Harmonic distortion and THD: what the figure tells you

What harmonic distortion adds to a tone

Play a pure 1 kHz tone through a perfect device and a pure 1 kHz tone comes out. Play it through a real amplifier, DAC or headphone and a little energy also appears at 2 kHz, 3 kHz, 4 kHz and on up. Those are harmonics, whole-number multiples of the tone, and they appear because no circuit or driver responds in exact proportion to its input. The more the output bends away from a straight copy of the input, the larger they get.

Clipping is the extreme case. When an amplifier runs out of voltage, the tops of the waveform are cut flat, and a flattened sine wave is a sine plus a long series of harmonics. Well below clipping, a good amplifier's harmonics are tiny, but they never reach zero.

How THD, THD+N and SINAD are defined

THD, total harmonic distortion, adds up the energy of all the measured harmonics and states it as a percentage of the fundamental tone. THD+N, total harmonic distortion plus noise, measures everything left at the output once the test tone is removed: harmonics, hiss and hum together. Most DAC and amplifier spec sheets quote this one. SINAD, signal to noise and distortion, states the same measurement the other way round, as how many decibels the unwanted output sits below the signal.

Percentages convert to decibels directly, because each factor of ten is 20 dB:

Makers use both forms. SMSL quotes the SMSL PS100 at 0.005% THD+N and also as −85 dB, where the exact conversion is −86 dB. A DAC with 0.001% THD+N has a SINAD of about 100 dB.

Low-order and high-order harmonics

The order of the harmonics matters as much as the total. The second and third lie an octave and an octave plus a fifth above the tone, intervals music is built on, and close enough to it that the tone masks them more easily. At low levels the ear tends to hear them as added warmth or density. Higher orders, the fifth, seventh and beyond, lie further up, where the tone masks them less, and several fall between the notes of the scale, so they sound harsh or gritty at levels that measure small. Two devices with the same THD can therefore sound different, and one percentage hides which kind of distortion you are getting.

Some designs are known for their harmonic signature. Single-ended tube amplifiers tend to produce mostly second harmonic, and crossover distortion in a poorly biased push-pull stage produces high orders, which is why the amplifier classes explainer treats it as the error Class AB exists to fix. Music also tests a device with many tones at once, and those tones mix into new frequencies unrelated to any of them, called intermodulation distortion, which a single-tone THD figure doesn't show.

Reading a distortion figure on a spec sheet

A distortion figure means little without its conditions. Most are measured at 1 kHz, where circuits behave best, and distortion usually rises at the frequency extremes and near full output. An amplifier measured into a high-impedance load can look far better than it does into a low-impedance planar or speaker, where it has to deliver current. A narrow measurement bandwidth gives a lower number than one that includes ultrasonic noise.

Power ratings are tied to distortion too. Tonewinner rates its AD-1PA+ power amplifier at 330 W into 8 Ω at 1% THD and 1 kHz, and separately quotes THD of 0.008% or less at 1 kHz. The first number marks where distortion has risen to 1%, close to clipping, and the second describes the amplifier at lower output. Read both. A power figure quoted at 10% THD describes an amplifier already well into clipping.

Why speakers and headphones usually decide it

Transducers distort far more than the electronics that drive them, because moving a diaphragm through air is much less linear than moving current through a circuit. The distortion of the HarmonicDyne Eris is given as 0.2% or lower, which its review calls clean enough for a headphone in its class. The HIFIMAN Serenade amplifier quotes 0.0015%. That puts the headphone's figure about 40 dB above the amp's, and headphone and speaker distortion climbs further in the deep bass and at high volume.

This is why the site's reviews quote published distortion figures without scoring on them. A figure of 0.001% puts the distortion 100 dB below the music, far beneath anything shown to be audible with music playing at normal levels. When two good DACs sound different, their THD figures are rarely the explanation, and the output stage, filters or a small level difference are likelier causes.

Testing whether you hear it

Distortion is one of the few things you can check with your own ears in controlled conditions. The ABX blind test includes a built-in pair for it: a 1 kHz tone, clean and with 1% of second and third harmonic added, which puts them roughly 40 dB below the signal. Try it first, then compare two sources of your own, level-matched, and the result tells you what your hearing resolves. For amplifiers where distortion is one line among many, the headphone amplifier guide ranks the models reviewed here.