Sound is measured in decibels (dB) because this is consistent with the way in which we hear sound. Decibels provide a manageable range to cover the range of audible sound levels (from 0dB to around 120dB) and correspond to the way in which we hear sound.
A decibel is one-tenth of a Bel, which is simply a logarithmic ratio to base 10. This means that a change of 1 Bel (10 dB) equates to a 10 fold increase in sound energy, so a 20dB increase equates to a 100-fold increase (from 10 x 10). A 3dB-increase equates to a doubling of sound energy.
The range of acoustic energy that we can hear covering around 120 dB equates to a ratio of 1012 i.e. the threshold of pain energy level is around one million million times that of the threshold of hearing (lowest sound level we can hear).