Wiki

Decibels (dB) are a measure of relative power, for example by what factor does an amplifier increase the power of a signal, or by what factor is a TV signal reduced by losses in the cable between the aerial and the receiver. It's a logarithmic scale, 3dB representing a doubling of power, hence the dB gains or losses in a signal path can simply be added. Note also that a doubling of the voltage into a given load also doubles the current, the power is increased four-fold, or by 6dB.