Total harmonic distortion

The total harmonic distortion (THD) is a measurement of the harmonic distortion present in a signal and is defined as the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. Distortion factor, a closely related term, is sometimes used as a synonym.

Total harmonic distortion - Wikipedia

Total Harmonic Distortion (THD) Explained

Total Harmonic Distortion | Universal Audio