This room, the ‘Anechoic Chamber‘ at Orfield Laboratories in the US.
Apparently it’s so quiet, staying in there for a time will drive you insane. Nobody has been able to be in the room for more than forty five minutes. The only noises you hear emanate from your own body. Fluids gurgling, joints creaking, your breathing and your heart pumping.
It’s 99.99 % sound absorbent and holds the Guinness World Records for the world’s quietest place, but stay there too long and you may start hallucinating.
It achieves its ultra-quietness by virtue of 3.3-foot-thick fiberglass acoustic wedges, double walls of insulated steel and foot-thick concrete.