed properly...
- There are shear-mode vibrations caused by poor corner-joint design.
- There are vibrations from the extensional and compressional forces applied via the woofer-mounting screws. There are ways to reduce those.
- Finally, there are the least-understood 200-300Hz "particle" vibrations. That's usually the main resonance shown in Stereophile- doesn't matter where JA would attach the accelerometer.
No braces can control these, nor can any conventional glue joint. Hardwoods, plywoods, particle boards, MDF, HDF, phenolic-board all have a particle resonance- just different in frequency, and a little different in self-damping. This is the vibration some above are thinking of (or should be) when they refer to "better damped".
How does one know this is a particle vibration and not the cabinet walls puffing in and out? A stethoscope shows that it's the same amplitude anywhere on the cabinet's surface- across a corner joint or not, across a brace or not, back, front, side, top... thus it can't be diaphragmatic pulsation. So these are surface vibrations- and it turns out, random ones, like the surface of boiling water.
Pointing cones "up" under a bookshelf speaker means touching fewer of these "bubbles"- keeps this 200-300Hz "noise" (which is what it sounds like) out of the stand and eventually out of the floor. But then far fewer of the cabinet's 200-300Hz particle vibrations are being damped on its bottom surface- hence the arguments about points up/down!
Place those cone-points down, under a floor-standing cabinet- and those same cabinet-surface vibrations are induced in the floor, whether wood or cement. But at least the points pierced the carpet, so the cabinet rocks less, making the bass tighter and the image sharper.
Put a stethoscope to the sides of MDF or oak, cherry or maple, carbon fiber or HDF- you will hear easily the 200-300Hz resonance; doesn't matter how thick the sides are nor how many braces. Are there ways to reduce this 200-300Hz problem? Yes. ~tain't saying how.
MDF is not very strong at the glue joints. The std. approach to counter this weakness is to make 2-3" thick sidewalls- for more surface area to glue. This means more bulk. Which means more cabinet exterior reflections. It does help the low bass but does nothing for the 200-300Hz ringing. But extra thickness is good for advertising...
The stiffest wood product beyond solid hardwood and birch plywood is phenolic board: layers of very thin paper impregenated with phenolic resin, cured under tons of pressure at very high temp- it's really expensive... ~$400/sheet, and really heavy. You can see blocks of it for machining at usplastics.com (usplastic.com ?).
The stiffest single-wood cabinet would be made with the Norwegian birch plywoods. It's about 4x more expensive than MDF (no big deal), but actually it's a bear to work with- likes to curl up when laying in the shop, and its cross-grain thin plys often splinter while being cut. Mostly we see it in PA speaker cabinets, as 15mm-thick birch ply is easily as stiff or stiffer than 1" MDF, and so the final cabinet weighs less. Birch ply also doesn't crumble into sawdust like MDF when a corner is banged.
Why do speaker companies advertise having veneer on the inside of a cabinet? They say it is better for resonance. Bah! Cheaper veneer is applied on the back side of every veneered MDF or plywood at the mill- necessary to equalize moisture absorption/loss while the sheets are in storage. It does nothing for the sound in any appreciable way.
The stethoscope shows all the vibrations I mention above.
Too bad reviewers won't buy a stethoscope for $50 off e-bay to verify a manufacturer's claims...
Too bad speaker designers don't use one to make better cabinets...
Too bad "isolation device" designers don't use one to make devices that actually work (does everyone really believe you can somehow "drain vibrations away"?? And what vibrations should they be tracking down? The same four as listed above, even in metals and plastics).
Good question Slappy! I really hope your Mom didn't name you that.
Best wishes,
Roy Johnson
Green Mountain Audio
