Orfield Laboratories has a room in their South Minneapolis facilities that has held the Guinness World Record for 'World's Quietest Room' since 2004. The room is built with fiberglass acoustic wedges that are 3.3-foot thick, and it has double walls of insulated steel and one-foot thick concrete. This makes it so that the room can absorb 99.99% of outside sound.
The room is used by various companies to test the loudness of their products without any outside distractions or sounds. This is a really good way to test if your company's latest phone is too loud, for example. The downside is that the room can drive you crazy. The laboratory says that the longest someone's been able to stay in the room is 45-minutes. But why?
Your body adapts to the silence. Your ears start picking up your heartbeat, your stomach or your lungs to levels that they aren't used to. The lack of sound also messes with your sense of perception. This is because our inner ear helps reach balance through sound cues when you walk. They say that people who are in there for half an hour have to be in a chair.
Source: Anechoic chamber