I've been in one. Back in HS, we took a trip to the GIANT anechoic chamber at Georgia Tech. It was incredible. If someone was facing away from you and yelling as loud as they could, you could barely hear it from behind them. You're standing on basically a giant net on top of a pit of foam blocks, and every wall is wildly shaped undulating foam.

It's really surreal and a number of people actually became nauseated at the sensation.