Hello dave. There is NO cheap way to stop sound from entering. Low frequency traffic rumble especially. Basically, there are only 2 methods. Mass and decoupling. In this case mass will help with the mid and high frequency, but traffic rumble is transmitted directly through the ground and up into the building. Only decoupling a room will help that, which basically translates to floating a room within a room. But this is all academic without exact information on your situation. But I can tell you this. Increasing sound transmission loss is NOT cheap. Nor easily accomplished. And if your in an apartment.....and can't build anything within nor attatch anything to anything, there is very little you can do for this type of sound intrusion, short of building a freestanding booth, or purchasing one. My .02
fitZ
