Originally Posted by Q.E.D.
20 dB is pretty quiet. It's about the level of background noise you'd expect in typical library during off-peak hours.
That's very, very optimistic. In our recording studio control room, the background noise is about 30dB. This is a rather typical, if slightly high, value. It is nevertheless very, very quiet. The noise criterion
standards give recomendations of 40 to 50 dBA for libraries and and 25 to 30 dBA for recording studios.
There is a difference between talking about ambient noise levels and the loudness of a particular source. If I say that my studio has a background noise level of 30 dB it means that if I stand somewhere in that room, the sound level is going to be at least 30 dB.
If I say that rustling leaves are about 20 dB, it means in practice... nothing. A jet taking off is 20 dB, and so is a gun going off, and the Krakatoa eruption... at the right distance. A noise measurement for a point source without a distance has no practical meaning, inverse square law and all.