Why a single number cannot tell you if your audio equipment sounds good

Countries & Cities · Published 15 October 2026 · Updated 25 October 2026 Swipe more like this →

In short: Total harmonic distortion measures how much a device alters an input signal by adding unwanted frequencies. While it is a standard metric for amplifiers and power grids, a single percentage often fails to capture how humans actually perceive sound, as our ears are far more sensitive to certain types of distortion.

Total harmonic distortion (THD) quantifies the additional signal content introduced by a non-linear system when a pure sine wave passes through it. When a device like an amplifier is imperfect, it creates harmonics—integer multiples of the original input frequency. THD is defined as the ratio of the power of these new harmonic components to the power of the fundamental frequency. In audio, lower THD generally indicates a more faithful reproduction of the original recording, while in power systems, it serves as a critical metric for grid stability, helping to minimize heating and electromagnetic emissions.

However, relying on a single THD value can be misleading. Research has long shown that human hearing does not perceive all harmonics equally. For instance, lower-order harmonics are typically harder to detect than higher-order ones, and even-order harmonics are generally less audible than odd-order ones. Furthermore, the nature of the distortion matters: crossover distortion is often more noticeable than clipping distortion because it produces stronger high-frequency harmonics that are harder for the brain to mask. Because of these psychoacoustic complexities, metrics like high-order harmonic distortion (HOHD) have been developed to better estimate how much distortion actually impacts the listener.

Calculating THD also involves methodological choices that can make comparing different products difficult. Manufacturers may use different definitions, such as THDF, which uses the fundamental frequency as the reference, or THDR, which uses the total signal. While these methods yield similar results at low distortion levels, they diverge significantly as distortion increases. Additionally, many professionals prefer using THD+N, which includes noise in the measurement, providing a more comprehensive view of signal degradation. Because THD is not a standardized specification, experts emphasize that measurements must be interpreted with care, accounting for the specific test conditions and the bandwidth used.

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