THD Test Online — Distortion Check
Check total harmonic distortion with a built-in synthetic self-test that verifies the FFT method first. Indicative, not calibrated. Free in your browser.
Protect your hearing. Start at a low volume. Never raise the volume to make a tone audible. Levels are shown in dBFS, relative to full scale — not SPL.
How it works
THD is computed in your browser with an FFT over a synthetic or captured buffer. Nothing is uploaded or recorded. Levels are reported in dBFS, relative to full scale.
Total harmonic distortion compresses a whole distortion spectrum into one number: the RMS sum of harmonic energy divided by the fundamental. This page demonstrates the method honestly, with a synthetic self-test and clear limits.
Harmonic analysis
The analyzer FFTs the signal, finds the fundamental bin, then sums bins at integer multiples. The table lists each harmonic with its frequency, level in dBFS, and individual ratio so you can see which overtone dominates.
Self-test and limits caveat
Run the self-test to confirm the math reports the injected harmonics. Browser DSP, output filtering, and the lack of a calibrated input mean results are relative and indicative — useful for learning, not for certifying hardware.
Frequently asked questions
What is THD?
Total harmonic distortion is the energy in integer multiples of a fundamental tone, relative to the fundamental. It is reported as a ratio, a percent, and in dB.
What does the self-test prove?
The self-test synthesizes a buffer with known 2nd and 3rd harmonics and runs the same FFT analysis used for measurement. It verifies the math, not your speakers.
Can this measure my headphones?
No. Without a calibrated microphone and loopback, browser results are indicative only. Use the harmonic table to learn how distortion spectra read.
Why is the result bandwidth-limited?
Only harmonics below half the sample rate can be observed, and the FFT window sets frequency resolution. Higher harmonics fold into the noise floor.