THD vs. Clipping in Car Audio Amps — Understanding the Difference and Why It MattersUpdated 13 hours ago
THD vs. Clipping in Car Audio Amplifiers
Summary
- Clipping happens when an amplifier hits its voltage ceiling and the waveform peaks flatten. It’s an event, not a percentage.
- THD (Total Harmonic Distortion) measures how much harmonic content exists relative to the main signal.
- THD rises gradually with output; once clipping starts, it increases sharply at the “knee.”
- 1–3% THD is common on sub amps before clipping.
- Clipping—not mild THD—is what overheats coils and damages speakers.
Understanding the difference
Clipping and THD are related but not the same. Clipping happens when the amplifier runs out of headroom, while THD measures the distortion percentage at any given level. You can measure THD before clipping occurs—but once the signal flattens, THD spikes.
What is clipping?
Clipping occurs when the amplifier output tries to exceed its supply rails. The sine wave flattens at the peaks, compressing dynamics and generating extra high-frequency content. This added energy increases voice-coil temperature and stress.
How to detect it: An oscilloscope displays clipping directly. A distortion analyzer will show a sudden jump in harmonic content at the onset of clipping.
What is THD?
Total Harmonic Distortion is the ratio of harmonic energy to the main frequency (fundamental). It’s expressed as a percent of the total signal. THD+N includes noise and usually measures slightly higher.
- Low output: often <0.1% on high-quality amplifiers.
- Near rated power: 1–3% THD is common before clipping.
- At or beyond clip: THD can rise to 5%, 10%, or even 20%+.
How THD relates to clipping
On a THD-vs-output graph, distortion increases slowly at first, then rapidly near the “knee”—the point where clipping begins and the waveform runs out of headroom.
| Output level (example) | Clip condition | Approx. THD% | Audible result |
|---|---|---|---|
| ~10 W | No clipping | ~0.05% | Clean, transparent |
| ~500 W | No clipping | ~0.5% | Clean |
| At rated output | No clipping | ≤1–2% | Normal spec range |
| +1–2 dB above rated | Slight clipping | ~3–7% | Edge of harshness |
| Well beyond rails | Severe clipping | ~15–40%+ | Distorted and compressed |
Why 2% THD doesn’t mean clipping
Many amplifiers—especially Class-D sub amps—show 1–3% THD before any visible flattening. Others maintain very low THD up to their voltage rails. A single THD number doesn’t confirm clipping; use a scope or watch for the THD curve’s knee.
Why clipping damages speakers
- Heat: flattened peaks increase duty cycle and coil temperature.
- Reduced cooling: clipped signals keep the coil at its extremes longer.
- Mechanical stress: squarer waves can drive nonlinear movement and unwanted resonances.
Detecting distortion correctly
- Oscilloscope: directly displays waveform shape; flat peaks indicate clipping.
- Distortion detectors (e.g., SMD DD-1): detect when distortion passes a threshold.
- Analyzers (e.g., JL Audio MAX, Audio Precision APx): perform THD/THD+N sweeps and reveal clipping visually.
Amp dyno interpretations
- Certified mode: stops at a preset distortion limit (often 1% THD) and reports clean power.
- Uncertified mode: continues to clipping to show maximum power output.
Tip: Publishing both “clean” (to ~2% THD) and “max” (at clip) values shows the full performance envelope.
Why THD may rise sooner on some amps
- Topology: Class-D designs may show more THD at higher frequencies but remain linear in bass.
- Feedback loop design: limited bandwidth can raise distortion at higher power.
- Power supply sag: voltage drop increases distortion before clipping.
- Load effects: impedance and phase can shift the knee location.
Power-rating practices
- Car audio: rated around ≤1% THD (CTA/CEA standards).
- Home/hi-fi: rated around ≤0.1% THD for precision.
- Reality: 1–2% THD in sub amps is inaudible and far from clipping.
Key takeaways
- Clipping is a voltage-limit event, not a THD percentage.
- THD exists at all levels and rises gradually until headroom is gone.
- 2–3% THD is normal and not automatically clipping.
- It’s clipped power—not small THD—that damages drivers.
- Use a scope or analyzer to see the waveform and knee directly.
Glossary
- THD: total harmonic distortion, ratio of harmonics to the fundamental (percent).
- THD+N: distortion plus noise, slightly higher than THD alone.
- Clipping: waveform flattening when output exceeds supply rails.
- Knee: the point on a THD curve where distortion rapidly increases—clipping onset.