Orfield Laboratories in Minneapolis has an anechoic chamber that has been certified as the quietest place in the world. It is so quiet that you can hear your blood flowing in your veins and your organs doing their thing. 99.99% of all sound created gets absorbed into the sound dampening and there is no echo. They say being in the room is exceptionally unnerving. The longest anyone has been in the space by themselves is 45 minutes.
The room is used by product designers to test the sounds of their machines.