This vCalc equation computes the total harmonic distortion (THD) of an electrical signal from the voltages (`V_n`) of the harmonic components comprising the signal.
This equation computes the THD from a set of five harmonics and the fundamental frequency as a simplified case. To consider this on the basis of n different harmonics, we could put the voltages of a large number of harmonics into a data set and allow the equation to pull any number n of the harmonics, but since this is an illustrative equation, we will limit this application to a case of five or less harmonics, when in fact the Fourier analysis of a waveform usually results in an infinite series.
In general, the Total Harmonic Distortion (THD) can be applied to any signal: electrical, acoustic or otherwise and the THD is defined as the ratio of the sum of the powers of the all the harmonic components to the power of the fundamental frequency. Since voltage is directly related to power, we are able in the case of an electrical signal to transform the equations to use voltages instead of power. If the equation using power in place of voltage were applied to the same case, the THD would compute the same.1