In theory, a room in which the walls (ceiling, floor, etc) absorb all sound. I want to build one someday, yet I fear that it may be hard to walk around inside one, since such surfaces aren't really designed for walking on.

Additionally, it appears that a similar technique works for EM radiation. A google search seems to reveal many interesting links.

In the ENGRI 118 - Design and Integration of a CD Player class of Cornell University (which both me and Uberfetus had to suffer through), one of the first labs we had to perform was a test of the dynamic range of the human ear, about the difference in sensitivity of the ear at different frequency levels. We performed this in an anechoic chamber.

Stepping into the little room and seeing the huge door slam shut behind you is pretty disturbing, add to the fact that the inside of the room was eerily quiet, you can hear your own heart beating. Any sound has multiple echoes, and it does not sound like a usual echo, it is hollow and tinny. As you walk around you can not only feel but hear all your movements. It is pretty weird inside one of those things. People with claustrophobia will die than prefer to be in one of those things.

Of course it is not a true anechoic chamber, but it gets pretty close. The thick spongy triangular shaped material covers the whole chamber except for the floor, which has a thick concrete base to block off noise.

After doing the experiment, which involved pressing a button when you hear a beep, I stepped out of the chamber. Even though I was not suffocating in there, I took a deep breath of air and sighed in relief, for some reason. Being inside of an anechoic chamber gives you a truly strange sensation, it is almost like an alternative dimension.

Anechoic chambers are also used for EM waves. The goal is to reduce the waves in the frequency of interest. These rooms are used to test antennas. They keep out interference from the outside and keep in signals from the antennas. Usually the electromagnetic anechoic chambers are completely surrounded in metal sheeting. Cheaper rooms are made using standard drywall with a sheet of aluminum foil covering it. All the metal sheeting must be connected together with no cracks or openings so as not to allow any EM waves to come in or leave the room. More expensive chambers use plywood or particle board with 1/16" metal sheeting glued to both sides and use metal connectors to attach the panels together. The insides are usually covered in special foam pyramids painted blue. The foam partially absorbs the incoming waves and reflects some of the power back out. The shape of the cone usually directs the reflected power toward another cone which absorbs and relects until any signal left will be so low power that is will be undetectable.

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