True silence will freak you out. Anechoic chambers are spooky; it feels as if something is being stripped bodily away from you. But they aren't "silent" because there's always something going on in them. A firm I worked for did design work for Paul Allen. He has a studio. And for some reason, he decided his drum room needed to be NC5. Here's a Noise Criterion explanation. You'll note it doesn't go down to NC5. That's because the presence of a human in an NC5 environment pushes it to NC10, even if they hold their breath. To measure an NC5 environment you have to put your device on a timer and close the door. It was the first time I'd been exposed to something that quiet. It's actually oppressive. You hear... you. If it was something you'd run across by accident you'd find it quite uncomfortable.