Ohm's Law
Definition
Ohm's law states that the current through a conductor is proportional to the voltage across it: V = I × R, where V is the voltage in volts, I the current in amperes and R the resistance in ohms. Combined with the definition of electric power, it gives P = V × I = V²/R = I²×R. At constant voltage, doubling the resistance halves the current. In alternating-current circuits and loudspeaker systems, the resistance is replaced by the impedance, which varies with frequency. Identical loads connected in parallel divide the impedance by their number, so four 8Ω loudspeakers in parallel present 2Ω to the amplifier.
In the field
The total impedance of a loudspeaker array is calculated before connection and compared with the minimum load stated by the amplifier manufacturer. A 2Ω load on an amplifier rated for 4Ω minimum draws about twice the current the amplifier was designed to deliver, which leads to overheating or protection shutdown at high output.
Sources
- IEC 60050-131 (Electropedia), IEV 131-12-04 : résistance · electropedia.org
- IEC 60050-131 (Electropedia), IEV 131-12-43 : impédance · electropedia.org
- Ohm's law (Wikipedia, pointeur secondaire) : V = I × R et formes de la puissance P = VI = V²/R = I²R · en.wikipedia.org
- Ballou, Handbook for Sound Engineers : impédance des haut-parleurs et association en parallèle