Editing Audio power (section)

=== Continuous power and "RMS power" {{anchor|RMS}}===
{{see also|Root mean square#Average power}}
[[File:RMS voltage average power.svg|thumb|A voltage waveform and its corresponding power waveform (resistive load).  RMS voltage in blue, peak power in red, average power in green.]]
''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,<ref>{{cite web|url=http://www.bcae1.com/speakrat.htm|title=Speaker Ratings|website=Basic Car Audio Electronics|access-date=2016-04-22}}</ref> 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).<ref>{{cite web|url=http://www.eznec.com/Amateur/RMS_Power.pdf|title=RMS Power|last=Lewallen|first=Roy|date=2004-11-18|quote=The RMS value of power is <u>not</u> the equivalent heating power and, in fact, it doesn’t represent any useful physical quantity.}}</ref><ref>{{cite web|url=http://www.hifi-writer.com/he/misc/rmspower.htm|title=Why there is no such thing as 'RMS watts' or 'watts RMS' and never has been|last1=Unknown|last2=Dawson|first2=Stephen|website=Hi Fi Writer|access-date=2016-04-22|quote=By contrast, RMS (root mean square) power, would have to be defined as the square root of the time average of the square of the instantaneous power, since this is what 'RMS' means. This ''could'' be done, but it is ''not'' the power as measured, and furthermore, it would have ''no'' technical significance (e.g. it doesn't measure heating power).}}</ref><ref>{{cite web|url=http://www.n4lcd.com/RMS.pdf|title=What's RMS Power or RMS Watts?|last=Quillen|first=Paul|date=1993|quote=the Voltage that's measured is RMS Voltage, but the resulting power is Average Power and it's measured in Watts.}}</ref><ref>{{cite web|url=http://www.doctorproaudio.com/doctor/temas/powerhandling.htm|title=Speaker power handling < Pro-Audio References|website=www.doctorproaudio.com|access-date=2016-10-28|quote=Often wrongly referred to as "RMS" power, since it is derived from RMS voltage readings.}}</ref>  The erroneous term "watts RMS" is actually used in an ANSI standard. This is also referred to as the [[nominal value]], there being a [[certification mark|product mark]] requirement to use it.<ref>{{citation|title=CEA-2006-A, Mobile Amplifier Power|url=http://www.ce.org/Standards/3108.asp|quote=Example of Product Mark Usage [...] Power Output: 30 Watts RMS|archive-url=https://web.archive.org/web/20110722193247/http://www.ce.org/Standards/3108.asp|url-status=dead|access-date=2011-08-13|archive-date=22 July 2011}}</ref>

''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<ref>{{cite web|url=https://www.ftc.gov/enforcement/rules/rulemaking-regulatory-reform-proceedings/amplifier-rule|title=Amplifier Rule 16 CFR Part 432 {{!}} Federal Trade Commission|website=www.ftc.gov|date=28 September 2014 |access-date=2016-10-28|quote=rated minimum sine wave continuous average power output, in watts, per channel [...] at the impedance for which the amplifier is primarily designed, measured with all associated channels fully driven to rated per channel power}}</ref><ref>{{citation|title=39 FR 15387|url=http://cfr.law.cornell.edu/cfr/cfr.php?title=16&type=part&value=432|archive-url=https://web.archive.org/web/20051130103725/http://cfr.law.cornell.edu/cfr/cfr.php?title=16|url-status=dead|archive-date=November 30, 2005}}</ref> 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.<ref>{{cite web|url=http://www.ti.com/product/LM4753|title=LM4753 Dual 10W Audio Power Amplifier|website=www.ti.com|access-date=2016-10-28|quote=capable of delivering 10W/channel at 10% distortion|archive-url=https://web.archive.org/web/20161028153858/http://www.ti.com/product/LM4753|archive-date=2016-10-28|url-status=dead}}</ref>

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&nbsp;dB crest factor [[Square wave (waveform)|square wave]]s) 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&nbsp;Hz to 20&nbsp;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 [[watt]]s 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.<ref>{{cite web|url=http://www.linkwitzlab.com/thor_splmax.htm|title=Electrical and mechanical limits|website=www.linkwitzlab.com|access-date=2016-10-28|quote=It should be clear [...] that amplifier power is not an issue at the low frequency end of the subwoofer range, but at high frequencies. The lowest frequency output is driver excursion limited.}}</ref>