Anechoic chambers are pretty damn awesome. Basically, they're rooms designed to be sound-proofed against outside noise, while, inside, sound is prevented from bouncing off the walls. There's no echo. There's a number of ways you can build this, but one system at the University of Salford in England, is actually a room within a room, with the innermost chamber actually mounted on springs, rather than the floor of the outer room.
Anechoic chambers are often used to test out audio equipment or to get accurate audio measurements on systems that are supposed to operate very quietly.
Minnesota Public Radio recently went inside the room that holds the title for world's quietest—an anechoic chamber at Orfield Laboratories in Minneapolis.
To get into the anechoic chamber, you go through two bank vault-like doors. The floor in the room is mesh like a trampoline so there's nothing on the floor for the sound to bounce off of. The walls are lined with sound-proofing wedges that are a meter long so they absorb the sound.
...A typical quiet room you sleep in at night measures about 30 decibels. A normal conversation is about 60 decibels. This room has been measured at -9 decibels.
Image: Photo of an anechoic chamber taken at the Kyushu Institute of Design's anechoic chamber by Alexis Glass. Free to use under GDFL.