poisednoise

The article implies that an anechoic chamber and a soundproof room are the same thing - they're not. There's often overlap: anechoic chambers have to be soundproof, but not the other way around.

An anechoic chamber has zero reverberation - that is, any sound is absorbed completely by the walls. A soundproof room can be highly resonant, soundproof just means no sound gets out. Recording studios tend not to be anechoic (although the vocal booth frequently gets close) but they are always soundproof.

To all intents and purposes a wide open space at altitude is anechoic - the sound has nothing to bounce off, so there's no return - but it's definitely not soundproof!