THD is total harmonic distortion. It represents the amount of harmonic distortion generated by an amplifier. Testing is normally performed with the amplifier operating at its maximum rated output, at a single frequency (often 1 kHz) and into a load resistor (usually 8 ohms). Harmonic distortion occurs when the amplifier creates frequency content that is not present in the input signal. For example, the second harmonic of a 1 kHz signal (F1) is 2 kHz (F2), the third at 4 kHz (F3), and so on. If any signal is present at harmonics of the input, it is classified as harmonic distortion. Total harmonic distortion is quantified using the equation: THD = 100 * sqrt( F2^2 + F3^2 + ... + Fn^2 ) / F1 Where: Fn = The signal amplitude at the corresponding harmonic. THD was more useful when it was introduced several decades ago, when some amplifiers were truly awful. Modern amplifier designs have no trouble producing astonishingly low THD levels, particularly as they are designed to perform well in this standard test. Note that THD in solid-state amplifiers is usually higher at lower power levels and rises suddenly just above the rated output power. Amplifers are measured under the best possible conditions for the manufacturer. THD was introduced as an simple, repeatable test, based on a simplified operational model. As a test, it is easy to perform, but does not consider many factors. - Speakers are not resistive. They present not only resistance, but capacitive and inductive impedances to the amplifier output. Speaker impedance varies with the signal frequency, unlike a resistor which has a fixed resistance. - Speakers generate back EMF, the effect of which is not unlike trying to shove a phase-shifted copy of the speaker input back into the amplifier's output. - Amplifiers are measured under optimum performance conditions, which may not always be replicated in real service. - Amplifiers can generate non-harmonic outputs, which are ignored by the THD calculation. - Amplifier performance varies with the frequency content of the signal and the speed at which it changes (transients). - The THD calculation does not consider the relative level of different harmonics. For example, one amplifier could generate output at the 2nd harmonic, with lower output at the 3rd. Another could generate equal levels at the 3rd, 4th, and 5th harmonics. The two amplifiers would not sound the same, but would have the same THD. These factors limit the usefulness of THD as a measure of amplifier quality.