Total Harmonic Distortion, or THD is an amplifier or pre-amplifier specification that compares the output signal of the amplifier with the input signal and measures the level differences in harmonic frequencies between the two. The difference is called total harmonic distortion. Music is made of fundamental and harmonic frequencies. For example, a violin playing a middle A note is producing a fundamental frequency of 440Hz or cycles per second. It is also reproducing harmonics of that frequency that are multiples of the fundamental frequency, such as 880Hz, 1220Hz, 1760Hz and so on. Each of the harmonic frequencies is lower in level or volume than the fundamental, but they serve to give each instrument its unique sound. A piano playing the same middle C note as the violin still sounds like a piano because of the fundamental frequencies plus its harmonic frequencies. When the total harmonic distortion of an amplifier is measured, the difference in the level of the harmonics at the output stage of the amp is compared to the level of the harmonics at the input stage, and the difference is the extent of the distortion.
How is it Expressed?
Total harmonic distortion is measured as a percentage, such as 0.004% THD. This means that the level of harmonic distortion is 0.004% of the total output signal. Lower percentages are better.
Why is it Important?
In reality, total harmonic distortion is hardly perceptible to the human ear. Every component adds some level of distortion, but most distortion is insignificant and small differences in specifications between components mean nothing. Some components have distortion so low it cannot be accurately measured. Listening to a component and evaluating its sound characteristics is the most important way to judge a product. Other considerations, such as room acoustics and selecting the right speakers are more important than the percentage of total harmonic distortion.