Total Harmonic Distortion. All electronic devices add some form of distortion to a signal passing through them. In audio, one form of distortion is the addition of harmonics that are not present in the original signal. THD is a measurement of this and is represented as a ratio of the total of all the harmonics added against the original signal at the output of the device. The closer THD is to zero, the less the circuit is distorting the source signal.