Total Harmonic Distortion, or THD, is one of the more regular little TLAs (or three-letter acronyms) you’re likely to see on most manufacturers’ specs. But what exactly is THD? Is it that important and what level of THD is acceptable for Hi Fi and AV equipment?
In the reproduction of audio between the input stage and the output stage of a hi-fi system, a certain amount of noise and distortion will be introduced into the signal. Some of this distortion is created by circuitry and hardware that introduce signal harmonics at twice and triple the frequencies of the original signal. These added signals are technically known as harmonic distortion. THD is a measure of the differential between the input stages of the system and the additional audible signal at each harmonic. The THD calculation does not consider the relative level of each harmonic. EG: The output of an amplifier at the second harmonic could be higher than the output at the third. Another could create equal levels on 3 and 5. The two amps would sound very different but could have the same measured total THD.
In most hi-fi circuits such as amplifiers and pre-amplifiers, less than 1% THD should be expected, as it is relatively easy to remove a large amount of THD from amplifier circuits using various techniques. Unfortunately, it is much more difficult to prevent physical elements of a setup such as hi-fi speakers from producing significant levels of THD. Much higher levels of THD are common in cheaper end speakers, especially at higher volumes. Speakers with high THD levels tend to cause audio “smearing” and result in poor picture and “muddy” sound. In reality, it is ideal to test a component in the room in which it will be used and at a volume level likely to be commonly heard to determine if the system is acceptable for the purpose.
