I'm just wondering why I have seen new homes built with fiberglass between the joists on the inside (e.g. upper floor or ground floor with insulated basement below) or in internal stud walls. Is this purely to reduce sound transmission? Reason being, I'm installing wood flooring upstairs in my 1930's house, and wondering whether I should lay fiberglass between the joists for sound proofing? It seems like a substantial expense for minimal gain.