B&W loudspeakers have done a lot of research in to the effects of cabinet material on speaker response, especially regarding the effects of stored energy and panel modes.
I'm sure you're all familiar with young's modulus, which is a measurement of stiffness and is the first thing to look at when assessing a material for the walls of an enclosure.
This modulus shows us that concrete is an extremely stiff material when compared to wood or board and, at first, seems the obvious choice when designing an ultra-rigid enclosure.
But, you have to step back and analyze what the point is of all this rigidity.
When a driver moves inwards, it pressurizes the cabinet and the cabinet will expand minutely and after, when the diver moves back out, the cabinet contracts again; but not instantly.
The box stores some of the energy from being pushed out, and when it's not being pushed by the driver any more, it releases this energy as sound.
The less rigid the box is, the more energy it stores and, consequently, the louder this sound is and the longer it goes on for.
The overall effect on the sound is that the bass sounds "boxy". It will sound loose and wallowy with no definition.
In contrast, a tight, rigid box will sound more defined.
When the bass stops, the sound stops dead with no ringing or over hang.
In general, a more rigid box will sound better in every aspect when compared to a less rigid one.
The point of B&W's experiments were to ascertain which method of increasing the rigidity gave the best results.
The first being to increase the rigidity of the enclosure walls, i.e: making the box out of concrete.
And the second being to increase the rigidity of the box itself by using reinforcement i.e: building the box out of 18mm mdf and using further panels the same size with holes cut in slightly spaced apart, running up and down the cabinet so as to create a honeycomb effect.
They call this Matrix(tm) bracing and use it in most of their expensive speakers such as the N801 monitor.
To understand why they chose such a complicated bracing system, you have to look at whats going on with a panel when it's being excited by a driver.
A totally unbraced panel will wobble around more or less in sympathy with the driver, acting kind of like a terrible ABR or port.
If it was braced with a batten, supported by a another panel like the opposite wall of the box, it would only really stiffen the panel in the region closest to the end of the batten, where it joins with the wood and the energy would avoid the stiff part of the panel and diffract around it and the overall loss in stored energy would be minimal.
You could brace a box with loads of battens and, although it would be physically very strong, because we are dealing with vibration as opposed to just stress, it still wouldn't be very much stiffer than the unbraced version.
A batten is only supported at each end and so has nothing stopping it moving side to side and the join has a very small surface area.
A panel on the other hand, is supported (preferably glued and screwed) on all four of it's sides and offers much more support, but is still capable of lateral wobble and so we use another panel to brace that, and another to brace that, etc. until we end up with a grid of panels (matrix) inside the box.
The end result is a structure whereby every point is supporting and being supported by every other point.
It is because crystals such as diamond have a molecular structure with the same properties as this (every point supporting every other) that makes them so rock 'ard.
Anyway, B&W did some comparison tests between the concrete box and the matrix box, to see which one sounded better.
The boxes were of the same size with same thickness of material (18mm). Each box was placed face down into a huge box of sand, so as to absorb all the energy from the front and a microphone was placed above the back panel, to record the sound output from the box itself.
The results were incredible. They found to start with that with your average hifi cabinet, up to 20-30 % of the sound you were hearing was being made by the box itself,
instead of the driver, but with regards to the comparison test, the matrix box was found to store ten times less energy than the same size concrete box.
In other words, it sounded ten times better. The amount of time and money that goes in to build a matrix cab is much more than the concrete box but the matrix has every other advantage.
Using concrete is a bit like cracking a nut with a sledgehammer. There are much easier and better ways to make wicked subs than to resort to such extreme materials.
Using a complete matrix is a bit over the top but the basic principle of the bracing is the most important thing.
If you're not going for a full matrix and just using some panels to stiffen the box, then avoid symmetry when placing the panels.
The more panels you use, the more you will have to increase the box size to make up for loss of volume, but this is only a small figure.
Many manufacturers use things like 22mm birch ply for their bass cabs, but I use 18mm mdf. It's stiffer than normal ply, but not birch because birch ply is like bloody titanium and blunts all your tools and gives you cramps and aches and weighs a ton and costs a fortune (but other than that, it's great) with a bracing system like described and my cabs pee all over professional kit when it comes to sound.
Another BIG advantage to a properly rigid box is the surprising increase in efficiency.
When the driver isn't wasting all that electricity on vibrating the box, it uses it to make sound and the difference is shocking.
There are many ways you can brace a box, but remember that the point is that the smaller the panel, the more rigid it is and by bracing the center of a panel with the end of another panel, you effectively turn that into two panels, each half the size.
A batten support doesn't do this. Use panels to reduce the surface area of other panels and arrange them in such a way so that they are all bracing each other but try to avoid too much obvious symmetry.
My 18" subs are bloody heavy (not like concrete though), but are so rigid that if you touch the sides when they are on full whack, you can't feel any vibration.
Minimal loss, maximum sound.
Remember: PA companies are marketing a product and sound quality is not their primary concern. Don't be tempted to copy what they do because you think it's the proper way to do it because you are spoiled with PA. Big amps, big drivers=loud music.
With hifi, you're limited to small boxes and have to get the most possible out of them which is why all the really cool advancements in speaker technology, that actually make things louder and sound better, come from the hifi world.
PA just use horns (yuk). If you apply this technology to PA the result is another level of quality when compared to Turbopants or some other macho brand.
Dig into hifi technology and you can build subs that make a mockery of professional kit.
The biggest thing that stands out with a really rigid box (with a quality driver of course) is the content. There is so much going on in the world of bass that we don't normally hear from so-called "subs" (subsonic bass means below hearing, sub is not just another word for bass) that it's a whole new world once you start to play tunes through a proper sub.
Concrete subs would look really cool though...