Specs & measurements

THD

Also called total harmonic distortion, harmonic distortion

THD, total harmonic distortion, is how much a device adds overtones that were not in the signal, given as a percentage or in decibels. Lower is cleaner; below about 0.1 percent it is generally inaudible on most music, though listeners argue about the exact threshold.

Illustrative spectrum of a 1 kHz test tone through a clean amplifier: the device adds harmonics at 2, 3, 4 and 5 kHz, here 109 to 118 dB below the tone. THD sums them: about 0.0005 percent, as amplifiers typically measure well under 0.01 percent.

When a device cannot reproduce a tone exactly, it adds multiples of that tone: the harmonics. THD sums them. Amplifiers and DACs typically measure well under 0.01 percent; headphones and speakers, being mechanical, measure far higher, often above 1 percent in the bass at loud levels.

Which harmonics matter is part of why tube amplifiers have fans. They produce more distortion than Solid-state designs, but mostly low-order harmonics that some listeners find pleasant. Measurements show the difference; whether it is a flaw is a matter of taste.

THD is usually reported with noise included as THD+N, and turned the other way up as SINAD.

Related terms