A student brought me an article on Orfield Laboratories from the Minneapolis Star Tribune. Unfortunately, the four-day-old article has already disappeared from the archives, but I can summarize it for you. This lab, which consults in sound engineering issues, has an anechoic chamber used to test microphones, speakers, hearing aids, and other sound equipment. This anechoic chamber has recently been designated as the Quietest Place on Earth by Guinness World Records, after engineers measured the room at negative 9.4 dB (decibels). The threshold for human hearing is placed at 0 dB, so this room has an absence of sound "so profound that a person standing in the room for more than a few minutes would begin to hear his or her own ear making noise as their brain struggled to understand what was happening." (from the article by Matt McKinney) There is no sense of space, because reverberation has been removed. The article mentions that John Cage was inspired by an experience in Harvard's anechoic chamber to compose 4'33". Sitting in the chamber, Cage could still hear sounds, caused by his own nerves (high pitch) and circulation system (low pitch). The owner of Orfield Labs suggests that the higher pitched sound is caused by the brain causing the ear drum to buzz, called an auto-emissive sound.
The room is constructed with 1 foot thick concrete walls. Inside these walls, a smaller room floats on springs. This smaller room has fiberglass acoustic tiles 3.3 feet thick. The result is a reduction of 150 dB from the outside world to the chamber. (This fact is from the article, but I don't understand how this was determined. Outside ambient noise should not be in the 140 dB range, 10,000 times more intense than the pain threshold. Plus, intensity is inversely related to the square of the distance, so are we comparing the sound intensity at the source to the sound intensity within the chamber, or the sound intensity from an outside location equally distant from the source as the chamber?)
The chamber sits in the same location as the now-defunct Sound 80 Studios, who pioneered digital recording with the St. Paul Chamber Orchestra ("Appalachian Spring"), Flim and the BB's, and Bob Dylan ("Blood on the Tracks"). The owner of Sound 80 Studios, Herb Pilhofer, composed the theme music for the Montreal Olympics.
Update: Owner Steve Orfield has sent me a link to the original article in the Star Tribune, and a brochure for the Lab. The brochure has a good picture of the anechoic chamber, showing the freakin' huge acoustic tiling. Many thanks to him. This interwebs thing provides some very cool interactions.