THD and Harmonic Distortion: How to Read the Spec, and When It Matters

What distortion adds

Feed a perfect device a pure 1 kHz tone and a pure 1 kHz tone comes out. Feed a real one and a little energy also appears at 2 kHz, 3 kHz, 4 kHz and upward: harmonics that were not in the input. That is harmonic distortion, and every amplifier, converter and transducer produces some.

The order of the harmonics matters as much as the amount. Low-order products, the second and third harmonic, sit close to the musical overtones an instrument already produces, and at low levels they tend to be heard as warmth, richness or density. High-order products, the fifth, seventh and beyond, have no such relationship with the music and sound harsh or gritty even at levels that measure small. This is why two devices with the same headline figure can sound different, and why some deliberately distorting designs, such as many tube amplifiers, are liked rather than tolerated.

THD, THD+N and SINAD

THD (total harmonic distortion) adds up the energy of all the harmonics and expresses it as a percentage of the fundamental. THD+N includes everything else the device adds as well, noise and hum, so it is the more complete and the more commonly quoted figure on DAC and amplifier spec sheets. SINAD (signal to noise and distortion) is the same measurement stated the other way round, in decibels: how far below the signal all the unwanted output sits.

The two scales convert directly. 1% is 40 dB below the signal, 0.1% is 60 dB, 0.01% is 80 dB, 0.001% is 100 dB and 0.0001% is 120 dB. So a DAC quoted at 0.0001% THD+N has a SINAD of about 120 dB, and one at 0.01% sits at 80 dB.

Reading a spec sheet

A distortion figure means little without its conditions.

At what frequency and level. Most figures are measured at 1 kHz, where circuits behave best. Distortion usually rises at the frequency extremes and near full output.

Into what load. An amplifier's figure into a high-impedance load can be far better than its figure into a low-impedance planar or speaker, where it has to deliver current.

With what bandwidth. A measurement limited to the audible band gives a lower number than one that includes ultrasonic noise.

Our reviews quote published figures where they exist, in the pros and cons below, and they range widely: from a few ten-thousandths of a percent on the best current DACs and amplifiers to figures a review calls modest for the price. What the reviews do not do is score a device on its spec sheet. A figure is where listening starts, not where it ends.

Why the electronics rarely decide it

A modern DAC or headphone amplifier at 0.001% THD+N puts its distortion 100 dB below the music. Under normal listening, with the masking effect of the music itself, that is far beneath anything audible. Transducers are another matter: headphones and speakers commonly produce distortion in the tenths of a percent, and much more in the deep bass or at high volume, because moving a diaphragm through air is a far less linear process than moving electrons through a circuit.

So when a review hears a difference between two good DACs, distortion figures are rarely the explanation, and when a headphone sounds clean at high volume, that is a real achievement of the driver. If you want to know whether a difference you think you hear between two devices is real, a level-matched blind test settles it far better than a spec sheet; the site's ABX test runs one in the browser.