Measuring Electrical Power
where W is the power in watts, E is the electromotive force in rms volts, I is the current in rms amperes, Z is the magnitude of the impedance in ohms [in audio (AC) circuits Z (impedance) is used in place of R (AC resistance)], and θ is the phase difference between E and I in degrees.
These equations are only valid for single frequency rms sine wave voltages and currents.
2.2.1 Most Common Technique
1. Measure Z and θ.
2. Measure E across the actual load Z so that
Expressing Power as an Audio Level
The reference power is 0.001 W (1 mW). When expressed as a level, this power is called 0 dBm (0 dB referenced to 1 mW).
Thus to express a power level we need two powers—first the measured power W1 and second the reference power W2. This can be written as a power change in dB:
