Unconventional & inexpensive bracing techniques

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I am building a pair of subwoofers, which I hope will be my last, or at least last for quite some time. Unfortunately I am also on a fairly tight budget for materials. Despite this, I really want to build the stiffest possible enclosures so as not to provide myself with any excuses to upgrade again ;)

External dimensions will be 23"h x 16"w x 18"d sealed. I want to use baltic birch plywood, for the increased stiffness over MDF, but since it is quite expensive here (~$55 for a 3/4" 5x5 sheet), doubling up on all the walls would add close to $120 to the cost. I don't want to use more than 2 sheets, and this gives me just enough material for .75" walls all around, with double width front baffles and a single front - back shelf brace per enclosure. My desire to avoid spending more on wood has led me to investigate some less conventional, affordable bracing techniques. A few ideas I had are listed below.

I managed to pick up some ~21" long aluminum 'beams' to use as 'ribs' on the longest walls of the box, these are so stiff I can't bend them even slightly over my knee, and take up very little volume. They have a screw/nail hole on either end for installation, and I will probably also use polyurethane glue along the whole 'beam' as well. I picked 4 of these up at the local surplus store for $ 2 each.

Another idea I had was to position the shelf brace in such a way that some of the remaining unbraced 'panels' are roughly the same size as large ceramic tiles. I would then use the polyurethane glue to attach ceramic tiles to any unbraced span large enough to accomodate them. This would be very cheap, as I have seen 1'x1' tiles selling for ~ $.60. Cermaic tile being very stiff, I thought this might be a good way to stiffen the walls on the cheap, with the added bonus of not taking up too much internal volume.

When I was at my local building centre the other day, I noticed they had some fairly substantial looking steel L and T braces typically used in home frame construction. Looks to be about 1/8" thick steel, with each side of the L being about 2-4" long, and with a few screw/nail holes for installation. They run about $1-2 each. I am wondering how useful these would be at internally bracing joints in a subwoofer enclosure, as compared to more conventional corner blocks? Certainly they take up a lot less volume, and seem to be extremely stiff.

Anyone have any thoughts/comments/criticisms on any of the methods proposed above?
Use cheap old partical board or MDF for your inner layer and you'll save $35 - $40 right there - and it provides a different consitency than the plywood so it's kind of like constrained layers. Triangle scraps are going to be just as effective or more so than $2 steel L-brackets, and way cheaper. I have thought of using tiles as well, used with a rubberized glue, it should provide some very good constrained layer damping. The aliminum I-beams could probably be effective if they are well glued and screwed to the walls - do them off center to spread resonant frequencies of the panels.

Good luck.
You're going to a lot of effort to save a few bucks.

Point taken! However, I have a lot more time than money. Furthermore, I don't think gluing a tile to a panel or screw/gluing an L brace is any more trouble or time than making perfectly fitting additional shelf braces and cutting out all the holes in them. So far, all the effort I've invested in this is $8 and a little thought.

I could gather large scraps, from a lumberyard, but I have only a jigsaw for cutting straight lines, and making perfectly sized panels with it is a bit hit and miss. I typically get all panels cut for me by home depot, and just do the driver /brace cutouts myself.

I do have lots of small MDF scraps lying around, and I suppose I could just glue some of them to the internal walls in the center of any unbraced panels. I had thought of doing that, and it's certainly cheapest, but it seems a bit arbitrary, and I wasnt sure how useful doubling up the thickness in only a few places like this would be at stiffening the box.

Plus, ultimately I guess I just like thinking about different ways to achieve the same goal, as I'm sure many of you do ;)
To stir the pot even more, the following is supposedly from North Creek's cabinet building guide:

Brace Material

Bracing material must possess qualities very different from side
wall material. The purpose of a brace is to prevent the side
walls from bowing sypathetically with the woofer. Therefore, the
job of a brace is to remain dimensionally stable under the forces
of tension and compression.

It was shown by Tappan in 1962 that solid soft wood is not
suitable along its width for bracing material (ref. 1). Modern
composite boards, such as particle board and MDF, possess far LESS
tension and compression strength than solid soft wood, and
therefore make far POORER braces.

The correct material to use for internal bracing is
multidirectional plywood. Plywood is strong in any direction
under tension, because wood does not stretch along the grain.
Although not as strong under compression, it is still superior to
soft wood and far superior to composite board.

Ref. 1: Tappan, "Loudspeaker Enclosure Walls", JAES, July 1962

I've made MDF 'windowpane' braces before, and found that while they certainly help, they don't feel all that stiff around the edges... This seems to agree with that.
When I was working on my TL, I didn't want to make the bracing from solid panels and make holes into that for weight reducing. So I glued the bracings together from smaller pieces (like those pieces you can get for free). It will only cost you glue then.


First I did use paper so I could clamp the bracing together whilst laying on the floor (as you can see the traces left on the bracing). Later on I used garbagebags that don't stick to PVA glue.

Mvg Johan
> Location: Guelph, Ontario
> I have a lot more time than money

Geez, if you can drive to Kingston this week, I have three well-weathered 8x4(?) sheets of regular ol' cheap plywood you could have for nothin'. Then all you'd have to buy is the birch veneer. I'm pitching them because they have insualtion glued to one side... Otherwise, I'd make shelves out of them.

'course the gas'll probably run you more than the wood is worth. :) Unless you have a compelling need for a bunch (I have 400 spare) 9 GB SCSI-II disks... mostly 5.25"...

Incidentally, is the plywood you're investigating birch through-and-through, or just the top layer? I made a desk years ago which had one ply of birch, the rest was pine I thin. I seem to recall 8x4 sheets being around $40, but then again, that was a LONG time ago.

Needing material that is strong in compression as well as tension, you could use metal pipe with screw-on flanges each end.


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collo - fantastic! I love the idea. Where does one get those tie rods and the flanges? I imagine you are limited to 'standard' lengths in the rods?

wes - thanks for the offer! I will be at a wedding this weekend, and you hit the nail on the head re: the gas working out to more than its worth! The wood I am looking at is void free baltic birch, AFAIK its birch all the way through, and only comes in 5x5 sheets at my local lumberyard, or 2x4 sheets at Rona.
morbo said:
I have a lot more time than money.

That breeds ingenuity.

but I have only a jigsaw [/B]

When I started, I took my allowance, went to the second hand shop and decided what was the best I could do for my $8.00. Guess what. You wouldn't believe the hours spent with a jigsaw and a straight edge.

That breeds patience.

Rademakers said:
So I glued the bracings together from smaller pieces (like those pieces you can get for free). It will only cost you glue then.

1X2, 2X2, 2X4, it's all going to be there at your local scrap yard. They're happy not to have to pay for disposal.

That breeds humility.

See? Many of lifes greatest lessons can be learned from building the humble loudspeaker.

Faithfully yours,
Father Cal ;)

In a AES apaper that goes back to 1972 Iverson published results that showed that most bracing used in loudspeaker enclosures is ineffective in achieving the primary purpose of replacing a single low frequencty panel resonance with two lower amplitude higher frequency ones.
The reason for this is that the brace must be a rigid boundary and most braces are not stiff enough to constitute a rigid boundary.
Steel is the stiffest of all common materials and even light steel sections are considerably stiffer than wood, so a steel section in very intimate contact by means of a rigid setting epoxy glue for instance, should at least in theory provide better bracing than a very much larger piece of wood in the same position.
thanks for that reference RCW, very handy, and very interesting considering how many people are using MDF for braces regularly. I wonder why there is not much interest in this aspect of box building?

Re: gluing the metal to the wood, does it have to be epoxy, or would say, a polyurethane glue (slightly flexible) work as well? And does anyone have any idea where to get the threaded tubing and 'footplates' as discussed here? I am assuming a plubming supply shop for tubing? but the footplate is a mystery to me...

Iversons data shows that panells that are glued and screwed into a box, along with glue block corner pieces have about 85% of the resonant frequency expected from an absolutely rigid boundary.
Typical bracing including cross bracing only increases panel resonant frequency by around 20%.
From this the closer to the ideal perfectly rigid boundary you approach the more effective the bracing. As mentioned only composite materials such as carbon and boron fibres are stiffer than steel, the only metal stiffer than it is beryllium, pvc pipe is not anything like as stiff.
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