“When Show THD is enabled and the RTA graph is set to a fractional-octave resolution of 1/12th-octave or higher, the notation THD: n%, will appear in the cursor tracking readout above the RTA graph, where n is the THD percentage value calculated for the current cursor frequency. THD values in Smaart are the ratio of the power in the fractional octave band at the cursor frequency, to the sum of the power in the next three harmonic frequencies. If (and only if) the cursor is positioned at the frequency of a single sine wave being used to stimulate the system under test, this value should be indicative of the total harmonic distortion present in the system at that frequency. Otherwise, it is generally meaningless” user manual pg. 84