What’s stiffer, internal bracing or thicker sides???

ScottG

Member
2003-02-04 12:23 am
US
It depends on a number of factors as to which method might be better. IF it's just stiffness you are looking for then bracing is the way to go.

However..

If you "wrap" the exterior of the cabinet with MDF (1/2" to 3/4") with viscous glue (something that doesn't stiffen-up much), then you will likely be better-off than bracing (..and this is coming from someone that hates MDF) overall - irrespective of stiffness. Most people wind-up using bracing that doesn't raise the panel modes enough and then don't damp the rest of the panel (usually internally).

The best thing you can do (mechanically) is use a baffle that is ultra-stiff: as-in 1/4" steel plate (that the driver is mounted to) - which radically improves clarity. (..Impedance compensation/flattening Impedance also does this electrically). I would rate both as a much higher priority than cladding/bracing. IF it's a ported design I'd also *damp the port and wax/polish the interior of the port. Fiber-fill is best used wrapped-around dowel bracing and kept OFF of the interior surface of the cabinet. (..waxing the interior cabinet surface is also a good idea).

*with port design I'll typically use a larger pvc/plumbing pipe that surrounds the exterior of the vent pipe but with a small amount of space between them - and fill that space with softer pure silicone caulk. Basically a "pipe within a pipe" where only the internal one vents to the exterior of the cabinet.
 
If it was my project I'd go mostly with bracing, since that:

(a) works
(b) is free - assuming you have scraps and offcuts

Also, I can't see the point of doubling up the entire cabinet wall. The box corners should be pretty rigid already.

Also also: Storing / displaying something heavy on your subs is free and easy, and makes them stiffer.
 

ViniWerkel

Member
2021-01-26 6:44 am
When you double the wall thickness of the cabinet you increase the area moment of inertia of your panel by a factor of 8 as I=w*t³/12 with t as thickness and w as your width.
If you brace the cabinet, your panel length halves in the calculation and you'll effectively have a two span beam with better stiffness than single span beam. If you run the numbers you'll get a deflection which is 40 times lower than in the non braced case.
Conclusion: bracing is about five times stiffer than doubling the wall thickness.

(I'm a german structural engineer, so excuse me if the wording is wrong, i used google translator for the technical terms)
 
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Conclusion: bracing is about five times stiffer than doubling the wall thickness.

I agree. This is a very generalized conclusion, and a real box is more complicated, but bracing is usually much more effective than increasing wall thickness.

The bracing must be of a good design in order for it to be effective. I have seen examples of bracing which, unfortunately, have little effect.

j.
 

jReave

Member
2012-10-30 4:34 pm
Doubling the wall thickness will increase stiffness by a factor of 8 but only doubles the panel's resonant frequency.

If the bracing is well implemented, ie. is itself stiff enough, bracing a square panel in the middle will theoretically increase the subdivided panels' resonant frequencies a little above double that of the original panel. So, more than when you double wall thickness. In reality, bracing results can be a little more complicated than that.

But increasing panel mass via thicker panels means more energy is required to activate the resonance or produce the same amplitude to the resonance.

With a sub, it's not too hard to raise the panel resonant frequencies with bracing above what the sub will actually be playing so the increase in difficulty of activating the panel resonances isn't really a factor. Not the case though when the panel resonances fall within the passband, like with a 2-way.

Panel resonances vary with the shape of the panel though, or more specifically with the ratio of the panel's length to width. So where you place the bracing and how much bracing you use matters.

For example, when you keep a panel's length the same but keep making the width smaller and smaller, the resonant frequency keeps going higher. Or to put it another way, if you have 2 panels of the same material and thickness with exactly the same surface area and therefore mass, but one is a square and one is a rectangle (eg. 12" x 12" vs 8" x 18") the rectangle will always have a higher resonant frequency. And the resonant frequency will be higher still the longer and skinnier the rectangle becomes. So with a 6" x 24"panel for example (still 144 sq in), now the resonance will theoretically be just a little more than double the 12" x 12" square's. So 2 panels with the same thickness, same surface area and same mass but one has a resonant frequency more than double the other (more in fact than if you doubled the original 12" x 12" square panel's thickness), just because of their dimensions.
 
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ScottG

Member
2003-02-04 12:23 am
US
With a sub, it's not too hard to raise the panel resonant frequencies with bracing above what the sub will actually be playing..
While true (as with the "bracing improves stiffness vs adding to wall thickness"),

..so the increase in difficulty of activating the panel resonances isn't really a factor.
..pressure within the box can activate the box's panels above the pass-band for the sub.

ie. if you have a 4th order low-pass at 80 Hz it's still very likely that the panels themselves can be generating sound several octaves above 80 Hz (..though how much it does that is something the constructor will have to measure).


Like Pano - I like both, but if I had to choose on a sub design it would be as I previously recommended.
 
These will be my rear subs. The cabs are very large as you can see. Each one will house a 15" sub and both will be driven by a Behringer NU3000 iNUKE High-Density 3000W Power Amplifier with DSP.

https://www.parts-express.com/Dayton-Audio-RSS390HF-4-15-Reference-Series-HF-Subwoofer-4-Ohm-295-468

Back sub L.jpeg
back sub R.jpeg
 
Retrofitting an existing cabinet depends on what you can actually do with it.

In the case of a woofer, you want to push resonances up above the bandwidth.

My first inclination is bracing as the more elegant and effective approach. But if you cannot retrofit decent ones…

If you halve the panel by 2 (something you should not do exactly), the potential resonance climbs by a factor of 2.

The stiffness of the box goes up by the cube of thickness (if i recall GM correctly), i don’t know off hand how much that pushed the potential resonance up, but googling should find you something that gives a good estimate.

Note: i would consider increasing wall thickness brute force.

Do consider that bracing can be added to the outside as well. Trickier to get to work well and certainly could have WAF issues.

dave
 
You can also get some Sonotube concrete former tube of different diameters and cut 1 inch rings of several tube diameters and using rigid glue glue it to the inside walls of your box. Use two per wall not symmetrically placed. You would be amazed at how it makes the panel think it’s much thicker than it is. It takes very little volume and reflects at angles breaking up resonances within the box. Use panel to panel bracing as well. The torsional rigidity of the tube glued to the cabinet wall raises the resonant freq much higher where there is less energy to excite it. It’s cheap too.
 
Don't 'brace' to make the panels stiffer. Do it to link opposite facing panels together


Making the [anels stiffer is good, but thanx for reminding me about ballooning. In a sealed woofer this can be an issue. So the brace should both stiifen and tie together the panels.

An iBeam is stronger than a sheet.

As i was talking about external braces my mind went to how much tricker it is to accomplish that function exteranlly than internally.

dave