Wendell,
Particle board is not rigid enough for a subwoofer. It will flex under pressure and add "coloration" to the sound of the sub. Also, it's even more brittle than MDF. So in the words of Nancy Regan, "Just say NO!"
Brian
I used high density particle board for my 10" sub. I was told i should double the walls to make up for it not being mdf, so I did. I'm no audiophile, but it sounds good to me.
you can us particle board at least 3/4 high grade there is some crappy stuff out there the good stuff will have a yellow tint to it and will have dence fibers but it is hard to shape and burns saw blades up, I would use MDF if you can fine it, it is only like $14.00 a sheet at home depot.
I think JBL uses particle board in some of their older speakers. I remember from when I looked at the inside, it wasn't a low grade particle board but it certainly didn't look like MDF.