sound loss by filling air gap between dual foundation

Can some one explain to me in simple terms what the effects of filling the gap between the two foundations (ring and slab) with polyeurethane expanding foam. These two foundations are both well below ground level, from one foot in places up to five foot. There seems to be some concern that it will interfere with the mass spring mass - how much?? The manufactures of the foam mantain that the foam priovides sound deadening properties upto 80 dB. TIA :)

Expanding PUR will, AFAIK, couple the slabs instead of decoupling 'm. All the acousticians I met will immedately shoot you when you come within 1 km from a building with expanding PUR-foam. :D

bert Stoltenborg wrote:

Expanding PUR will, AFAIK, couple the slabs instead of decoupling 'm. All the acousticians I met will immedately shoot you when you come within 1 km from a building with expanding PUR-foam. :D

I hear you loud and clear!! I'm just not clear WHY :?: :?: