The term “ground” is used broadly in audio to describe an electrical or electronic conductor that is connected to the earth; usually indirectly. This is typically done for safety reasons in electrical connections, and for many different reasons in electronics; including safety. The DC power used in audio electronics is almost always referenced to ground by connecting the “zero volt” circuit of the power supply to the chassis and AC power ground. The connection is either direct at one carefully chosen place or through a high-current low impedance resistor or inductor to allow de-coupling at radio frequencies.
In many electrical systems; one conductor of the electrical supply is connected to the earth by a stake driven into the ground near the power transformer that supplies power to the building. In a basic two-phase system typically used in residential wiring in the USA, a large wire connects the stake to the center tap of the power transformer secondary coil, and the two ends of the secondary coil supply the “hot” voltages to the building. Another grounding stake is located near the circuit breaker box of the building; and a large wire is used to connect the stake to the ground buss bar inside the circuit breaker panel. The two “hot” conductors carry voltage waveforms that are of opposite polarity. High powered devices like ovens and heating units are connected to these two conductors through circuit breakers, and receive 230 volts AC (the voltage difference between the two voltage waveforms).
This is analogous to balanced audio in that neither signal is “ground” and the receiving device “sees” the voltage difference of the two conductors without a reference to ground.
For low powered devices and lighting in a residence; only one “hot” conductor is connected through a circuit breaker to the outlets or fixtures, and the second conductor is referred to as “neutral” (or “cold”) because it is connected to ground ONLY at one point in the circuit breaker panel. This is necessary for safety reasons because, although the neutral conductor is at “ground” voltage potential; it carries the “full power” of the current used by the devices or lamps (referred to as the “load”) on that circuit. The “load sees” the voltage difference between one hot conductor and “ground;” or 115 volts AC.
This is analogous to unbalanced audio; where one conductor serves as both the ground reference between devices and the signal return. The shock hazard and risk of damage is low due to the low voltages and currents involved.
Contemporary audio devices can contain both analog and digital circuitry. Because a ground conductor has finite resistance and can carry currents returning from different circuitry to the power supply; the physical layout of “ground” connections is critical to good performance.
Even though two audio devices have circuitry referenced to ground; small “leakage” currents in their power supply can cause current to flow between the devices via cables that interconnect them. This one of the primary reasons why the shield of an audio cable is not used as a signal conductor in balanced audio. The other reason is that electrostatic interference can induce noise currents on the shield conductor; which can be amplified by the audio input if the shield is also used an audio conductor (as in unbalanced coaxial audio connections).
Ground currents that flow on the interconnects can also result in "hum" in the audio or unreliable operation in digital audio interfaces. Please see ground loop for more information on this subject.