Fan noise - sponge to sound deaden?

I had a few pieces of sponge 2'x2'x2" laying around and wondered if they'd deaden the sound of my PC in the closet - a piece behind it against the wall and the side of it across from the intake fans killed some of the noise - they're not impeding airflow. Definitely seemed to kill some noise reflecting off the walls.

Anyone else used this to kill some of the fan noise? Egg carton foam maybe?

somegeek

Out of curiosity I bought some of that "Spire" Sounddeadening material online (same as the Akasa Paxmate) and all it is is a really dense, adhesive backed foam rubber. I guess it has enough gas trapped in it and is dense enough to do the job, because it killed pretty much all the vibration in my case, and reduced the noise by around 5db (my guesstimate).
Nonetheless, its the same principle as what all the luxury car makers do...lets line our Lexus with 50lbs more than the Toyota version and people will think it's better built! The stuff makes ur case feel like a vault, solid and quiet, but heavy and, well, heavy. It also is an insulator, so not the best if you want the maximum in cooling...

I used a 2" thick mattress pad to sound insulate my desk cabinet (whrer my case resides)-

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