Most Efficient Bracing ? (less space taken)

Like this:


If you prefer a window brace the figure below left is the usual arrangement, but it places most of its material where it does the least good, in the corners. The figure below right is twice as effective using less material.

Question in this matter, I know in years past in the car audio world they would use all thread for bracing, yet here I see incredible wood working braces made, why is that? is it more of visual reason's or ...?

Allthread works fine so long as sufficiently anchored to baffle walls. The only real drawback is that as it is typically much smaller in diameter than a wooden brace might be, it will almost certainly have a smaller bending moment of inertia and thus be more prone to buckling during compression.

The most efficient use of material would actually be something like a decent sized (4" or so) thin walled PVC pipe with staggered holes cut in the sides to allow the inner space to acoustically contribute to total box volume.

couldnt that potentially cause inner box resonances unless you stuff the pvc as well?

A matrix of dowels like Bill's , or B&W Matrix 801... I dont think there is a better example

I used 1/2"-13 " thd rod for my IB sub line array baffle bracing:



Just last week downloaded the iPhone VR Mobile "free" Vibration Meter, need to attach that to one of those braces to see if any in plane flexing is happening, to my eyes/hands I cound not sense any.
http://itunes.apple.com/tn/app/vr-mo...469459743?mt=8

A little more time consuming but otherwise this is a great method.

Another is carbon fiber tubes, which you can find used as arrow shafts, golf club shafts, ski poles. Preferably broken, but still long enough to work. Very strong, very stiff, very light.

Mathematically, super thick walls are what prove to be the most wasteful of space. The frequency of panel vibrations (for a given material & thickness) come from the largest unbraced square area.

I personally have mixed feelings on the single point bracing, as I question if it really is equivalent in damping what panel vibrations do remain as compared to a single point. No question though, many use excessively thick and complicated bracing.

Another consideration is box construction. In some cases, you can use bracing to help guide construction and alignment, but that depends greatly on the construction methods you are using.

So the cabinet act like a speaker contributing to the overal sound. I say work it in to the FR and you will be fine, unless it moves as much as the cones.

I used 2" MDF, bracing is 2" MDF. Drivers are all front facing and they have little movement or vibration. I think there comes a point of a deminished return.

How bout we get more people to buy measurement gear and set up their subs the right way !!! I cant believe there isnt a sticky for the benefits ? Much bigger fires to put out. Tons of wasted money on great subs sitting in corners sounding like garbage and people arguing over specs that mean nothing if they are not utilized........ok im done

Yes. The point being made is that the diameter or square size of the brace shouldn't matter.


Exactly what I stated. The question is if there is any practical difference in damping higher frequency vibrations with a small contact area vs a larger strip of say a window style brace. I'm not saying the difference is huge, especially for subwoofers, but I wouldn't call it equivalent.

Of course the real consideration is that wall thickness tends to take up MUCH more volume than the differences in more commonly seen bracing methods.

For a given volume of material, single point bracing to the opposing side is much more efficient than blade stiffeners. This is easily shown with simple FEA models (the braced plate bending calcs get messier than I care to trudge through). The same is true compared to panel thickness but only globally. It of course does no good to build a box from kraft paper only to have to brace it every 1/16". The primary benefit of increased wall thickness is allowing higher stiffness in unbraced spans and thus allowing bracing to be spread wider. The optimum use of material can be found with a simple spreadsheet or mathcad calc, but to be honest I've never bothered as (1) mdf and ply comes in standard thickness and (2) I don't need to optimize a design for either weight or external volume for any of my diy applications. Easy enough to just use simple rules of thumb that work. Unless I really need portability, and that seems never to be the case, I'm happy enough to just overbuild and call it done.

This all applies only to sub enclosures. The situation is more complex for full range enclosures where increasing panel stiffness may only push a resonance higher within the audible passband. Whether it is better to have a higher q resonance with less total energy or a much broader q resonance with higher total energy, or whether resonance or non-resonant sound transmission drive audibility, are not particularly straightforward to answer given that each choice demands a different distribution of mass, stiffness, and damping. Some have proposed that well damped panels should dominate and allowing some flexibility is fine. I'm not sold on the arguments that stiffness is much much less important, though it is clear that for example an efficiently stiff but undamped [corrected - my phone apparently didn't like "undamped"] panel, say a steel wall with minimal blade stiffeners, which rings like a bell is no good. In fantasy land the panels can just be stiff, massive, and well damped, like granite with a thick damping layer. In the real world it is difficult to come to a clearly best compromise.

Curious why you assume lower resonant freq has lower q

Far too much emphasis is placed on that aspect. It doesn't matter what the resonant frequency of the panel is if there's insufficient pressure within the box to excite it.

Those were two separate ideas you joined together. Sorry if my sentence transition was less than clear there.

Box resonances may be excited by direct coupling of the driver frame as well, probably more often the case in fact. In any case, the amount of energy required to excite a resonance is determined by damping. For an undamped panel (idealized case), the energy may be infinitessimal if the excitation is sustained long enough. This is the argument many make in favor of high damping at the cost of panel stiffness. On the other hand, transmission at non-resonant frequencies (or for single or few cycle input functions) is highly dependent on both mass and stiffness as well, which is perhaps what you were referring to instead of resonance, and is why I think neglecting to build a sufficiently stiff panel is a mistake. Too many people get hung up on talking about only resonances.

AS bit more more precisely:

If there are modes, and there are, and if there's energy, which there is, the modes will be excited.

The question is how many dB of panel radiation relative to driver radiation is significant.

It would be interesting to compare that with how much panel vibration is perceptible by feel; given our extreme sensitivity to tactile vibration, I suspect what feels like too much isn't.

To clarify what I wasn't clear enough with above, I well agree a cross brace is much better than simple thickness reinforcement from a brace. The exact example would be comparing only two dowel rods to the image on the left Bill posted earlier:


I well agree the example on the right to be more effective, but what if the number of cross braces are the same but one has the full perimeter contact area vs. just the cross members? For many subwoofers it won't matter much, but some empirical examples I've done have suggested that the tone of the ringing to be much better damped (from a strike or knock) with the perimeter braced as well. As I suggested above, this does have more impact in full range speakers than subwoofers.

In that case you are comparing four cross braces (two for each direction) to four cross braces + blade stiffeners. Of course there is increased stiffening due to the contribution of the blade stiffeners.

But for essentially the same amount of material, it would be more effective to now have eight cross braces (four each direction) instead of four cross + blades.

That was the point Bill was making and I agree. I'm not sure if I have any FEA tools currently installed or not (having made a near complete transition from engineering to medicine now). I'll look and see... maybe I can install a simple tool like Algor if I get a chance. It would be helpful to show modal analyses of the various options being discussed.

But again, rarely do I care to find the optimized solution for either weight or external volume. Were it easier for some reason to cut holes in a panel to make window bracing, that's what I'd do. Were it easier to grab dowels or scrap wood to make cross bracing that's what I'd do.

Especially when you consider the OPs question, 'What would be the best way to bracing , taking the least amout of valuable space'.