Total Harmonic Distortion (THD)
Definition
Total Harmonic Distortion (THD)
Total Harmonic Distortion (THD) quantifies the nonlinear distortion in an audio system by measuring the ratio of harmonic overtone energy to the fundamental frequency energy, expressed as a percentage or in decibels. Defined by IEC 60268-5 Clause 24, THD indicates how faithfully a system reproduces a pure tone. SonaVyx measures THD through spectral analysis of individual harmonics.
THD = √(V₂² + V₃² + V₄² + V₅²) / V₁ × 100%, where Vₙ = harmonic amplitude
How Total Harmonic Distortion (THD) Is Measured
THD is measured by feeding a pure sine tone into the system under test, performing FFT analysis on the output, and comparing the energy at harmonic frequencies (2f, 3f, 4f, 5f) to the fundamental frequency f. SonaVyx measures harmonics H2 through H5 individually and computes total THD per IEC 60268-5. THD+N adds noise floor energy to the measurement for a more complete distortion picture.
Practical Example
A power amplifier is tested at rated output with a 1 kHz sine tone. SonaVyx FFT analysis reveals the fundamental at -3 dBFS, H2 at -62 dBFS, H3 at -68 dBFS, H4 at -75 dBFS, and H5 at -80 dBFS. The resulting THD is 0.12%, well below the 1% threshold that is generally considered audible for music reproduction.
Harmonic Order and Character
Even harmonics (H2, H4) are often described as warm or musical because they reinforce octave relationships. Odd harmonics (H3, H5) are perceived as harsher and more noticeable at equivalent levels. Tube amplifiers typically produce predominantly even harmonics, which is one reason they are considered musically pleasing. Solid-state amplifiers with crossover distortion tend toward odd harmonics, which sound more objectionable even at lower THD percentages.
THD vs THD+N
THD measures only harmonic distortion products. THD+N (Total Harmonic Distortion plus Noise) includes all non-fundamental energy — harmonics, noise floor, hum, and interference. THD+N is a stricter metric because it captures the complete deviation from a pure tone. For professional audio equipment, THD+N below 0.01% (-80 dB) is considered excellent. Consumer equipment typically achieves 0.1% to 1%.
THD at Different Drive Levels
Distortion typically increases as the system approaches its maximum output capability. Manufacturers specify THD at rated power, but measuring at multiple drive levels reveals the onset of clipping. SonaVyx can sweep the input level while measuring THD at each step, producing a THD-vs-level curve that shows the system's linear operating range and maximum clean output.
Speaker THD
Loudspeaker THD is measured per IEC 60268-5 Clause 24, typically at rated power with sine tones at multiple frequencies. Speaker THD is significantly higher than electronic equipment — 1 to 3% is common for woofers at rated power, particularly at low frequencies near the speaker's resonance. Tweeters typically achieve lower THD (below 0.5%) within their operating range.
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