If you play metal with high-gain on a guitar with a tremolo system, you will have faced the ringing spring issue: you mute the strings but it continues ringing after the mute... that's because the springs in the back of the guitar are sympathetically ringing.
Various solutions exist to fix this issue, usually involving foam that you stuck in there or special coated springs, etc...
I wanted to tried this rather cheap and clean solution. You will receive six dampeners in total, and I would recommend using two per spring to completely dampen the ringing. With only one, it was not enough. So with one pack, you can successfully dampen one guitar ig it uses three springs.