View source for Audio power

{{Sound measurements}}

'''Audio power''' is the [[electric power|electrical power]] transferred from an [[audio amplifier]] to a [[loudspeaker]], measured in [[watt]]s.  The [[electrical power]] delivered to the loudspeaker, together with its [[Electrical efficiency|efficiency]], determines the [[sound power]] generated (with the rest of the electrical power being converted to heat).

Amplifiers are limited in the electrical energy they can output, while loudspeakers are limited in the electrical energy they can convert to sound energy without being damaged or [[Distortion|distorting]] the audio signal.  These limits, or [[power rating]]s, are important to consumers finding compatible products and comparing competitors.

==Power handling==
In [[audio electronics]], there are several methods of measuring power output (for such things as amplifiers) and '''power handling''' capacity (for such things as loudspeakers).

=== Amplifiers ===
Amplifier output power is limited by voltage, current, and temperature:
* Voltage: The amp's [[power supply]] [[voltage]] limits the maximum amplitude of the waveform it can output.  This determines the peak momentary output power for a given load resistance.<ref name=":1">{{cite web|url=http://www.rocketroberts.com/techart/powerart_a.htm|title=Understanding Power Amplifier Power Ratings|website=www.rocketroberts.com|access-date=2016-10-28|quote=The peak of the signal is just touching [...] the values of the power supply rails. This signal is still clean and undistorted, however it is the maximum clean signal that is possible for this amplifier. [...] Peak Power: [...] the maximum amount of power that can be delivered to a load [...] For the amplifier we have been using in our example, the peak power (for an 8 ohm load) is 200 watts. This amount of power is delivered to the 8 ohm load at the instant when the output voltage of the amplifier is at +40 volts}}</ref><ref>{{cite web|url=http://www.meyersound.com/support/papers/amp_power.htm|title=Making Sense of Amplifier Power Ratings|website=www.meyersound.com|access-date=2016-10-28|quote=Using this rail voltage, we could calculate the instantaneous peak power...|archive-url=https://web.archive.org/web/20161019214511/http://www.meyersound.com/support/papers/amp_power.htm|archive-date=2016-10-19|url-status=dead}}</ref>
* Current: The amp's output devices ([[transistor]]s or [[Vacuum tube|tubes]]) have a current limit, above which they are damaged.  This determines the minimum [[load resistance]] that the amp can drive at its maximum voltage.<ref>{{cite web|url=http://www.sengpielaudio.com/calculator-AmplifierLoudspeakerAndOhm.htm|title=Amplifier, Loudspeaker, and Ohm|last=Sengpiel|first=Eberhard|website=www.sengpielaudio.com|access-date=2016-10-28|quote=If speaker impedance is too low [...] Too much current will run through the AV receiver's output transistors, causing the receiver to overheat and shut down.}}</ref>
* Temperature: The amp's output devices waste some of the electrical energy as heat, and if it is not removed quickly enough, they will rise in temperature to the point of damage.  This determines the continuous output power.<ref name=":0">{{Cite book|url=https://books.google.com/books?id=G-MDAAAAMBAJ&q=average+continuous+power+abbreviation&pg=PA101|title=Popular Mechanics|last=Magazines|first=Hearst|date=1987-12-01|publisher=Hearst Magazines|language=en|quote=The Federal Trade Commission has long been aware of this fact, and in 1975 stepped in to halt the abuses of companies that wanted you to think your amp could launch a speaker into orbit. [...] all published power specs have to state continuous power in watts [...] the average amount of power the amp is capable of producing over an extended period of time.}}</ref>
As an amplifier's power output strongly influences its price, there is an incentive for manufacturers to exaggerate output power specs to increase sales. Without regulations, imaginative approaches to advertising power ratings became so common that in 1975 the US [[Federal Trade Commission]] intervened in the market and required all amplifier manufacturers to use an engineering measurement (continuous average power) in addition to any other value they might cite.<ref name=":0" />

=== Loudspeakers ===
For loudspeakers, there is also a thermal and a mechanical aspect to maximum power handling.
* Thermal: Not all energy delivered to a loudspeaker is emitted as sound. In fact, most is converted to heat, and the temperature must not rise too high. High level signals over a prolonged period can cause thermal damage, which may be immediately obvious, or  reduce longevity or performance margin. 
* Mechanical: Loudspeaker components have mechanical limits which can be exceeded by even a very brief power peak; an example is the most common sort of loudspeaker driver, which cannot move in or out more than some [[Excursion (audio)|excursion]] limit without mechanical damage.
There are no similar loudspeaker power handling regulations in the US; the problem is much harder as many loudspeaker systems have very different power handling capacities at different frequencies (e.g., tweeters which handle high frequency signals are physically small and easily damaged, while woofers which handle low frequency signals are larger and more robust).

== Power calculations ==
[[Image:Peak-power-average-power-tau-T.png|thumb|A graph of instantaneous power over time for a waveform, with peak power labeled P<sub>0</sub> and average power labeled P<sub>avg</sub>]]

Since the [[instantaneous power]] of an [[alternating current|AC]] waveform varies over time, [[AC power]], which includes audio power, is measured as an average over time.  It is based on this formula:<ref>{{cite web|url=http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/ACCircuit/AC-AvePower.html|title=Average Power in an AC Circuit|last=Vawter|first=Richard|archive-url=https://web.archive.org/web/20100327161721/http://www.ac.wwu.edu/~vawter/PhysicsNet/Topics/ACCircuit/AC-AvePower.html|archive-date=2010-03-27|access-date=2016-04-22}}</ref>
:<math>
P_\mathrm{avg} = \frac{1}{T}\int_{0}^{T} v(t) \cdot i(t)\, dt \,
</math>

For a purely [[resistive]] [[Electrical load|load]], a simpler equation can be used, based on the [[root mean square]] (RMS) values of the voltage and current waveforms:
:<math>
P_\mathrm{avg} = V_\mathrm{rms} \cdot I_\mathrm{rms} \,
</math>

In the case of a steady sinusoidal tone (not music) into a purely resistive load, this can be calculated from the [[peak amplitude]] of the [[voltage]] waveform (which is easier to measure with an [[oscilloscope]]) and the load's resistance:
:<math>
P_\mathrm{avg} = \frac{{V_\mathrm{rms}}^2}{R} = \frac{{V_\mathrm{peak}}^2}{2R} \,
</math>

Though a speaker is ''not'' purely resistive, these equations are often used to approximate power measurements for such a system. Approximations may be used as reference on a specification sheet of a product.

=== Example ===
An amplifier under test can drive a sinusoidal signal with a peak amplitude of 6 V (driven by a 12 V battery). When connected to an 8 [[Ohm (unit)|ohm]] loudspeaker this would deliver:
:<math>P_\mathrm{avg} = {(6~\mathrm{V})^2 \over 2(8~\Omega)}\,\ = 2.25~\mathrm{W}</math>

In most actual car systems, the amplifiers are connected in a [[bridge-tied load]] configuration, and speaker impedances are no higher than 4 Ω.  High-power car amplifiers use a DC-to-DC converter to generate a higher supply voltage.

== Measurements ==

=== 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 green, average power in violet.]]
''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 [[Power (physics)|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]]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>

===Peak power===

''Peak power'' refers to the maximum of the instantaneous power waveform, which, for a sine wave, is always twice the average power.<ref>{{Cite news|url=http://proaudioblog.co.uk/2015/04/pa-power-ratings-1-know-whats-watt/|title=Power Ratings: Know What's Watt - ProAudioBlog.co.uk|date=2015-04-13|newspaper=ProAudioBlog.co.uk|language=en-US|quote=For a sine wave, the peak power is twice the continuous average power.|access-date=2016-10-28|archive-url=https://web.archive.org/web/20161028154124/http://proaudioblog.co.uk/2015/04/pa-power-ratings-1-know-whats-watt/|archive-date=2016-10-28|url-status=dead}}</ref><ref name=":1" /><ref>{{cite web|url=http://www.ti.com/lit/pdf/SLEA047|title=Power Rating in Audio Amplifiers|last=Luu|first=Tuan|date=March 2005|publisher=Texas Instruments|quote=Peak power ratings are obtained by using peak voltage. [...] If the dynamic headroom of an audio amplifier is specified at 3 dB, then the amplifier can deliver two times its average power; e.g., if the amplifier is rated for 200 W average power, then the peak power is 400 W.}}</ref><ref>{{cite web|url=http://www.eetimes.com/document.asp?doc_id=1274915|title=Understanding Class-D amplifier power supply requirements {{!}} EE Times|website=EETimes|access-date=2016-10-28|quote=The peak power that an amplifier can deliver to the load is [...] PSE(PEAK) = [...] 2 • PSE(RMS)}}</ref> For other waveforms, the relationship between peak power and average power is the [[Peak-to-average ratio|peak-to-average power ratio]] (PAPR).

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.<ref>{{cite web|url=http://www.prestonelectronics.com/audio/Speakers.htm|title=Speaker Selection and Amplifier Power Ratings|website=www.prestonelectronics.com|access-date=2016-10-28|quote=In an amplifier, the maximum peak power output to a speaker is limited by the amplifier power supply. [...] In an amplifier, the peak power rating is useful for describing the maximum instantaneous limit of its capability for pulse sounds such as drumbeats and bass notes.}}</ref>

It always produces a higher value than the average power figure, however, and so has been tempting to use in advertising without context, making it look as though the amp has twice the power of competitors.

===Total system power===
'''Total system power''' is a term often used in [[Audio engineering|audio electronics]] to rate the power of an audio system. Total system power refers to the total [[Energy consumption|power consumption]] of the unit, rather than the power handling of the [[Loudspeaker|speakers]] or the power output of the [[amplifier]]. This can be viewed as a somewhat deceptive [[marketing]] ploy, as the total power consumption of the unit will of course be greater than any of its other power ratings, except for, perhaps, the peak power of the amplifier, which is essentially an exaggerated value anyway.{{citation needed|date=December 2011}} [[Shelf stereo]]s and [[surround sound]] receivers are often rated using total system power.

One way to use total system power to get a more accurate estimate of power is to consider the [[amplifier class]] which would give an educated guess of the power output by considering the efficiency of the class. For example, [[class AB amplifier]]s can vary widely from 25% to 75%[http://www.rockfordfosgate.com/products/details/t400-2] efficiency while Class D amps are much higher at 80% to 95%[http://epsilonelectronicsinc.com/news/2015/08/11/soundstream-is-now-shipping-stealth-124-marine-amplifiers/]. An exceptionally efficient Class D amp, the [[ROHM]] BD5421efs, operates at 90% efficiency.<ref>{{cite web|url=http://www.eetasia.com/ART_8800464222_499501_NP_c0bbb082.HTM|archive-url=https://archive.today/20120730184544/http://www.eetasia.com/ART_8800464222_499501_NP_c0bbb082.HTM|url-status=dead|archive-date=2012-07-30|title=Class-D amplifier guarantees 90% efficiency|date=2007-05-14|website=EE Times-Asia|access-date=2016-04-22}}</ref>

In some cases, an audio device may be measured by the ''total system power'' of all its loudspeakers by adding all their peak power ratings. Many [[home theater in a box]] systems are rated this way. Often low-end home theater systems' power ratings are taken at a high level of [[harmonic distortion]] as well; as high as 10%, which would be noticeable.<ref>{{cite web|url=http://hometheater.about.com/cs/audiocomponents/a/aapowermada.htm|title=Don't Get Seduced by Amplifier Power Specifications|website=About.com Tech|access-date=2016-04-22|archive-date=2016-05-13|archive-url=https://web.archive.org/web/20160513004005/http://hometheater.about.com/cs/audiocomponents/a/aapowermada.htm|url-status=dead}}</ref>

=== PMPO ===
''PMPO'', which stands for ''Peak Music Power Output''<ref>{{Cite book|url=https://archive.org/details/wordsworthdictio0000dale|url-access=registration|title=The Wordsworth Dictionary of Abbreviations and Acronyms|last1=Dale|first1=Rodney|last2=Puttick|first2=Steve|date=1997-01-01|publisher=Wordsworth Editions|isbn=9781853263859|page=[https://archive.org/details/wordsworthdictio0000dale/page/127 127]|language=en|quote=PMPO peak music power output}}</ref><ref>{{Cite book|url=https://books.google.com/books?id=d4iuIdjZBBMC&q=%22peak+music+power+output%22&pg=PA246|title=Dictionary of technical abbreviations: SPAIN - German|last=Huzau|first=Alexander C.|date=2000-01-01|publisher=BoD – Books on Demand|isbn=9783831112999|page=246|quote=PMPO acoustic. peak music power output}}</ref> or ''Peak momentary performance output'',<ref>{{cite web|url=http://www.ti.com/general/docs/litabsmultiplefilelist.tsp?literatureNumber=slea047a|title=Power Rating in Audio Amplifier (Rev. A), slea047a - TI.com|website=www.ti.com|access-date=2016-11-04}}</ref> is a much more dubious [[figure of merit]], of interest more to advertising copy-writers than to consumers.<ref>{{cite web|url=https://www.audioholics.com/audio-amplifier/amplifier-power-ratings|title=The Truth About Amplifier Power Ratings|website=Audioholics Home Theater, HDTV, Receivers, Speakers, Blu-ray Reviews and News|date=27 April 2009 |access-date=2016-04-22}}</ref>  The term PMPO has never been defined in any standard,<ref>{{Cite magazine|url=https://books.google.com/books?id=WwsEAAAAMBAJ&q=%22peak+music+power+output%22&pg=PA33|title=Silly Season On in Phono Field|last=Lachenbruch|first=David|magazine=Billboard|date=1963-07-20|publisher=Nielsen Business Media, Inc.|page=33|language=en|issn=0006-2510|quote=It was called "Music Power Output."  So now that we have a good, workable standard, what do we see in the industry's advertising material?  Something brand new. It's called — of all things — "Peak Music Power Output".}}</ref> but it is often taken to be the sum of some sort of peak power for each amplifier in a system. Different manufacturers use different definitions, so that the ratio of PMPO to continuous power output varies widely; it is not possible to convert from one to the other. Most amplifiers can sustain their PMPO for only a very short time, if at all; loudspeakers are not designed to withstand their stated PMPO for anything but a momentary peak without serious damage.

== Power and loudness in the real world ==
{{unreferenced section|date=January 2015}}
Perceived "[[loudness]]" varies approximately [[logarithmic scale|logarithmically]] with the acoustical output power. The change in perceived loudness as a function of change in acoustical power is dependent on the reference power level. It is both useful and technically accurate to express perceived loudness in the logarithmic [[decibel]] (dB) scale that is independent of the reference power, with a somewhat straight-line relationship between 10&nbsp;dB changes and doublings of perceived loudness.

The approximately logarithmic relationship between power and perceived loudness is an important factor in audio system design. Both amplifier power and speaker sensitivity affect the maximum realizable loudness. Sensitivity is typically measured either suspended in an anechoic chamber in 'free space' (for full range speakers), or with the source and receiver outside on the ground in 'half space' (for a subwoofer).

While a doubling/halving of perceived loudness corresponds to approximately 10&nbsp;dB increase/decrease in speaker sensitivity, it also corresponds to approximately 10X multiplication/division of acoustical power. Even a relatively modest 3&nbsp;dB increase/decrease in sensitivity corresponds to a doubling/halving of acoustical power. When measuring in 'half space', the boundary of the ground plane cuts the available space that the sound radiates into in half and doubles the acoustical power at the receiver, for a corresponding 3&nbsp;dB increase in measured sensitivity, so it is important to know the test conditions. ±3&nbsp;dB change in measured sensitivity also corresponds to a similar doubling/halving of electrical power required to generate a given perceived loudness, so even deceptively 'minor' differences in sensitivity can result in large changes in amplifier power requirement. This is important because power amplifiers become increasingly impractical with increasing amplifier power output.

Many high-quality domestic speakers have a sensitivity between ~84&nbsp;dB and ~94&nbsp;dB, but professional speakers can have a sensitivity between ~90&nbsp;dB and ~100&nbsp;dB. An '84&nbsp;dB' source would require a 400-watt amplifier to produce the same acoustical power (perceived loudness) as a '90&nbsp;dB' source being driven by a 100-watt amplifier, or a '100&nbsp;dB' source being driven by a 10 watt amplifier. 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&nbsp;Hz before clipping.

Like perceived loudness, speaker sensitivity also varies with frequency and power. The sensitivity is measured at 1 watt to minimize nonlinear effects such as [[power compression]] and harmonic distortion, and averaged over the usable bandwidth. The bandwidth is often specified between the measured '+/-3&nbsp;dB' cutoff frequencies where the relative loudness becomes attenuated from the peak loudness by at least 6&nbsp;dB. Some speaker manufacturers use '+3&nbsp;dB/-6&nbsp;dB' instead, to take into account the real-world in-room response of a speaker at frequency extremes where the floor/wall/ceiling boundaries may increase the perceived loudness.

Speaker sensitivity is measured and rated on the assumption of a fixed amplifier output voltage because audio amplifiers tend to behave like voltage sources. Sensitivity can be a misleading metric due to differences in speaker impedance between differently designed speakers. A speaker with a higher impedance may have lower measured sensitivity and thus appear to be less efficient than a speaker with a lower impedance even though their efficiencies are actually similar. Speaker efficiency is a metric that only measures the actual percentage of electrical power that the speaker converts to acoustic power and is sometimes a more appropriate metric to use when investigating ways to achieve a given acoustic power from a speaker.

Adding an identical and mutually coupled speaker driver (much less than a wavelength away from each other) and splitting the electrical power equally between the two drivers increases their combined efficiency by a maximum of 3&nbsp;dB, similar to increasing the size of a single driver until the diaphragm area doubles. Multiple drivers can be more practical to increase efficiency than larger drivers since frequency response is generally proportional to driver size.

System designers take advantage of this efficiency boost by using mutually coupled drivers in a speaker cabinet, and by using mutually coupled speaker cabinets in a venue. Each doubling of total driver area in the array of drivers brings ~3&nbsp;dB increase in efficiency until the limit where the total distance between any two drivers of the array exceeds ~1/4 wavelength.

Power handling capability is also doubled when the number of drivers doubles, for a maximum realizable increase of ~6&nbsp;dB in total acoustic output per doubling of mutually coupled drivers when the total amplifier power is also doubled. Mutual coupling efficiency gains become difficult to realize with multiple drivers at higher frequencies because the total size of a single driver including its diaphragm, basket, waveguide or horn may already exceed one wavelength.

Sources that are much smaller than a wavelength behave like point sources that radiate omnidirectionally in free space, whereas sources larger than a wavelength act as their own 'ground plane' and beam the sound forward. 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.

Likewise, speaker proximity much less than 1/4 wavelength to one or more boundaries such as floor/walls/ceiling can increase the effective sensitivity by changing free space into half space, quarter space, or eighth space. 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.

Sound absorbing structures, sound diffusing structures, and digital signal processing may be employed to compensate for boundary effects within the designated listening area.

==Matching amplifier to loudspeaker==
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. Chuck McGregor recommended a rule of thumb in which the amplifier's maximum power output rating was twice the loudspeaker's continuous (so-called "RMS") rating, give or take 20%. In his example, a loudspeaker with a continuous power rating of 250&nbsp;watts would be well-matched by an amplifier with a maximum power output within the range of 400 to 625&nbsp;watts.<ref>[http://www.prosoundweb.com/article/how_many_watts_loudspeaker/ ProSoundWeb, Study Hall. Chuck McGregor, ''How Many Watts : Amps vs. Loudspeakers: The eternal question answered - what's the "right" wattage for my loudspeakers.''] Retrieved February 27, 2009.</ref>

[[JBL (company)|JBL]], which tests and labels their loudspeakers according to the [[International Electrotechnical Commission|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:<ref name="jbl">[http://www.jblpro.com/pub/technote/spkpwfaq.pdf JBL Speaker Power Requirements]</ref>
# For "carefully monitored applications where peak transient capability must be maintained, a system should be powered with an amplifier capable of delivering twice its IEC rating." As an example, a [[studio monitor]] rated at 300 watts IEC, can be safely driven by a 600-watt (RMS) amplifier, provided that "peak signals are normally of such short duration that they hardly stress the system's components".<ref name="jbl"/>
# For "routine application where high continuous, but non-distorted, output is likely to be encountered, a system should be powered with an amplifier capable of delivering the IEC rating of the system". This includes most consumer systems. "Such systems can often be inadvertently overdriven, or can go into feedback. When powered with an amplifier equal to their IEC rating, the user is guaranteed of safe operation."<ref name="jbl"/>
#  "For musical instrument application, where distorted (overdriven) output may be a musical requirement, the system should be powered with an amplifier capable of delivering only one-half of the IEC rating for the system." This necessary because, for example, an amplifier normally outputting "300 watts of undistorted sinewave" can reach closer to 600 watts of power when [[Clipping (signal processing)|clipping]] (i.e. when its output is closer to a [[square wave]]). If such a scenario is plausible, then for safe operation of the loudspeaker, the amplifier's (RMS) rating must no more than half the IEC power of the loudspeaker.<ref name="jbl"/>

== Power handling in 'active' speakers ==
{{unreferenced section|date=January 2015}}
'''Active speakers''' comprise two or three speakers per channel, each fitted with its own amplifier, and preceded by an electronic [[audio crossover|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. An additional advantage is that peak power handling is greater if the signal has simultaneous peaks in two different frequency bands. A single amplifier has to handle the peak power when both signal voltages are at their crest; as power is proportional to the square of voltage, the peak power when both signals are at the same peak voltage is proportional to the square of the sum of the voltages. If separate amplifiers are used, each must handle the square of the peak voltage in its own band. For example, if bass and midrange each has a signal corresponding to 10 W of output, a single amplifier capable of handling a 40 W peak would be needed, but a bass and a treble amplifier each capable of handling 10 W would be sufficient. This is relevant when peaks of comparable amplitude occur in different frequency bands, as with wideband percussion and high-amplitude bass notes.

For most audio applications more power is needed at low frequencies. This requires a high-power amplifier for low frequencies (e.g., 200 watts for 20–200&nbsp;Hz band), lower power amplifier for the midrange (e.g., 50 watts for  200 to 1000&nbsp;Hz), and even less the high end (e.g. 5 watts for 1000–20000&nbsp;Hz). Proper design of a bi/tri amplifier system requires a study of driver (speaker) frequency response and sensitivities to determine optimal crossover frequencies and power amplifier powers.

== Regional Variations ==

===United States===
Peak momentary power output and peak music power output are two different measurements with different specifications and should not be used interchangeably.  Manufacturers who use different words such as pulse or performance may be reflecting their own non-standard system of measurement, with an unknown meaning.  The [[Federal Trade Commission]] is putting an end to this with Federal Trade Commission (FTC) Rule 46 CFR 432 (1974), affecting Power Output Claims for Amplifiers Utilized in Home Entertainment Products.

In response to a Federal Trade Commission order, the [[Consumer Electronics Association]] has established a clear and concise measure of audio power for consumer electronics. They have posted an FTC approved product marking template on their web site and the full standard is available for a fee.
Many believe this will resolve much of the ambiguity and confusion in amplifier ratings.
There will be ratings for speaker and powered speaker system too. This specification only applies to audio amplifiers. An EU counterpart is expected and all equipment sold in the US and Europe will be identically tested and rated.<ref>[http://www.ce.org/Standards/3108.asp CEA-490-A: Test Methods of Measurement for Audio Amplifiers] {{webarchive |url=https://web.archive.org/web/20110722193247/http://www.ce.org/Standards/3108.asp |date=July 22, 2011 }}, Federal Trade Commission (FTC) Rule, Power Output Claims for Amplifiers Utilized in Home Entertainment Products, 46 CFR 432 (1974). Accessed 2011-08-13.</ref>

This regulation did not cover automobile entertainment systems, which consequently still suffer from power ratings confusion.  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.<ref>{{cite web|url=http://www.ce.org/Standards/browseByCommittee_2648.asp |title=CEA Standard for testing mobile audio equipment |access-date=2011-08-13 |url-status=dead |archive-url=https://web.archive.org/web/20110718065137/http://www.ce.org/Standards/browseByCommittee_2648.asp |archive-date=2011-07-18 }}</ref>

===Europe===
{{expand section|date=August 2011}}
DIN ([[Deutsches Institut für Normung]], German Institute for Standardization) describes in DIN 45xxx several standards for measuring audio power. The DIN-standards (DIN-norms) are in common use in Europe.<ref>{{cite web |url=http://www.epanorama.net/documents/audio/amplifier_power.html |title=Understanding amplifier power ratings |access-date=2011-08-13| archive-url= https://web.archive.org/web/20110629025051/http://www.epanorama.net/documents/audio/amplifier_power.html| archive-date= 29 June 2011 | url-status= live}}</ref>

===International===
{{expand section|date=August 2011}}
[[International Electrotechnical Commission|IEC]] 60268-2 defines power amplifier specifications including power output.<ref>{{cite web |url=http://webstore.iec.ch/preview/info_iec60268-3%7Bed3.0%7Den.pdf |archive-url=https://web.archive.org/web/20120319222951/http://webstore.iec.ch/preview/info_iec60268-3%7Bed3.0%7Den.pdf |url-status=dead |archive-date=2012-03-19 |title=IEC 60268-2 (preview) |publisher=IEC |date=August 2008 |access-date=2011-08-24}}</ref>

== See also ==
*[[Programme level]]
*[[Audio system measurements]]
*[[Sound level meter]]
*[[Audio noise measurement]]

== References ==
{{reflist|colwidth=30em}}

== External links ==
*[http://sound-au.com/power.htm Amplifier Power Ratings] (and How to calculate satisfactory PMPO values) by Rod Elliott
*[http://www.epanorama.net/documents/audio/amplifier_power.html Understanding amplifier power ratings]
*[http://www.sengpielaudio.com/calculator-levelchange.htm Audio power and the corresponding factors: Subjectivly sensed loudness (volume), objectively measured sound pressure (voltage), and theoretically calculated sound intensity (acoustic power)]
*[https://jbltopspeakers.com/the-5-best-jbl-speakers-of-2021/  The 5 Best JBL Speakers of 2021]
{{DEFAULTSORT:Audio Power}}
[[Category:Power (physics)]]
[[Category:Audio electronics]]
[[Category:Audio amplifier specifications]]