Page "Audio system measurements" Paragraph 9

; Total harmonic distortion ( THD ): Music material contains distinct tones, and some kinds of distortion involve spurious tones at double or triple the frequencies of those tones.

Such harmonically related distortion is called harmonic distortion.

For high fidelity, this is usually expected to be < 1 % for electronic devices ; mechanical elements such as loudspeakers usually have inescapable higher levels.

Low distortion is relatively easy to achieve in electronics with use of negative feedback, but the use of high levels of feedback in this manner has been the topic of much controversy among audiophiles.

Essentially all loudspeakers produce more distortion than electronics, and 1 – 5 % distortion is not unheard of at moderately loud listening levels.

Human ears are less sensitive to distortion in the bass frequencies, and levels are usually expected to be under 10 % at loud playback.

Distortion that creates only even-order harmonics for a sine wave input is sometimes considered less bothersome than odd-order distortion.