Audio Fundamentals
Total harmonic distortion, or THD, measures the amount of unwanted harmonic content added to an audio signal by electronic components. It expresses how faithfully a device reproduces the input without generating additional harmonics or distortion products.
Total harmonic distortion (THD) quantifies the degree to which an audio system or component introduces harmonics not present in the original signal. When an analogue or digital device processes audio, nonlinearities in its circuitry can create multiples of the original signal’s frequencies — for example, 2×, 3×, or 4× the fundamental tone. These extra frequencies are harmonic distortion.
THD is calculated by comparing the sum of all harmonic energy to the energy of the fundamental frequency, typically expressed as a percentage or in decibels. For instance, a THD of 0.01% means that the total harmonic content is one ten-thousandth the level of the main signal. Lower values indicate cleaner, more accurate reproduction.
In professional audio, THD values below 0.1% are considered excellent. High-quality preamplifiers, converters, and power amplifiers often achieve figures as low as 0.001% or better. Some analogue devices, such as valve or tape circuits, deliberately introduce higher THD because the resulting even-order harmonics can produce a pleasing, musical warmth.
Total harmonic distortion is usually specified alongside THD+N (Total Harmonic Distortion plus Noise), which includes both harmonic distortion and random noise in the measurement. Together, these metrics help engineers assess the linearity, transparency, and overall fidelity of an audio device.
