https://www.youtube.com/watch?v=mXVGIb3bzHI This guy stayed in there for barely over an hour. He didn't say anything about hallucinations, only that he could hear his own heartbeat and it was a good place to think.
the one i know of is at UC California at Berkeley, apparently you can hear distant earthquakes and underground noises occasionally as well as your own blood and heart beat. I've never heard of hallucinations and I think it is possible to stay in for much longer although it is probably difficult. Most babies refuse to go in and start crying instantly though. Also, when someone else in the room speaks to you, they have to look directly at you because otherwise they will sound very quiet as the room absorbs the noise.
http://en.m.wikipedia.org/wiki/Dynamic_range
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