Test Tones: THD & Sine Sweep
Two listening tests in one page. Step a 1 kHz tone up through exact distortion levels — from a pure sine to an obvious 5% THD — and mark the first one you can hear. Then run a 20 Hz–20 kHz sweep whose level follows the hearing-threshold curve, so at whisper volume it should stay barely audible from end to end.
Play the 1 kHz tone and raise the THD until the sound changes.
Step up from 0% and press this at the first level that no longer sounds clean.
Start at a low volume — the sweep’s low and high extremes are generated much louder than the mids, and sustained pure tones are fatiguing at high level.
Give each level a few seconds before moving up — clean turns edgy gradually, and quick switching smears the comparison.
The perceptual sweep is calibrated for threshold-level listening. Play it comfortably loud and the compensation stops matching your ear, so the mids disappear.
Speakers and headphones add THD of their own — often around 0.1%. Below that floor every level sounds identical, because the gear is the limit, not your ear.
Spec sheets are full of distortion figures — “THD < 0.01%” on a DAC, “1% THD” on a speaker — but almost nobody knows what those numbers sound like. These signals let you test the claim on your own ears and your own gear instead of taking the datasheet’s word for it. Everything is synthesized live by the Web Audio API at your device’s native sample rate: nothing is downloaded or uploaded, and the harmonic ratios are exact by construction rather than limited by a compressed audio file.
How does the THD listening test work?
Total harmonic distortion is how much of a signal is not the tone you asked for: fed a pure sine, a device adds overtones at multiples of the input frequency, and THD is their level relative to the fundamental. This test inverts that — it plays a 1 kHz sine with 2nd and 3rd harmonics added at an exact ratio. Start at 0%, give each level a few seconds, and step upward until the clean whistle picks up an edge or buzz; press “I hear distortion” there to record your threshold. Most people find anything below roughly 0.5% indistinguishable from pure on a sine tone.
How does the perceptual sine sweep work?
A constant-level sweep suits measurement microphones, not ears: hearing is far more sensitive around 2–4 kHz than at 30 Hz or 15 kHz, so a flat sweep sounds like silence, a swell, then silence. The perceptual sweep corrects both axes — time runs logarithmically, so 20 to 40 Hz lasts as long as 10 to 20 kHz, and the level rides the hearing-threshold contour, playing the extremes far louder than the mids. Turn the volume down until the opening rumble is only just audible and the rest should stay that quiet: a stretch that vanishes is your gear or your hearing rolling off, a stretch that jumps out is a resonance. The Flat option plays the same sweep at constant level.
What different THD levels mean
| THD | Level | Typical of | Can you hear it? |
|---|---|---|---|
| 0.001% | −100 dB | High-end DACs and studio amplifiers | No — far below any human threshold |
| 0.01% | −80 dB | Good audio interfaces and headphone amps | Inaudible |
| 0.1% | −60 dB | Quality headphones and speakers | Inaudible to virtually everyone — and many transducers sit at this floor themselves |
| 0.5% | −46 dB | Small speakers pushed hard | The keenest ears begin to hear a pure tone thicken |
| 1% | −40 dB | Phone and laptop speakers near full volume | Audible to attentive listeners — the classic rule-of-thumb line |
| 5% | −26 dB | Overdriven or clipping output | Plainly audible as buzz and edge |