The percentage of unwanted extra tones a system adds to the original signal. Lower THD means cleaner, more accurate sound.
THD measures how much a component adds frequencies that weren't in the original signal — mathematically related “harmonics” of the input tone. It's expressed as a percentage of the total output.
THD generally rises near a component's power or excursion limits, which is why your THD reading often climbs sharply at very low frequencies (where excursion is highest) or near maximum volume. A small THD bump at one frequency, surrounded by a clean curve elsewhere, usually points to a specific mechanical or electrical limit being approached — not a system-wide problem.
| Context | Typical Home Audio | Typical Car Audio |
|---|---|---|
| Clean operating range | < 0.5% | < 1% |
| Audible / noticeable | > 3 – 5% | > 3 – 5% |
