When it comes to soundproof chambers, the quietest place on earth is a studio at Orfield Labs. Quoting from this article from the Smithsonian Magazine, the studio is So silent that the background noise measured is actually negative decibels, -9.4 dBA. The studio is in fact in the Guinnes book of records for being 99.99% sound absorbent. Yes, staying in the room for more than an hour will drive you crazy.
Below is an image of the lab, and its record-winning certificate (courtesy of Daily Mail Online):
