My only two criteria for sound proofing the Antec case were that the sound absorbing material would have to be less than 3/4" thick, and not too expensive. With constraints like that, more than half of the commercially available sound absorbing foams and pads were out of the question.

Many of them were hundreds of dollars a sheet, and 3"-6" thick for the really good material.Some 'Melamine Foam Sound Absorbing Ceiling Tiles' I stumbled upon caught my eye because it had a listed NRC value of 0.95, (where 1.0 is the most sound absorbing you can get), was just 1/2" thick, and was reasonably priced for a good-sized sheet...