I believe you saw xps or “extruded polystyrene”
At the depot they are 4’x8’pink or blue boards
Not to be confused with eps or “expanded polystyrene” which is usually the white boards that seem to beak off in white beads floating through the air.
Neither of these are ideal for broadband absorption but might absorb a touch of bass. I’m guessing most midrange and high end would reflect back but I’ve never tested (measured) with them.
I agree with kota 1 just find some owens 703 it will be sold at any insulation supply center.