THD

THD vs Clipping in Car Audio Amplifiers

THD vs Clipping in Car Audio Amplifiers

How they differ, how they relate, and why 2% THD does not automatically mean you’re clipping.

Beginner + Intermediate Mobile-first Includes visuals

TL;DR

  • Clipping is a hard limit where the waveform hits the rails and flattens[7]. It’s an event, not a percentage.
  • THD is a measured percentage of harmonic content[1]. All amps have some THD even at low power.
  • THD usually rises slowly with power; at clip it spikes sharply[1].
  • 2–3% THD without clipping is common on many sub amps[1].
  • Clipping is what overheats voice coils and kills subs; THD alone doesn’t[7].

1) Introduction

“Clipping” and “THD” are related but not the same. Clipping is the point where the amplifier can no longer increase peak voltage[7]. Total Harmonic Distortion (THD) quantifies how different the output is from a perfect sine wave[1].

You can have measurable THD well before clipping. Clipping is where THD stops rising slowly and skyrockets[1].

2) What is Clipping?

Clipping occurs when the output tries to exceed the amp’s supply rails. The tops and bottoms of the sine wave flatten[7]. This compresses dynamics, adds high-frequency energy, and increases heat in voice coils[7].

Waveform before and after clipping Clean sine (no clip) Clipped (flat tops/bottoms)
Before clipping the sine is smooth. After clipping, peaks flatten against the rails.
Detection: An oscilloscope shows flat-topping directly. Full analyzers also reveal a sharp jump in harmonic content at clip[1].

3) What is THD?

Total Harmonic Distortion (THD) is the ratio of all harmonic energy to the fundamental, expressed in percent[1]. Even a non-clipping amplifier has non-zero THD[1].

  • Low power: often <0.1% on good designs[1].
  • Near rated: 1–3% without clipping is common on sub amps[1].
  • At clip: THD spikes fast (5%, 10%, 20%+)[1].
How instruments compute it: RMS of harmonics divided by RMS of the fundamental. Many systems report THD+N (distortion plus noise); it’s typically slightly higher than THD alone[1].

4) How Clipping and THD Relate

THD usually rises gradually as output power increases, while clipping is a sudden geometric change when the waveform runs out of headroom. On a graph, the THD curve shows a clear “knee” where clipping begins[1].

Typical THD% vs Output Power Low power High power THD% Clipping onset → fast THD rise
THD rises gradually, then spikes at clipping onset (knee).
Output LevelClipping?THD% (typical)Listening Note
10 WNo0.05%Clean
500 WNo0.5%Clean
Rated powerNo≤1–2%Acceptable per many specs[3]
+1–2 dB above ratedJust starting3–7%Edge appears
Well above railsHard clip15–40%+Harsh, compressed

5) 2% THD Does Not Mean You’re Clipping

Many subwoofer amplifiers reach 1–3% THD before clipping; others keep THD very low until right at the rails. Either way, 2% THD can be clean power with no flat-topping visible on a scope[1].

Rule of thumb: Use the knee in the THD curve and visible flat-topping as your clip indicators. A single THD number is not a clip flag[1].

6) Why Clipping Damages Subwoofers

  • Heat: Flat tops inject strong harmonic content and increase effective duty cycle. Coils run hotter[7].
  • Quasi-DC behavior: Severe clipping holds the coil near extremes longer, reducing cooling time[7].
  • Mechanical stress: Square-ish waveforms can excite unintended resonances and increase excursion nonlinearly[7].
Bottom line: THD alone doesn’t blow drivers. Clipped power does[7].

7) Detecting Clipping and Distortion Properly

Oscilloscope

Shows waveform shape directly. Flat peaks = clipping. Gold standard for visual confirmation[1].

SMD DD-1

Distortion detector. Designed for gain setting by detecting when distortion exceeds its threshold; not a scope and not a clip meter[5].

JL Audio MAX

Generator + analyzer with FFT, scope view, and logging. Useful for mapping THD curves and seeing the knee[6].

Audio Precision APx

Lab-grade analyzers with APx500 software for THD/THD+N, sweeps, and clipping analysis[2].

Note: Many analyzers report THD+N. It includes noise along with distortion and reads slightly higher than THD alone[1].

8) SMD AD-1 Modes and Our Bench Protocol

AD-1 Modes (relevant ones)

  • Certified: test stops when distortion reaches the specified threshold (commonly 1% THD) and reports clean power[4].
  • Uncertified: test continues to the onset of clipping and reports maximum power at clip[4].

Other modes exist (e.g., Dynamic), but the two above map cleanly to “to 1% THD” vs “to clipping.”

Our Protocol on This Site

  • Rated Power (forgiving): we report power at 2% THD (slightly above the typical 1% THD rating standard)[3].
  • Max Power: we also report the maximum power at clipping (equivalent to an “uncertified” result)[4].

9) Why Some Amps Show Higher THD Before Clipping

  • Class topology: Class D sub amps may have higher THD at HF yet remain linear in the bass band[1].
  • Loop design: Feedback bandwidth, dead-time control, and output filter quality affect distortion growth[1].
  • Supply regulation: Sag or ripple raises distortion under load before hard clipping[1].
  • Load dependency: Impedance and phase angle shift the knee position versus frequency[1].

10) “Clean Power” Ratings and Standards

  • Car audio: Many brands cite power at ≤1% THD in line with CTA/CEA-2006 practice[3].
  • Home/hi-fi: Often rate at ≤0.1% THD to emphasize low-distortion mid/high performance[1].
  • Reality: A sub amp at 1–2% THD can still be audibly clean in the bass range and well shy of clipping[1].

11) Final Takeaways

  • Clipping is a rail-limit event[7].
  • THD is a measured percentage that exists at all power levels[1].
  • 2–3% THD can be normal without clipping on many amps[1].
  • Clipping is what damages subs by heat and stress[7].
  • Trust the scope and the THD knee, not any single THD number[1].

Appendix: Quick Glossary

  • THD: Ratio of all harmonic energy to the fundamental, in percent[1].
  • THD+N: THD plus noise; typically a bit higher than THD alone[1].
  • Clipping: Flattening of peaks when the waveform hits supply rails[7].
  • Knee: Point on the THD-vs-power curve where distortion rises sharply[1].
Tools referenced: SMD DD-1 (distortion detector)[5], SMD AD-1 Amp Dyno[4], JL Audio MAX measurement system[6], Audio Precision APx analyzers (APx500 software)[2].

References

  1. Audio Precision. “What are THD and THD+N?” and application notes on THD vs level and clipping. https://www.ap.com/technical-library/what-are-thd-and-thdn/
  2. Audio Precision. APx500 Series analyzers and APx500 software product documentation. https://www.ap.com/analyzers-apx500/
  3. CTA / CEA-2006. Mobile Audio Amplifier Power Rating standard (overview). https://en.wikipedia.org/wiki/CEA-2006
  4. D’Amore Engineering / SMD. AD-1 Amp Dyno product page describing Certified vs Uncertified modes. https://damoreengineering.com/product/ad-1-amp-dyno/
  5. SMD. DD-1 Distortion Detector owner's manual (purpose and operation). https://www.smddealers.com/dd-1-manual
  6. JL Audio. MAX Measurement System overview/specs (generator, FFT, oscilloscope, logging). https://www.jlaudio.com/pages/max-measurement-system
  7. Elliott Sound Products (Rod Elliott). “Clipping” – amplifier overload and loudspeaker damage article. https://sound-au.com/clipping.htm

Note: Where vendor manuals require login or purchase, the public product pages listed above provide the same functional descriptions.