Continuous average sine wave power ratings are a staple of performance specifications for audio amplifiers and, sometimes, loudspeakers.
As described above, the term average power refers to the average value of the instantaneous power waveform over time. As this is typically derived from the root mean square (RMS) of the sine wave voltage, it is often referred to as "RMS power" or "watts RMS", but this is incorrect: it is not the RMS value of the power waveform (which would be a larger, but meaningless, number). (The erroneous term "watts RMS" is actually used in CE regulations.) This is also referred to as the nominal value, there being a regulatory requirement to use it.
Continuous (as opposed to "momentary") implies that the device can function at this power level for long periods of time; that heat can be removed at the same rate it is generated, without temperature building up to the point of damage.
On May 3, 1974, the Federal Trade Commission (FTC) instated its Amplifier Rule to combat the unrealistic power claims made by many hi-fi amplifier manufacturers. This rule prescribes continuous power measurements performed with sine wave signals for advertising and specifications of amplifiers sold in the US. (See more in the section Standards at the end of this article). This rule was amended in 1998 to cover self-powered speakers such as are commonly used with personal computers (see examples below).
Typically, an amplifier's power specifications are calculated by measuring its RMS output voltage, with a continuous sine wave signal, at the onset of clipping—defined arbitrarily as a stated percentage of total harmonic distortion (THD), usually 1%, into specified load resistances. Typical loads used are 8 and 4 ohms per channel; many amplifiers used in professional audio are also specified at 2 ohms. 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.
Continuous power measurements do not actually describe the highly varied signals found in audio equipment (which could vary from high crest factor instrument recordings down to 0 dB crest factor square waves) but are widely regarded as a reasonable way of describing an amplifier's maximum output capability. For audio equipment, this is nearly always the nominal frequency range of human hearing, 20 Hz to 20 kHz.
In loudspeakers, thermal capacities of the voice coils and magnet structures largely determine continuous power handling ratings. However, at the lower end of a loudspeaker's usable frequency range, its power handling might necessarily be derated because of mechanical excursion limits. For example, a subwoofer rated at 100 watts may be able to handle 100 watts of power at 80 hertz, but at 25 hertz it might not be able to handle nearly as much power since such frequencies would, for some drivers in some enclosures, force the driver beyond its mechanical limits much before reaching 100 watts from the amplifier.