ok had a look at this link and it seems to be for acoustic foam. it says on their site:

Acoustical foam (and fiberglass and acoustical blankets and mattresses and curtains, and so on) is an acoustical absorber. Foam is ideal for improving the sound in the room, but does little to stop sound from leaking into or out of a room.

I definitely need to know about reducing the sound leaking out of the room - not improving the accoustics in the room (although this will come later)