Total harmonic distortion (THD): Measuring and correctly classifying distortion
Total harmonic distortion (THD) is the key indicator of how cleanly an amplifier, converter, or loudspeaker reproduces an audio signal. This glossary entry explains what THD and THD+N mean, which values are common in studio and hi-fi systems, how to measure distortion yourself – and why a touch of it is beneficial in mixing and recording. professional mastering It may even be desirable.
Table of Contents
Everything you need to know about THD and distortion levels at a glance:
What is total harmonic distortion (THD)?
The total harmonic distortion (THD) describes how many harmonics a system adds to a signal that are not present in the original. Every real circuit operates with a slight non-linearity: a pure sine wave will produce harmonic distortions at the integer multiples of the fundamental frequency (k2, k3, k4...).
The ratio of the RMS values of these harmonics to the total signal is measured, expressed as a percentage: 1% means that one hundredth of the signal energy consists of additional overtones. The mathematical definition and the differentiation of the variants are explained by the Wikipedia background on measurement technology (English) in detail.
Important for practical purposes: The value alone says nothing about the sound. What matters is which harmonics are produced – even harmonics (k2) sound round and musical, odd harmonics (k3) sound wiry to aggressive – and at what level the measurement was taken.
Total harmonic distortion in numbers: THD vs. THD+N
THD Total Harmonic Distortion (THD) only adds up the distortion – that is, the harmonic components. THD + N (plus Noise) also includes the noise of the device in the measurement – the value is stricter, more practical and is therefore found in most datasheets.
| device class | Typical value (THD+N) | classification |
|---|---|---|
| Studio converter (AD/DA) | <0,001% | Virtually distortion-free – at the limit of measurability |
| HiFi amplifier | <0,05% | Clean and uncritical – far below the hearing threshold |
| tube devices | 0,5-5% | Deliberately high – the distortion is an intentional tonal characteristic here. |
| Loudspeakers | 0,5-3% | Largest source of distortion in the entire eavesdropping chain |
What is striking is the huge gap between electronics and acoustics: the weakest link in the chain is almost always the loudspeaker – not the interface or the amplifier in front of it.
When harmonic distortions are disruptive – and when they are part of the overall tone
Whether harmonic distortion is perceived as a flaw or as a characteristic depends on its type and intensity. Tube and tape machines primarily produce even harmonics, which the ear perceives as warmth and density – this is precisely the effect sound engineers use as saturation (Saturation) is used intentionally to make voices and instruments sound fuller.
The counterpart is hard ClippingAbruptly cutting off peak levels creates strong odd and dissonant components that quickly sound harsh and unpleasant. Therefore, a high THD value is not automatically a flaw – as long as the coloration suits the material and is a deliberate choice.
Whether the desired color comes from real hardware or emulation is secondary – our article compares the strengths of both worlds. Analog vs. digitalThe crucial point is that the coloring remains a conscious decision and that clanging does not occur as an unwanted defect in the chain.
Measuring THD in practice: sine tone, analyzer, measuring microphone
Electronics (interface, amplifier): A pure sine wave – typically 997 Hz or 1 kHz – is sent through the device, and the output is looped back in. An FFT analyzer (e.g., in REW or the plugin analyzer) displays the harmonics as clean lines over the noise floor; the software then calculates the THD or THD+N value.
Speaker: Here comes a calibrated Measurement Microphone in front of the box. Measurement is taken at a defined temperature. Sound Pressure Level and in several frequencies, because the distortion increases significantly with the level – a value without a level specification is worthless.
For everyday studio work, a quick check is usually sufficient: play a sine tone, vary the level and observe in the analyzer when overtones visibly emerge from the noise.
Common mistakes and misunderstandings about total harmonic distortion
THD figures appear objective, but can quickly lead to misunderstandings in datasheets – you should therefore be aware of the following five points:
- Comparing datasheets without measurement conditions: A THD value is only valid for the specified level, frequency, and load. Without this information, figures from two manufacturers are not comparable.
- Confusing THD with THD+N: The same amplifier might excel in THD but disappoint in THD+N if it produces noise. Always check which value is specified.
- "The lower the pitch, the better it sounds": Below the threshold of hearing, adding another zero after the decimal point makes no difference – 0,001% and 0,0001% sound identical.
- Interpreting tube values as a defect: The 0,5–5% variation in a tube stage is not a defect, but rather the reason why the device is purchased: the desired tone color.
- Ignore the wiretapping: Anyone judging loudspeakers with 2% distortion doesn't need to worry about 0,001% in the converter – optimize the listening chain first, then the electronics.
Conclusion: Low THD values are the foundation, tonal quality is the icing on the cake.
Total harmonic distortion (THD) makes distortion comparable: it shows how neutrally converters, amplifiers, and loudspeakers truly operate – provided the level, frequency, and measurement (THD or THD+N) are known. Modern studio electronics have long since transcended any audible doubt in this regard; the real differences arise in loudspeakers and deliberately colored components such as tube and tape amplifiers. A low THD is therefore the foundation – the real challenge lies in allowing distortion precisely where it is desired as a tonal characteristic.
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FAQ – Frequently Asked Questions about THD and Distortion
What is a good total harmonic distortion (THD) value?
For electronics, a THD+N value below 0,05% is uncritical; modern studio converters achieve values below 0,001%. Loudspeakers, due to their design, typically have a THD+N value between 0,5% and 3% – this is normal. The level and frequency at which the measurement was taken are also crucial.
What does THD+N mean?
Total Harmonic Distortion plus Noise: In addition to distortion, the device's own noise is also included in the measurement. This value is therefore more precise than pure THD and describes real-world performance better – which is why it is included in most datasheets.
Why do tube amplifiers sound good despite high distortion?
Tubes predominantly produce even harmonics (k2), which the ear perceives as warmth and fullness – the high THD value therefore consists of musical, not harsh, components. This coloration is deliberately used as saturation and is the main reason why tube amplifiers are valued in studio applications.
How can I measure THD myself?
For electronics: send a pure sine wave (1 kHz) through the device via loopback and read the harmonics in an FFT analyzer – software like REW calculates the value automatically. For loudspeakers, a calibrated measurement microphone and a defined sound pressure level are required.
Can you even hear 0,01% distortion?
Generally not: Music completely masks such low levels of distortion, and the speakers distort many times more. Furthermore, the distortion factor only becomes audible significantly above this level – or when inharmonic components such as hard clipping occur.