There's a small room in Minnesota that blocks out 99% of all external sound. That's an impressive number! Also impressive: nobody can take more than 45 minutes alone in the room before they go nuts.
The Daily Mail describes Orfield Labs' anechoic chamber—perfect for making extremely sensitive audio measurements. But also perfect for sending you into a hallucinatory hell so hellacious you'll need a chair:
‘When it's quiet, ears will adapt. The quieter the room, the more things you hear. You'll hear your heart beating, sometimes you can hear your lungs, hear your stomach gurgling loudly. ‘In the anechoic chamber, you become the sound.' And this is a very disorientating experience. Mr Orfield explained that it's so disconcerting that sitting down is a must. He said: ‘How you orient yourself is through sounds you hear when you walk. In the anechnoic chamber, you don't have any cues. You take away the perceptual cues that allow you to balance and manoeuvre. If you're in there for half an hour, you have to be in a chair.'
That sounds swell. Just the serene quiet of you, your thoughts, and the unceasing pounding of the human heart. Your brain can't take it, apparently, and begins to fabricate sounds that aren't really there—completely delusional noises meant to block out the churning of your own horrid biomass.