Is your residence a single tenant or multi family structure? It is quite common for apartments or condominiums to be stick construction but have gyp-crete (up to 2" or so) poured over a wood sub floor. It is not as inert as concrete on a steel sub floor (as
@KEW suggested) but is loads (no pun intended because, you know, concrete) better than a straight plywood on wood joists. I'd have to look up the STC, which may prove interesting.
EDIT: I reread your comment earlier - third floor, etc.,
