Audio power is the electrical power transferred from an audio amplifier to a loudspeaker, measured in watts. The electrical power delivered to the loudspeaker, together with the speaker's efficiency, determines the sound power generated (with the rest of the electrical power being converted to heat).
Amplifiers are limited in the electrical power they can output, while loudspeakers are limited in the electrical power they can convert to sound power without being damaged or distorting the audio signal. These limits, or power ratings, are important to consumers in 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 maximum possible peak output power for a given load resistance. Current: The amp's output devices (transistors or tubes) have a current limit based on the internal impedance of the amplifier. Excessive current may also cause damage to the amp or trip protection circuits within the amp. The maximum current determines the minimum load resistance that the amp can drive at its maximum voltage. 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. The temperature rise can limit maximum continuous output power. As an amplifier's power output strongly influences how much consumers are willing to pay for it, there is an incentive for manufacturers to exaggerate output power specs. 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.
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 may 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 limit without mechanical damage. Unlike with amplifiers, 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., the tweeters in loudspeaker systems handle high-frequency signals, are physically small and easily damaged, while woofers handle low-frequency signals and are larger and more robust).
Power calculations
Since the instantaneous power of an AC waveform varies over time, AC power, which includes audio power, is measured as an average over time. It is based on this formula:
For a purely resistive load, a simpler equation can be used, based on the root mean square (RMS) values of the voltage and current waveforms:
In the case of a steady sinusoidal tone 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:
Though a speaker is not purely resistive and audio program is not a sinusoid, these equations are sometimes used to approximate power measurements for such a system.
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 loudspeaker this would deliver:
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
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 may be referred to as RMS power or watts RMS. This should not to be interpreted as the RMS value of the power waveform. Watts RMS is used in an ANSI standard where it is also referred to as the nominal value and where there is a product mark requirement to use it.
Continuous (as opposed to peak) 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. This rule was amended in 1998 to cover self-powered speakers, such as those commonly used with personal computers.
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.
The sine wave used for continuous power measurements does not actually describe the highly varied signals found in audio reproduction, but these measurements 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. 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 may force the driver beyond its mechanical limits much before reaching 100 watts from the amplifier.
Peak power Peak power refers to the maximum of the instantaneous power waveform, which, for a sine wave, is always twice the average power. For other waveforms, the relationship between peak power and average power is the 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 instantaneously without damage. This characterizes the ability of equipment to handle quickly changing power levels, as many audio signals have a highly dynamic nature.
Peak power is always a higher value than the average power figure, however, and so it 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 electronics to rate the power of an audio system. Total system power refers to the total power consumption of the unit, rather than the power handling of the 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 possibly the peak power of the amplifier. Shelf stereos 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 amplifier output power is to consider the amplifier class, from which one could derive an estimate of the power output by considering the efficiency of the class. For example, class AB amplifiers 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/
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.
PMPO PMPO, which stands for Peak Music Power Output or Peak momentary performance output, is a much more dubious figure of merit, of interest more to advertising copy-writers than to consumers. The term PMPO has never been defined in any standard. 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.
Power and loudness in the real world
Perceived loudness varies approximately 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 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 (for full-range speakers), or with the source and receiver outside on the ground in (for a subwoofer). 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 dB increase in measured sensitivity, so it is important to know the test conditions.
While a doubling or halving of perceived loudness corresponds to approximately 10 dB increase or decrease in speaker sensitivity, it also corresponds to approximately a 10-fold multiplication or division of acoustical power. Even a relatively modest 3 dB increase or decrease in sensitivity corresponds to a doubling or halving of acoustical power. ±3 dB change in measured sensitivity also corresponds to a similar doubling or halving of electrical power required to generate a given perceived loudness, so even deceptively differences in sensitivity can result in large changes in amplifier power requirement.