Hey all, I'm in the process of re-building my riser. It's 60" deep by 96" wide by 16" tall. I learned a few things last time around and am ready to enclose it (my current riser is an open design).

Originally I had planned to fill the riser with fluffy pink fiberglass insulation packed in as tight as I could get it. However, I have access to a LOT of furniture-grade foam (like foam used in seat cushions) and am wondering if there would be any advantage to using this stuff over the insulation. I can get as much of the stuff as I want for free and am tempted to go this route as I believe I can pack it much more densely than fiberglass.

I am also thinking of adding transducers to the riser if that makes a difference.

I think if there is no electrical in your riser that could potentially ignite the foam I think it would work plus you said it was free nothing better then free :p

So no worries on it performing differently acoustically?

And I never thought about foam starting on fire. I figured would melt before it ever hit a flash point.

throw a piece outside on a cinder block, and hit it with a grille lighter....

And put that on YouTube.

Seriously, find out... but don't do it if it's windy.
Wear some safety glasses and keep an extinguisher nearby.

Goggles are in case this stiff turns into embers and starts floating, the last thing you want is burning goo in your eye.

Polyurethane foam is very flammable. Cigarette-started furniture fires are a major source of house fires (polyurethane foam is sometimes referred to by firefighters as "solid gasoline").

That said, I'm not sure I'd be too concerned about it in a riser, where it's surrounded by concrete/wood. If you have a flame that burns through 5/8" plywood to get to the foam, you already have problems. And there is already a ton of this stuff in your couch, which is only covered with a thin layer of fabric. (Note, I'm not a fire or code expert. I don't even play one on TV.)

Learn something new every day I guess. Funny that when I went through my pyro stage as a kid I never got around to furniture. Another fun fact: light a laserdisc player on fire and the plastic runs like molten lava. Also, burned furbies laugh, even when all that remains is a charred chassis. Freaky.

I'll keep all of this in mind though, thanks guys. The riser floor will be 1" thick, sides are probably 1/2" plywood. I was thinking of some accent lighting through vents for the steps. Might not be a bad idea to go with the fluffy pink stuff.

Very flamable and very toxic when burning.

I don't think this will affect the riser as far as acoustics go unless your planing to use it as a bass trap. You will want to use unfaced fiber glass or like material if you intend on making this into a broadband absorber. Also a word of advice be sure you tune right and located it in the proper spot. It can cause issues if you don't.