Sine Wave Power
The term sine power is used in the specification and measurement of audio power. A meaningful and reliable measure of the maximum power output of an audio amplifier – or the power handling of a loudspeaker – is continuous average sine wave power. The peak power of a sine wave of RMS value X is √2*X; conversely, the RMS value of a sine wave of peak X is (1/√2)*X. For a resistive load, the average power is the product of the RMS current and RMS voltage.
Harmonic distortion increases with power output; the maximum continuous power output of an amplifier is always stated at a given percentage of distortion, say 1% THD+N at 1 kHz. Considerably more power can be delivered if distortion is allowed to increase; some manufacturers quote maximum power at a higher distortion, like 10%, making their equipment appear more powerful than if measured at an acceptable distortion level.
In the US on May 3, 1974, the Amplifier Rule CFR 16 Part 432 (39 FR 15387) was instated by the Federal Trade Commission (FTC) requiring audio power and distortion ratings for home entertainment equipment to be measured in a defined manner with power stated in RMS terms. (See more in the section Standards at the end of this article). The erroneous term "watts RMS" is actually used in CE regulations.
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