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 its 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 distorting the audio signal. These limits, or power ratings, 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. Current: The amp's output devices (transistors or 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. 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. 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. 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 limit without mechanical damage.