[John Casler]

Acording to MicroSoft Encarta:

Human ears are capable of perceiving an extraordinarily wide range of changes in loudness, the tiniest audible sound being about 1 trillion times less intense than a sound loud enough to cause the ear pain.

[Rob Babcock]

That makes sense- we generally discuss SPL in dB/a, like that's a unit, but it's really a logrithmic ratio, not a unit of sound, per se.