THD — Total Harmonic Distortion

Total Harmonic Distortion (THD and THD+N)

For amplifier selection and power specifications, see audio/amplifiers-and-power. For gain structure and clipping, see glossary/gain-structure.

THD (Total Harmonic Distortion) measures the harmonic distortion that an audio device adds to a pure sine wave input — the higher the THD, the less faithfully the device reproduces the original signal. All audio components (amplifiers, DACs, preamps, DSPs) add some THD; the goal is to keep it inaudible (typically < 0.1% for professional audio). THD+N (Total Harmonic Distortion plus Noise) is the more complete specification, including both harmonic products and the noise floor in the measurement. In AV system design, THD matters most when selecting amplifiers, evaluating loudspeaker systems, and identifying clipping or overload conditions.

How THD Is Measured

A pure sine wave at a specific frequency (commonly 1 kHz) and level (typically rated output or 1W) is input to the device. The output contains the fundamental frequency plus harmonic overtones (2nd harmonic at 2 kHz, 3rd at 3 kHz, etc.) caused by nonlinearity. THD is the ratio of the sum of all harmonic amplitudes to the fundamental amplitude:

THD (%) = √(V₂² + V₃² + V₄² + ...) / V₁ × 100

Or expressed in dB:

THD (dBc) = 20 × log₁₀(THD%)

0.1% THD = −60 dBc (60 dB below the fundamental).

Typical THD Values in AV Equipment

Loudspeakers are the highest-distortion component in most audio systems by a large margin — amplifier THD of 0.01% is inaudible; speaker THD of 3% at high SPL is audible as harshness.

THD and Clipping

When an amplifier or DSP is driven into clipping, the output waveform is truncated — the tops and bottoms of the sine wave are cut flat. Clipping produces very high odd-order harmonic distortion (3rd, 5th, 7th harmonics), which the human ear finds harsh and fatiguing. Even brief clipping events (a transient peak that clips for < 1 ms) are audible because they generate broadband harmonic content.

In AV systems, clipping is the most common cause of distorted audio complaints. It is always a gain structure problem — too much gain somewhere in the chain. The correct fix is to reduce gain at the offending stage, not to insert a limiter as a permanent "fix" (limiters mask the symptom but don't solve the underlying gain structure error).

THD vs. IMD (Intermodulation Distortion)

IMD (Intermodulation Distortion) occurs when two or more tones interact in a nonlinear system to produce sum and difference frequencies that were not in the original signal. While THD tests with a single sine wave, IMD tests with two or more tones. IMD is generally considered more audibly offensive than THD because IMD products are not harmonically related to the input and appear as inharmonic noise.

SMPTE IMD and CCIF IMD are two standard tests: SMPTE uses a 60 Hz + 7 kHz mix; CCIF uses two high-frequency tones (e.g., 18 kHz + 19 kHz, producing a 1 kHz difference product). In practice, for professional installed AV, THD specs are more commonly specified than IMD, though both matter for critical listening environments.

Common Pitfalls

• Confusing THD at rated output with THD at typical operating level. Amplifiers are specified at rated power (e.g., 0.05% THD at 100W into 8Ω). At typical operating levels (10–20% of rated power), THD may be much lower or much higher depending on the amplifier design. Fix: evaluate THD vs. power curves, not just the rated-power spec, for critical applications.

• Tolerating persistent low-level clipping. Small clip events at DSP input stages or amplifier inputs appear as brief THD spikes. Over time these indicate a gain structure problem that will eventually cause hardware damage or listener fatigue. Fix: eliminate clipping at every stage; use metering to verify no stage clips during typical program material.

• Blaming amplifier THD for speaker harshness. Loudspeaker harmonic distortion at high SPL (3–10%) far exceeds amplifier THD (0.01–0.05%). The harshness is from the driver being driven beyond its linear range, not the amplifier. Fix: reduce playback level or specify higher-sensitivity speakers; consider line arrays or distributed speaker systems that spread the load.