Located in Building 87 of Microsoft’s Redmond, Washington campus is the “anechoic chamber.” Its name means “echo free,” and that is precisely what it is. The room is used for testing Surface tablets and digital personal assistant Cortana. It has been designed to achieve as close to silent as possible.
Underneath the largest chamber, there’s an entirely separate foundation from the rest of Building 87. The room sits on springs and has an air barrier from the rest of the structure. The surfaces are covered with special sound-absorbing wedges that prevent echos and produce a disorienting feeling for the room’s occupants. A reporter who went into one of the chambers wrote that voices sounded clipped, because they lacked the barely perceptible echoes that normally accompany speech. “My own voice sounded like it was having trouble coming out of my head,” Wilson Rothman wrote. “For a moment, I felt genuine disorientation, like the light-headedness you can get with low blood sugar.”
In theory the quietest sound produced is Brownian motion, the sound of atoms bouncing off each other. This has been estimated at -23 decibels. The anechoic chamber measures at -20.6 decibels. To put this in contrast, normal human speech is in the neighborhood of 60 decibels, and normal breathing is 10 decibels.