THD

Total harmonic distortion (THD) is a measure used in electrical engineering and signal processing to quantify the extent to which a waveform deviates from a pure sinusoid due to the presence of harmonics. Harmonics are sinusoidal components at integer multiples of the fundamental frequency.

THD can be expressed for voltage, current, or power. For a periodic voltage waveform, THD is defined as the root-mean-square of all harmonic components divided by the RMS value of the fundamental component: THD = sqrt(V2^2 + V3^2 + V4^2 + …) / V1. Expressed as a percentage, THD% = 100 × THD. When using power quantities, a similar form applies with powers or currents.

Measurement relies on spectral analysis of a signal, typically with a fast Fourier transform. A measurement instrument such as a spectrum analyzer or a specialized THD meter performs the decomposition and reports THD, THD+, N, or individual harmonic magnitudes. Standards and acceptance criteria vary by application; in electrical power systems, IEEE 519-2014 provides limits for voltage and current THD depending on service voltage and network impedance.

Applications and implications: in power distribution, high THD can cause equipment overheating, reduced efficiency, and malfunction of sensitive devices; in audio, lower THD generally indicates higher fidelity, though perceptual audibility depends on level and context. Typical consumer audio amplifiers target low THD values, often below 0.1% at rated power; laboratory and high-fidelity gear may report THD in the 0.001–0.01 range. The term THD+N includes the effect of the device’s noise floor.

Limitations: THD does not capture intermodulation distortion or phase relationships between harmonics, and it does not directly reflect perceived quality in all cases. It is one metric among many used to assess waveform quality and power quality.