So what is the deal with the sandbox?
The idea is to increase the mass of the box to a point where the driver can not exert enough energy to induce any sort of resonance in the box.
That's mainly why MDF is popular for subs and speakers. It's easily machinable and it's pretty dense. This means you can build boxes that are very hard to induce resonance in due to their mass. The result is a cheap but heavy box.
The increase of bracing is to
stiffen the box to prevent the panels from resonating. By breaking up the panels into smaller sections with bracing, their resonant frequency jumps up tremendously, and hopefully, out of the passband.
Mass is the ONLY thing that works effectively at preventing the
transmission of low frequencies.
Another approach is to use something like a void free baltic birch. BB is much lighter and much more stiff. This yields a box that has points of resonance at rather high frequencies. In the case of a sub these frequencies aren't present in the content, so they're not induced. The result here is a more expensive, but lighter and more durable box.
Baltic Birch is only about 15 to 20% lighter. Any inherent stiffness advantage it may have, is rendered negligible in a well braced enclosure. Bracing is the
key. Mass loading, decoupling, thermal dissipation and resonance control are the other main things that add tremendous performance gains. Sand does all three very effectively, and inexpensively.
FYI, typical sands seem to take up about 17-19 cuin per pound, or 90-100 lbs per cuft.
To give you an idea on how quickly the mass adds up...
I had this custom stand welded up. It weighed 50 lbs when I got it. After filling with sand, it weighed over 80.
Cheers
