Sound deadening

Hi, so I was looking into putting some sound deadening in the car as the decat+non resonated cat back sounds awesome, but it's just a bit droney inside. I saw some sheets of sound deadening foam/material on eBay that looked promising. Anyone done anything like this? And if so what products/types did you use? Cheers in advance guys
 

Well I was thinking put it underneath seats and boot mat, And shape it around the fixing points etc. Might look into putting on the exterior too then, Cheers for the links bud
 
So a follow up on this... marginal difference to the droning noise, easier to maintain a conversation inside the car... but no a massive difference to be honest.