THD measures how much an audio device adds unwanted harmonic content to a pure signal. Lower THD means cleaner sound. Audiophile gear typically targets <0.1%.
Total Harmonic Distortion (THD) is a measurement of how much an audio device (headphone, amplifier, speaker) corrupts an audio signal by adding unwanted frequencies (harmonics) not present in the original. When you play a pure 1 kHz sine wave into a headphone, THD measures how much energy appears at 2 kHz, 3 kHz, 4 kHz, etc. — the unwanted harmonics. THD is expressed as a percentage: a headphone with 1% THD adds harmonic content equal to 1% of the original signal's energy. Lower is cleaner; higher means more "distortion" or "grunge."
**How THD arises technically:** Headphone drivers have mechanical and electrical nonlinearities. At low volumes, a driver's diaphragm moves in the linear zone (motion proportional to voltage), producing clean output. As volume increases, the diaphragm reaches mechanical limits (maximum excursion), and motion becomes nonlinear — the driver can't push as hard on the "down" stroke as the "up" stroke, creating asymmetry. This asymmetry generates harmonics. Amplifiers add THD through saturation (clipping at high volume) and component nonlinearities. Bass frequencies push drivers harder, so THD is highest in the 20–200 Hz range at high SPL (sound pressure level). Measuring THD at 94 dB SPL (moderately loud) vs 114 dB SPL (very loud) yields drastically different results — specs usually cite 1 kHz at 94 dB for fair comparison.
**Why THD matters to buyers:** Below 1% THD, distortion is inaudible to most listeners in A/B tests. Above 3%, distortion is clearly audible as "grunge," "muddiness," or a harsh quality. The jump from 0.1% to 0.05% THD is imperceptible even to trained ears. Studio monitors and high-fidelity speakers target <0.5% THD across the frequency range. Consumer headphones typically range 0.5–2% depending on how hard the driver is pushed. Gaming and casual listening don't benefit much from ultra-low THD. Critical listening, classical music, and mixing require lower THD for accuracy.
**What to look for / common pitfalls:** - THD varies with frequency and SPL; always check at what loudness specs are measured (94 dB is standard, 100 dB reveals worse THD) - Bass-heavy headphones (large drivers) naturally have higher THD at high volume due to physics - "THD+N" (distortion plus noise) is the complete measurement; THD alone omits background hiss and hum - Low THD specs are meaningless without frequency response data — a device could have <0.1% THD but sound wrong due to frequency imbalance - Marketing myth: THD <0.01% is not perceptually different from <0.1% in consumer gear
Real-world 2026 measurements: High-end headphones (Audeze, HiFiMAN) achieve <0.5% THD below 1 kHz at 94 dB. Premium earbuds (AirPods Pro 2, Sony WH-1000XM5) measure 1–2% THD across the spectrum. Budget earbuds ($20–50) often exceed 3% THD, especially at high volume or in bass. Amplifiers for headphones (audiophile portable amps) target <0.01% THD. Gaming headsets (SteelSeries, HyperX) rarely publish THD specs, prioritizing mic quality over accuracy.