"Bus-proof" Iso booth

Hi, I just got my new place. It's on a cobblestone street, and a bus line goes by this street, which means lots of noise (air transmitted and mechanically transmitted). My iso booth (2m by 2m, mostly singers) will be less than 20 meters from the passing buses (actually, nothing in this room will be farther than that!). The floor is ceramic tiling over a concrete slab. I will float the iso booth, but I'm concerned with the stiffness of the neoprene that i will put, I'm affraid of ending up at one end of its flexibilty range. So this came to my mind: what if I lay 2 pieces of neoprene with different stifnesses? Thanks

Different stiffnesses will mainly make it harder to figure out how many pucks you need for your booth - have you checked out the REFERENCE section, specifically the first page of Aaron's floating floor thread http://www.johnlsayers.com/phpBB2/viewtopic.php?t=839 Where I outlined a method of getting your compression right for ANY material? Steve