The metal asspect of it doessn't matter. It's the thinkness or gauge of the metal that matters relaative to the amount of air the speaker or sub is going to move. For instance I would imagine you're trying to stay as small as possible, I will also assume that your housing a sub (I'll refer to it being a 12" sub). If a 12 moves 1.5 cu. feet, as an example, and you have 1.5 cu. feet within the box that is ideal however, if the metal being used isn't rigid enough relative to the force being used to move the air it is going to move in and out creating an awful sounding rattle. Which actually sounds as if you've blown your sub almost or it will give a popping sound every time the speaker moves. It has nothing to do with magnetics and the need for the lack of as pointed out above. You simply need a bit more air space combined with a more rigid structure. Build one out of 1/4 inch and youy won't have this problem, however, the weight is obsurd! HAHA /forums/images/graemlins/rotfl.gif No really if your useing 18 ga. try 14 or 16 maybe. Or you can make a better structure. It's like trying to suck the air out of an empty plastic pop bottle, pretty easy, try an empty pop can and it is possible but is much thougher not due to the thin metal but the rigidity.(Is that a word?) It's a stronger structure. Good Luck.
Use underlayment and be done with it.
