Best way to decrease the internal volume of a sealed speaker?

If you use Flex Seal, it may have petroleum distillates in it, as the more expensive rubber sprays at Wally World (10-12 bucks) will have. However, the cheaper stuff (2-4 bucks) doesn't have pd's. Pd's are rather corrosive to electronic circuitry.

Automotive undercoating spray (same thing as Flex Seal really and is what you'd be looking for at Walmart or your hardware store) will dry overnight to roughly half the thickness at which it was applied. However, after several days, the depth of the coating will continue to shrink and then be no thicker really than a coat of paint, so it might require simply too many coats and too much drying time to be of practical use as a way to reduce volume on its own...but good as a sealant, though. 

Flex Seal could possibly be thicker, but I have no idea if it contains distillates.

Styrofoam, I would think, would not absorb too much, although something that low mass may be excited to resonate a bit maybe, coating it with the rubber spray may help somewhat. 

Any object left inside the cabinet must Not be hollow, fill it with sand, if nothing else!

+1 Audiotweak

Best cutting edge advise so far

The last thing you want is flexible materials. They will increase your internal volume.

If you want to permanently increase it, and don't have room for the usual brick / wood block ideas I suggest a 2-part mixable epoxy that dries hard.

You may also experiment less permanently with buck shot. Placed in bags it's relatively easy to arrange and move.

Styrofoam will do exactly what you need it will  not absorb no need to coat it and best of all you can infinitely tweak by cutting off material  until you have the correct volume before you make anything permanent. 

Best of luck

Peter

ivan_nosnibor,

Thank you for that info. I was thinking of just using the flex seal over the Styrofoam IF the Styrofoam alone would act as an absorbent. But it sounds like it does not.

theaudiotweak,

I will order some of the Cascade to be used as a inside lining.

eric_squires,

Wouldn't a mixable 2 part epoxy require putting it into some type of mold? What type of thickness do you suggest?

pbn,

Not sure what type of Styrofoam thickness I should be using. I was thinking 2-4", does that sound about right? I'll try Hobby Lobby first.

ozzy