Total Harmonic Distortion, abbreviated THD, is a specification that identifies the amount of unwanted harmonic frequencies created when a signal passes through an audio component, most commonly an amplifier. Here's a brief definition of THD: All musical instruments have a tonal characteristic that consists of fundamental and harmonic frequencies. The combination of the two gives the instrument its unique sound quality and allows the human ear to distinguish between different instruments. For example, a violin producing a fundamental frequency of 500Hz will also produce multiples of that frequency, 1000Hz, 1500Hz, etc., which are multiples of the fundamental frequency. The harmonics are produced at lower levels than the fundamental frequency, but they are present and distinguish the violin from other instruments. When an audio signal is passed through a component, unwanted harmonic frequencies are generated or added to the original signal producing what is known as harmonic distortion. THD is expressed as a percentage of the original signal, for example .05% total harmonic distortion. Total Harmonic Distortion is one method of comparing audio components, but generally is not the most important, because all audio components have some amount of THD and minute differences in distortion between components is usually not audible. A listening test is the best way to evaluate sound quality. For more terms and definitions, check out my glossary of terms.