The total harmonic distortion (THD) (also called distortion factor or THD – Total Harmonic Distortion) describes how much an audio signal is distorted by non-linear distortions.
Simply Explained
Ideally, an amplifier or speaker reproduces a signal exactly as it goes in – just louder. In practice, however, additional frequencies arise, called overtones or harmonics, which are not present in the original signal.
The distortion factor indicates how large the proportion of these unwanted overtone components is relative to the useful signal.
Technical Meaning
- The distortion factor is measured in percent (%)
- It relates the sum of the overtones to the fundamental frequency
- The lower the distortion factor, the more faithful the reproduction
Audible Effects
- Low distortion factor → clean, natural sound
- High distortion factor → sound appears harsh, shrill, or distorted
Generally, a distortion factor of at least 1 percent is noticeable to trained listeners, while 5 percent distortion factor is a critical limit for normal ears. Modern hi-fi amplifiers typically have a distortion factor of < 0.1%, making it practically inaudible.
Practical Relevance
Today’s amplifiers and audio devices usually have very low distortion values thanks to high-quality components. Distortions mainly occur when:
- the amplifier is overloaded (e.g., at too high volume)
- speakers are operated outside their optimal operating range, meaning they are exposed to conditions they were not designed for.
In short:
The distortion factor is a measure of how “honest” an audio device sounds. The smaller the value, the closer the sound remains to the original.
Find out more in our blog: Distortion: What is it and is it always a bad thing?