Noel you got it almost right. Threshold of hearing at the quietest end is roughly 20 micropascals. That's the reference which results in 0 dB. This is from a human hearing use of dB, whereas our VU meters are not referencing human hearing but voltage. Both uses employ dB because of the convenience of relating numbers across many orders of magnitude down to just a few orders of magnitude. VU use of dB has negative numbers in most of the range because the reference unit used is on the highest end of the scale rather than the lowest.