The basic rule is: A heavy wooden box well braced gets you
closer to the goal of making a dead cabinet.

You can do whatever you want to achieve this goal. Thick wood,
tiles on the inside, sand, concrete, composites, kitchen sink.

The problem is easy to solve when you are building a small
speaker box as you don't care if the speaker box weighs 50 - 100
pounds. When building a line array, it's a hard pill to swallow
that you might need a 500 pound cabinet to solve the problem to
your satisfaction.

You need to find a compromise, weight vs. performance. It's your
call.

The problem is easily solved with 1.5" thick MDF cabinet walls +
internal bracing. You need excellent bracing methodologies and
you can use plywood bracing to keep it lighter, but still a very heavy box.

Replace the 1.5" MDF with 1.5" plywood and you cut your cabinet
weight in 1/2, but performance is also reduced, catch 22.

A hybrid 3/4" MDF + 3/4" plywood is another choice, better performance than 1.5" plywood, but not as good as 1.5" MDF.

http://home.pacbell.net/lordpk/robarray/Rear_chamber-2.JPG

Example; This line array is 3/4" oak plywood. Four isolated chambers, 3 pieces of wood to make those chambers and to
brace the cabinet. To keep the weight down and to make
a dead box, I installed smaller 12" x 12" pieces of 5/8" MDF to
every wall in each chamber except the front baffle. I could have
used 3/4" + 5/8" lamination but I chose this method to trim weight.

After the build, the hammer test revealed that each chamber can be improved as the ringing wasn't as good as I wanted. To solve this,
a 1" dowel was inserted in each chamber to couple the left and right
sides of the cabinet.

I don't see any reason to use sand, or to add tiles because in the end, the cabinet will be heavy and to get a heavy cabinet you
can just start off with thick walls with bracing and the construction
project becomes easier. Fine tune the cabinet later with dowels or
whatever piece of wood.

I'm not from the double thickness wall box school (a double thick baffle would be appropriate) but you can get by with a single 0.75" thick wall on the sides, back, etc if you design the enclosure correctly. The secret is to partition or brace the box so that you minimize the wall pressure and length of any unsupported wall. What I do with my line arrays is to partition so that I have full width and depth shelves between every pair of drivers (I'm thinking 4",5", or 6.5" drivers) . For a six foot high box I'll use a shelf every 12 inches so you'll have support within the box. If the box is more than 12 inches deep, I would suggest vertical braces to support the internal panels between each other and the top and bottom of the enclosure. Thus my rule is to have bracing or shelving for no more than a 12 inch width or depth for any unsupported panel.

Bottom line is to design your enclosure like an iron bridge--lots of partitions and braces and you'll be good to go. None of my line arrays boxes don't have vibrational or any box talk issues.

Jim

My point is that by partitioning the box you spread the load thoroughout the volume. So my point is that you really don't need the double thick box walls if you have enough braces. The added mass just makes your enclosure heavy and hard to move. Now nothing is wrong if you wish to build the ultimate box but just make sure that you have plenty of help moving it into place. I'm just thinking that you are going to an overkill situation vs. a good enough solution.

Your angled corners should work to disperse the load among the structural elements in the box.

Jim

Dead cabinet - to reduce wood vibrations that color the sound.

Echo - Have you ever been in an empty room and talked? Echo.
Make a small box and stick your head partway inside, make a loud
burst of sound, you hear echo. This echo needs taming with the fiber
material that you install inside the box to reduce that echo that
smears your sound.

Audio is simple. Hammer to test dead cabinets, your head inside
the box to 'test for echo'.

http://www.amazon.com/Test-Echo-Rush/dp/B000002JAI

.. LOL ..