Audio Power: RMS value & PMPO

In the case of DC signal, it is easy to calculate as it is always positive & unvarying. But in the case of AC, it is sinusoidal in form, which is continuously changing in amplitude & shifting between positive & negative values. If the values are just averaged out, the positive & negative may just cancel each other out, giving a fallacious value.

So the Root Mean Square (RMS) value is computed to get a more realistic value. First each number of the dataset (of the sine wave) is squared as even the square of the negative numbers in the dataset (or the portion of the sinewave below the X axis) get a positive magnitude. Then a mean or average is worked out to get a single workable value. Then the (square) root is taken of this value to correct the squaring done at first (to absorb the negative values). This is the RMS value of the power rating of the audio amplifier, which gives a realistic approximation.

However, some manufacturers quote a PMPO (Peak Music Power Output) which is the maximum sine wave power obtainable before presence of the signal has time to degrade performance. However, this is just a marketing tool to impress the gullible customer. For example, a popular brand markets its system as having 4000 Watts PMPO whereas it's RMS value is 45 Watts.