Speaker Box Material — Solid Wood or MDF?

      Solid wood turns out to be a poor choice for speaker cabinets. This discussion explains why, and covers other ground concerning speaker construction. November 20, 2005

Question
I have recently been approached by an engineer about building solid wood speaker boxes - 11" W x 9 3/4" D x 28" T. The problem is that I am not sure how to keep the boxes from tearing themselves apart with wood movement. I was thinking dovetails on the outer case, but I am not sure about the back. Would biscuits work? If this pans out to his target sales 80-120 pairs a month I can see it being very lucrative. The problem is he doesn't want MDF or plywood - only solid wood. Any suggestions would be appreciated.

Forum Responses
(Cabinetmaking Forum)
From contributor B:
If he insists on solid wood without any MDF or ply, I'd politely walk away. He'll be out of business within six months anyway. Speaker boxes require that everything be tight and airtight. You can't account for wood movement without compromising the airtight requirement. What is he going to do when the customers start returning boxes because the wood is obviously moving? Advent (and I'm sure others) put a solid wood top on a MDF or ply box.



From contributor F:
Acoustically solid wood is a very bad choice as every pair of speakers will sound different as the density of each board will not be the same and the frequency response will shift. Try veneered 1" MDF (or HDF if you can get it) sides and solid corners and or solid top, or have some custom MDF laid up with 1/8" veneer to get the solid effect.


From contributor S:
Without question, both posts from Contributor B and Contributor J are absolutely correct. If the engineer needs proof, show him any other speaker box in the world. There's a reason they're made out of ply or MDF and it's not because it's less expensive.

If he's concerned about the perception that veneered substrates equal cheap, take him on a tour of 18th century furniture that incorporated shop-made plywood and extensive use of veneering. If that's not enough, reiterate Contributor B's point about wood movement. There is no practical force on the face of the earth that can prevent it. You're essentially being asked to build an immovable object out of an irresistible force.



From contributor P:
This seems like a tall order. One thought I had is to make the top, bottom, and sides like you would a drawer box - dovetailed, splined, box joint, or whatever. Then make the front and back of the cabinet floating in a dado, like you would a drawer bottom. You could do it in such a way that the front and back overlaid the sides partially, with say a 1/8" shadow line around the perimeter. You would have to have some method of sealing the dado on all 3 sides with some sort of closed cell sponge, or other airtight, flexible material. The front and back would be decoupled acoustically from the sides, and also they could expand and contract independently of the sides.

Not all speakers require an airtight cabinet. Some woofers are designed to be in an open or ported case. The interior volume is critical, as are a number of other factors. What I'm getting at is that the cabinet is intrinsically linked to the transducer in determining the overall performance of the speaker, so it seems to me that the construction of the case - materials, shape, size - should be determined by the engineer. Or he/she should at least be able to determine what the critical factors are likely to be. Speaker designing is not a completely precise science, and what I have seen is that after engineering the speaker, a significant amount of tweaking of minor variables has to happen to approach the desired audio result.



From contributor B:
To contributor P: Technically you're correct about boxes not being airtight; the box construction should be airtight. A port must be precisely sized for the box volume, and is sized relative to the resonant frequency of the drivers to maintain the proper system "Q". Any air leakage from anywhere in the box besides the port will compromise the sound of the box. Nobody likes to hear boxes buzzing.


From contributor E:
To the original questioner: We actually used to make the boxes for Duntech Speakers in Australia and they were all veneered. Another alternative I would suggest checking out would www.ambro.com.au

They're an Australian company, but check out the product called stayblewood - maybe something similar would suit?



From contributor G:
Those are pretty large speaker boxes, and solid wood is not recommended. Use MDF veneered or particle boards. Solid wood has an inherent resonance that will wreck the flat frequency response of any traditional cone-type speaker, and thus is not used. The less expensive stuff will produce a much better sonic quality.


From contributor S:
Solid wood is certainly not out of the question for movement reasons. Dovetail 5/4 or thicker for the four sides, and float a panel in a groove for the front and back. A double groove would work better, and machined tightly so there will be no loose panels. Don't even think about spaceballs. Use quartersawn face and back panels - and let the joinery show to reflect the use of solid wood. How the solids will work acoustically, I don't know. But don't rule out solid just because it "isn’t the way it’s done."


From contributor D:
All of the people that I have seen making musical instruments are always shaving and tuning the wood for the best sound because it all resonates different. I have always used MDF for speaker boxes because of the density and consistency of it and they always sound good, and each one sounds the same. Also, how much would you charge for 5/4" mahogany speaker boxes?


From contributor C:
The question of MDF or solid wood has absolutely nothing to do with perceived quality and everything to do with sound. The boxes have been engineered for optimum sound (carbon fiber rods, exact speaker location) specific radius cut onto the front edge to facilitate sound transmission).

As far as the construction goes, I was thinking I would construct the sides, top, and bottom with dovetails or biscuits. The front can't be a panel because it actually acts as a speaker itself transmitting sound out of its edges and off the front face. I was thinking I would use biscuits to hold the face on. I am not sure about the back. I was also planning on sealing the inside with either silicone or construction adhesive. Anyway, MDF is out of the question, but does anyone have any experience with "lumber core" sheet goods?



From contributor R:
I've built horn loaded speakers using 1/2" Baltic birch for the inner box, then covered it with solid wood overbox and silicone between the 2 layers to allow some wood movement. I've also done some where the front face has a stepped dado around the outside edge, which mates into another dado around the inside faces of the sides, which were mitered. The back was done essentially the same way – I used silicone to allow flexibility in these joints. Assembly is done all-at-once, so it's a bit tricky.


The comments below were added after this Forum discussion was archived as a Knowledge Base article (add your comment).

Comment from contributor J:
Making speakers out of solid wood is possible but is more than tricky. We make our high end OCTO bookshelf speakers from solid wood. This is expensive and required a new way of thinking in the way of joints, expansion and speaker shape. Basically this accomplishment required lots of engineering and FEA analysis, special gluing techniques and etc. Unless you are willing to invest in exhaustive research I would veneer it. Our other speakers feature solid wood end caps and MDF or HDF middles that are veneered. This would be your best bet.



Comment from contributor M:
Solid vs. man-made. The basic trouble with solid wood, for any practical application, is a drastic and almost uncontrollable change in the density. This density can be changed at will and without warning from temperatures and humidity in the immediate area where the cabinets would be located. Imagine the sound changing during a long drenching rain, or changing during a long, hot summer afternoon. Even relocating to another room within the same building could alter the sound. Unless the climactic conditions could be controlled and assured, solid wood is too unmanageable.


Comment from contributor C:
I think your best bet would be a veneer. Enclosure resonance does play a major factor so if you did decide to build the enclosures out of hardwood, choose as dense as possible. One solution for the air tight problem could be coating the interior of the enclosure with a solidifying resin or fiberglass type material. You can use tnuts and weatherstripping material to seal and mount the subwoofer/driver.

As previously mentioned, the integrity of the box as far as air tightness goes will play a big role in frequency and resonance. If ported the port is designed to act as a crossover of sorts, this is especially true of bandpass designs. If your enclosure is leaking air/sound waves through unintended/undirected pathways, you will get mediocre to poor results, not to mention a high amount of "fluttering noise" from the leaks.



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