I've just installed my desktop PC case in a cupboard in my desk and, predictably, was suffering a lot of noise as the three hard disk drives set up a resonance in the enclosed area.
This mat just stopped that dead.
The mat was a couple of inches too long for the space, but easily trimmed to size with a Stanley knife, and I used the spare to make four small footpads for the case feet, which I put on top of the mat, giving a double layer of mat under the feet.
It's gone from very loud vibration/resonance noise to silence, as quickly as that.
My PC case is already lined with sound-deadening sheets, which as far as I can tell have never made it quieter, so I wouldn't bother with those. But if your PC is noisy, I'd suggest putting it on one of these mats as the first thing you try.