THDn

THDN is an acronym used in electronics and audio engineering to refer to Total Harmonic Distortion plus Noise. In practice, it describes how much of a system’s output contains distortion products from nonlinearity (harmonics) and random noise, relative to the intended fundamental signal. The term is often treated as synonymous with THD+N in datasheets and test reports.

Measurement of THDN involves injecting a pure sine wave at a specified level and frequency, then analyzing the output within a defined measurement bandwidth. THDN is typically expressed as a ratio of the combined power of distortion components and noise to the power of the fundamental, either as a percentage or in decibels. The standard form is THD+N = sqrt(P2 + P3 + … + Pnoise) / P1, with results reported for a given bandwidth. The bandwidth, level, and gating method used during measurement can significantly affect the reported value, since more noise or higher harmonics may be captured at wider bandwidths.

Applications of THDN include assessment of audio components such as DACs, ADCs, preamplifiers, power amplifiers, and headphones. It provides a compact indicator of linearity and noise performance, complementing other metrics like signal-to-noise ratio and THD alone. Limitations include sensitivity to measurement conditions and the fact that THDN does not capture all aspects of audible performance; different listening scenarios, frequencies, and dynamic ranges may yield different perceptual results. For these reasons, THDN is typically considered alongside a broader set of specifications.