The Decibel
It is useful to understand the use of the decibel. It is simply a convenient way of expressing differences in signal levels. In terms of power, if a signal is reduced to half power we say “it is -3 dB”. If a signal is reduced to half voltage, we say “it is -6 dB”.
A decibel is also convenient for expressing ratios, such as signal to noise ratio. A 20 dB signal to noise ratio sounds like listening to music on a car radio speaker with the windows open and a jack hammer operating close by, or that a 90 dB ratio allows us to listen to even the quietest passages of a symphony with little disturbance.