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:
P a v g = 1 T ∫ 0 T v ( t ) ⋅ i ( t ) d t {\displaystyle P_{\mathrm {avg} }={\frac {1}{T}}\int _{0}^{T}v(t)\cdot i(t)\,dt\,}
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:
P a v g = V r m s ⋅ I r m s {\displaystyle P_{\mathrm {avg} }=V_{\mathrm {rms} }\cdot I_{\mathrm {rms} }\,}
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:
P a v g = V r m s 2 R = V p e a k 2 2 R {\displaystyle P_{\mathrm {avg} }={\frac {{V_{\mathrm {rms} }}^{2}}{R}}={\frac {{V_{\mathrm {peak} }}^{2}}{2R}}\,}
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:
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