Ok. Typical foam won't work for low end, since even the deepest Fonac (75 mm) only absorbs 17% of the incoming energy at 125 hz. In comparison, a 50 mm fiberglass panel will absorb 30% at 125 hz, 75% at 250 hz and close to 95-100% at 500 hz and beyond.
Not only you need MASS, you also need DENSITY. A 20 cm concrete wall is great, but 20 cm of wood agglomerate would work tremendously well in low frequencies.
Let's break this down into numbers:
A panel of wood agglomerate has an α (absorption coefficient) of 47 (base 100, 0 means 0% absorption, 100 means full absorption) at 125 hz and 52 at 250 hz. In terms of absorption, nothing beats that except (guess what?), PEOPLE. People are incredibly dense.
Fiberglass insolation or glass wool (50 mm) has an α of 30 at 125 hz and 75 at 250 hz.
Typical fonac or sonex foam (75 mm) has an α of 7-13 at 125 hz. Is pretty much useless for low end.
So, this is what a good construction combination would be:
(TL means transmission loss, and it represents, on a base of 100, how good insolation provides at different frequencies)