How this instrument works
Total harmonic distortion measures how much of a signal's energy has leaked into unwanted harmonic frequencies rather than staying at the pure fundamental. It's calculated as the square root of the sum of the squared harmonic amplitudes — an RMS combination — divided by the fundamental's amplitude, then expressed as a percentage: THD% = sqrt(h2² + h3²) / fundamental x 100.
A lower THD means a cleaner signal that more faithfully reproduces the original waveform, which is why hi-fi amplifiers, DACs, and studio monitors are often specified and marketed by how low their THD figure is. A higher THD means the signal has picked up audible harmonic content — sometimes an unwanted flaw, and sometimes, as with tube amp overdrive or analogue saturation, a deliberately sought-after coloration.
This instrument uses the 2nd and 3rd harmonics, the two that typically dominate a distorted signal's harmonic content and the pair most commonly measured or specified in simplified THD figures. A full THD measurement in a lab setting can sum contributions from many more harmonics, so this is a practical approximation rather than an exhaustive spectral analysis.
- Enter Fundamental amplitude — the signal's amplitude at its base frequency.
- Enter 2nd harmonic amplitude — the amplitude measured at twice the fundamental frequency.
- Enter 3rd harmonic amplitude — the amplitude at three times the fundamental frequency (leave at 0 if you only have a 2nd-harmonic reading).
- Read Total harmonic distortion (%) directly beneath the inputs.
- Fundamental amplitude must be greater than zero, since it's the denominator of the ratio.
Worked example — fundamental 1.0 with 2nd and 3rd harmonics
Enter Fundamental amplitude = 1.0, 2nd harmonic amplitude = 0.03, 3rd harmonic amplitude = 0.02. That's sqrt(0.03² + 0.02²) / 1.0 x 100 = sqrt(0.0013) x 100, approximately 3.6056%. Total harmonic distortion reads 3.6056.
A THD figure around 3.6% is well above what a clean hi-fi amplifier would typically target (often well under 1%), but it's a modest, musically plausible amount of harmonic content for something like a mildly overdriven guitar signal or an inexpensive consumer audio device.
Questions
What counts as a 'good' THD figure for audio gear?
It depends heavily on the type of gear and its purpose. High-fidelity amplifiers, DACs, and studio monitors often aim for THD figures well under 1%, sometimes fractions of a percent, because transparency is the goal. Guitar overdrive pedals and tube amp circuits, by contrast, intentionally run much higher THD, because that added harmonic content is exactly what listeners hear as 'warmth,' 'grit,' or 'crunch.'
Why does this calculator only use the 2nd and 3rd harmonics?
Because in most real-world distorted signals, the 2nd and 3rd harmonics carry the large majority of the added harmonic energy, making them the most practically useful pair to measure or estimate quickly. A rigorous lab THD measurement can include many more harmonics, but a 2nd-and-3rd-harmonic estimate is a common, useful simplification.
Does a higher THD always sound worse?
Not necessarily — it depends heavily on which harmonics dominate. Even-order harmonics (like the 2nd) are generally described as adding a warmer, more musically pleasant character, while odd-order harmonics (like the 3rd) tend to sound harsher and more obviously 'distorted.' Two signals with identical THD percentages but different harmonic makeups can sound very different.
Can THD be exactly zero?
In theory, yes — a perfectly pure sine wave with no harmonic content at all would have 0% THD. In practice, virtually no real audio circuit achieves exactly zero; even very high-end equipment has some small, non-zero THD figure, just one small enough to be inaudible or negligible for practical purposes.
What's the difference between THD and THD+N?
THD alone measures only the harmonic distortion components of a signal, as this calculator does. THD+N ('THD plus noise') adds in the contribution from non-harmonic noise across the measurement bandwidth as well, so THD+N figures are typically slightly higher than THD-only figures for the same piece of equipment.