Audio power meter

Most audio output measurements are concerned with the voltage delivered across the loudspeaker by the output stage. If the loudspeaker impedance were known, it would be easy to calculate the power. But the impedance is known only approximately and, moreover, varies with frequency. Measuring both the current through, and the voltage across, the loudspeaker voice coil is, therefore, a far more reliable method of determining the power delivered to the loudspeaker.