Zero is not nothing

Decibels are odd things and you would think that zero decibels, 0dB, would mean silence but nope, that's way too obvious.

In a hearing test, the scale goes down to -10 or -20dB depending on the equipment being used and a lot of people can hear down to that level.

Audiogram example

Zero decibels was set by taking a reference group of people and finding the quietest sound they could hear and that was set as zero, meaning 0dB is considered to be the 'normal' threshold of human hearing. Zero decibels therefore is not an absence of sound but is in effect the reference point from which we assess whether hearing is deteriorating or not. 

As with any group-study though, there are always people who can hear better than the average and some who hear worse and typically it is young people who hear the best. Young women up to their early 20s particularly are usually very good with hearing levels that can hit -5, -10 or even -15dB at some frequencies, which then, like men, starts to drop away as they get older.

The theoretical value for complete silence by the way is -23dB, although nobody has ever measured that or got a room to achieve that level.