Audio power
Continuous average sine wave power ratings are a staple of performance specifications for audio amplifiers and, sometimes, loudspeakers.
[11] 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.
This rule prescribes continuous power measurements performed with sine wave signals for advertising and specifications of 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), usually 1%, into specified load resistances.
[14] 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.
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.
The peak power of an amplifier is determined by the voltage rails and the maximum amount of current its electronic components can handle for an instant without damage.
This characterizes the ability of equipment to handle quickly changing power levels, as many audio signals have a highly dynamic nature.
[citation needed] Shelf stereos and surround sound receivers are often rated using total system power.
The approximately logarithmic relationship between power and perceived loudness is an important factor in audio system design.
A good measure of the 'power' of a system is therefore a plot of maximum loudness before clipping of the amplifier and loudspeaker combined, in dB SPL, at the listening position intended, over the audible frequency spectrum.
The human ear is less sensitive to low frequencies, as indicated by equal-loudness contours, so a well-designed system should be capable of generating relatively higher sound levels below 100 Hz before clipping.
The sensitivity is measured at 1 watt to minimize nonlinear effects such as power compression and harmonic distortion, and averaged over the usable bandwidth.
This beaming tends to make high frequency dispersion problematic in larger venues, so a designer may have to cover the listening area with multiple sources aimed in various directions or placed in various locations.
When the distance to boundaries is > 1/4 wavelength, delayed reflections can increase the perceived loudness but can also induce ambient effects such as comb filtering and reverberation that can make the frequency response uneven across a venue or make the sound diffuse and harsh, especially with smaller venues and hard reflective surfaces.
Charles "Chuck" McGregor, while serving as senior technologist for Eastern Acoustic Works, wrote a guideline for professional audio purchasers wishing to select properly sized amplifiers for their loudspeakers.
[27] JBL, which tests and labels their loudspeakers according to the IEC 268-5 standard (called IEC 60268-5 more recently) has a more nuanced set of recommendations, depending on the usage profile of the system, which more fundamentally involves the (worst case) crest factor of the signal used to drive the loudspeakers:[28] Active speakers comprise two or three speakers per channel, each fitted with its own amplifier, and preceded by an electronic crossover filter to separate the low-level audio signal into the frequency bands to be handled by each speaker.
This approach enables complex active filters to be used on the low level signal, without the need to use passive crossovers of high power handling capability but limited rolloff and with large and expensive inductors and capacitors.
This is relevant when peaks of comparable amplitude occur in different frequency bands, as with wideband percussion and high-amplitude bass notes.
Manufacturers who use different words such as pulse or performance may be reflecting their own non-standard system of measurement, with an unknown meaning.
However, a new Approved American National Standard ANSI/CEA-2006-B which includes testing & measurement methods for mobile audio amplifiers is being slowly phased into the market by many manufacturers.
