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 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 (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: 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 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 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). The erroneous term "watts RMS" is actually used in CE regulations. This is also referred to as the nominal value, there being a regulatory requirement to use it. 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