What Is Speaker Total Harmonic Distortion? (THD Explained)

Speaker total harmonic distortion is a speaker specification that can tell us a lot about the quality of a speaker driver build. It is a fundamental tool of speaker design and development. 

The total harmonic distortion (THD) measured curve can tell a speaker designer a lot about their design and any non-linear distortion effects the speaker driver may be introducing. 

Total harmonic distortion (THD) compares the input signal to the output signal of a speaker driver, highlighting any non-linear distortions or unwanted effects introduced by the speaker driver itself. As a general rule, the lower the THD value, the better. 

For speaker designers, total harmonic distortion measurements give a good indication of any nonlinear build problems with the speaker system.

The aim of most speakers is to replicate the input audio signal as accurately as possible. However, this is impossible to do in the real world as the speaker driver will introduce some effects. It can’t be perfect. 

In its simplest form, THD compares the input audio signal to the output audio signal and tells us how well the speaker is replicating the sound without introducing too much distortion. 

Total harmonic distortion measurement curves can be confusing to interpret and understand as a new speaker design engineer, so in this article, I will cover:

  • What is speaker total harmonic distortion (THD)?
  • What is non-linear distortion?
  • How is speaker total harmonic distortion measured?
  • How to read a total harmonic distortion speaker curve?
  • What is 2nd & 3rd harmonic distortion?
  • What is a good THD for a speaker?

What Is Speaker Total Harmonic Distortion (THD)?

In a perfect world, the output from a speaker will reproduce the input signal perfectly. However, in reality, the output signal from a speaker will have noise and additional audible effects because of the construction and motion of the speaker parts. 

Total harmonic distortion is how much a speaker’s output signal deviates from the input signal as a result of non-linear distortion. Non-linear distortion includes unwanted effects created by the speaker mechanics, such as overheating or poor suspension movement.

In very simple terms, everything on the output signal of a speaker that differs from the input signal is a distortion.

We express total Harmonic Distortion as a dB level below the input signal or as a percentage of the input signal, which is the most common representation of this number.  

The lower the THD number, the better.

What Is Non-Linear Distortion?

Non-linear distortion causes harmonic distortion. 

As a simple definition, non-linear distortion describes the phenomenon of a non-linear relationship between the “input” and “output” signals. [source]

When you send a simple audio signal to a speaker and measure the total harmonic distortion output, any distortion effects or deviation from the original input signal are classified as non-linear distortion.

Here are just a few examples of speaker-build problems which can create non-linear distortion and impact our total harmonic distortion result. 

  • Poor voice coil former construction. 
  • Poor voice coil construction. 
  • Off-centre voice coil.
  • Non-symmetric movement of the voice coil. 
  • Non-symmetric suspension placement
  • The voice coil moves out of the gap. 
  • Non-rigid pistonic motion of the cone. 
  • Diaphragm not moving coherently
  • Air compression problems

The total harmonic distortion measurement is just a symptom of a problem; it does not tell us explicitly what is causing the distortion. 

Therefore, if you have a poor total harmonic distortion measurement, the poor result could be caused by a range of problems, including some, a few, or none of the speaker build problems already mentioned. 

This debugging of speakers during speaker design and development, where problems can be created by so many variables, is a common problem faced by loudspeaker designers. 

How Is Speaker Total Harmonic Distortion Measured?

We measure speaker total harmonic distortion by applying a sine sweep signal to a speaker, measuring the output, and then comparing the input and output signal to check for any differences. 

In the speaker audio industry, professional audio measurement equipment is used to measure total harmonic distortion. 

For example, the Klippel acoustic measurement system is a professional audio measurement system used by speaker manufacturers to diagnose system nonlinearities with speaker designs. [source]

Generally, an input signal, which is a sine sweep (called the fundamental frequency) is sent to the speaker and the output signal is then compared to this fundamental frequency.  

A curve of total harmonic distortion, 2nd, and 3rd harmonic distortion is plotted so speaker designers can inspect and understand how their design is performing. 

How To Read A Total Harmonic Distortion Speaker Curve?

The following image is a total harmonic distortion measurement as completed on a Klippel system. This is a measurement of Kali audio’s IN-8 V2 speaker as completed by the Audio Science Review. Check out the Audio Science Review here to learn more about this speaker, learn more about measurements and support a wonderful audio site. 

Image via Audio Science Review

From this image, we can see the fundamental input signal and the THD measurement. As a general rule, the THD measurement should always be significantly lower than the fundamental frequency. 

I have seen THD measurements where the THD is higher than the fundamental. This tells us that something is significantly wrong with the measurement setup and external noise is getting into the system. (Unless you designed a really bad speaker!)

What Is 2nd & 3rd Harmonic Distortion?

Every input signal will have harmonics. In very simple terms, 2nd order harmonic distortion is sound distortion added at twice the input frequency. 

For example, if we input a signal of 1kHz, 2nd order harmonic distortion will happen at 2kHz. We may have unwanted sounds in this region. [source]

In a similar way, 3rd order harmonics is sound distortion added on the 3rd harmonic, which can be listened for in the 3kHz region. 

Total Harmonic Distortion is the sum of all this distortion.

Although there is 4th, 5th, and 6th harmonics etc, by concentrating on controlling or improving the 2nd and 3rd harmonic distortion, the higher harmonic distortion will be controlled by default. 

What Is A Good THD For A Speaker?

A single THD number will not give us an indication of the sound quality or tell us if we have a good loudspeaker.

For example, although we often percieve 2nd order harmonic distortion as a warmer sound and 3rd order harmonic distortion as being harsh or aggressive, as the actual THD number includes both figures as a total, you cannot determine if there is more 2nd or 3rd order harmonic distortion from one number. 

Therefore, a single THD number is not an indication of sound quality. 

Most listeners don’t need to concern themselves with speaker THD. Let’s say you have a speaker with a specification of 0.1 THD. In the real world, this means that less than 0.1% of the output signal has distortion effects added by the speaker. 

If the THD is less than 1%, the vast majority will not notice this, although some audio experts may claim they can hear distortion at this level. From my experience, if you can hear distortion at this level, you must be a trained listener or a die-hard audiophile.

Final Thoughts

Total harmonic distortion (THD) is one of these numbers that are more important for speaker designers than for speaker users. 

In music production, when we want to replicate a recording as accurately as possible, it is worth paying attention to THD as we need to know how much our speakers are adding unwanted effects as distortion to the audio recording we are mixing. 

As a speaker design engineer, reading THD curves is a great way to see how well your speaker design and build are performing. Speaker design engineers generally focus on the 2nd and 3rd harmonic when developing a speaker. 

If everything works well, then your speaker THD curve will show nothing too abnormal and be at a low level. A good THD level to aim for as a speaker engineer is a distortion level of at least -40dB from your fundamental frequency, which equates to a distortion level of 1%.

As an audiophile, if you are concerned about THD and comparing speakers, then the lower the THD value the better.

Happy listening!

Engineer Your Sound

We love all things audio, from speaker design, acoustics to digital signal processing. If it makes noise, we are passionate about it.

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