Audio Engineering

38 Chapter 2

For a power of  12 dBm:

0.001 10 0.00006309 W.

1012

    



The voltage across 600 Ω is

EWR

   



0 00006309 600

0 195

.V.

Note that this  12-dBm power level can appear across any impedance and will always
be the same power level. Voltages will vary to maintain this power level. In constant-
voltage systems the power level varies as the impedance is changed. In constant-
current systems the voltage changes as the impedance varies (i.e.,  12 dBm across
8 Ω 0 00006309..V). 8 0 022

2.2 Measuring Electrical Power .....................................................................................

WEl cosθ (2.8)^

WZI^2 cosθ (2.9)^

W

E

Z



2

cosθ

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 ofR (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

W E

Z



2

cosθ.