Continuous Power
Continuous power ratings are a staple of performance specifications for audio amplifiers and, sometimes, loudspeakers. Continuous power is sometimes incorrectly referred to as RMS power and is derived from Root mean square (RMS), a method for measuring AC voltage or current.
In its 1974 Amplifier Rule meant to combat the unrealistic power claims made by many hi-fi amplifier manufacturers, the Federal Trade Commission prescribed continuous power measurements performed with sine wave signals on advertising and specification citations for amplifiers sold in the US. 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)—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.
Continuous power measurements do not actually describe the highly varied signals found in audio equipment but are widely regarded as a reasonable way of describing an amplifier's maximum output capability. Most amplifiers are capable of higher power if driven further into clipping, with corresponding increases in harmonic distortion, so the continuous power output rating cited for an amplifier should be understood to be the maximum power (at or below a particular acceptable amount of harmonic distortion) in the frequency band of interest. 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. The continuous ("RMS") value is also referred to as the nominal value, there being a regulatory requirement to use it.
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