Power and Electricity
All electrical systems need a reference voltage; a good reference voltage is 0 (zero) volts. Ground is defined as the 0-volt reference in a circuit. In AC power systems, we use the earth as our reference voltage because it almost always remains at 0 volts. The voltage at each point in an AC electrical system is measured relative to the earth ground. In an audio circuit, the voltage at each point in the circuit is measured relative to a 0-volt reference called ground, whether or not it is attached to the earth.