Many foams, even those called acoustic, are not very effective. There are exceptions, such as Auralex, which have a high co-efficient, but these also tend to be costly.
One of the highest co-efficient materials also happens to be dirt cheap: 6-8 pound per cubic foot density mineral wool board. You can purchase this material in bails of 6 pieces of 2" thick x 24" wide x 48" high for about $35-$40 at your local commercial insulation contractor. 2" thick piece against a flat surface will absorb nearly 100 percent of acoustic reflections from about 250Hz on up. A 4" piece(2 x 2" pieces) will absorb down to 140Hz or so, nearly 100 percent of reflected acoustic energy.