What is Total Harmonic Distortion (THD)

What is Total Harmonic Distortion?

TLDR: Total harmonic distortion (THD), is a measurement that shows how much distortion a voltage or current has due to harmonics.

Total harmonic distortion (THD), is a measurement that shows how much distortion a voltage or current has as a result of harmonics. THD is an important aspect of audio, communications and power systems. Generally speaking, THD  should be as low as possible, but that is not always the case.

 

Total Harmonic Distortion Defined

Total harmonic distortion (THD), is an accumulation of electrical current distortion that is greater than the ideal. The majority of household electrical systems draw linear loads. The sine curve of linear current shows smooth, sinusoidal peaks and valleys. Residential circuits can experience some distortion, but not enough that it causes significant efficiency problems.

Many commercial and industrial applications, however, use non-linear loads. Harmonic distortion is caused by the way power supplies, converters and generators draw power. This can cause serious problems for power systems’ efficiency. These loads are inductive and have low power factors: The non-working power of a load is a large percentage of its total apparent power.

 

THD Measurement

Figure 3 shows an example block diagram showing a 1kHz sinewave passing through an amplifier. This creates a new 1kHz sinewave with some crossover distortion. The new wave is fed into a spectrum analyzer, which displays the amplitude of various harmonics.

I measured each harmonic frequency individually from a typical frequency spectrum and then recorded the data in this table:

Amplitudes in Harmonics of Distorted Sinewave

Harmonic Amplitude

  • 1 3.08V
  • 3 0.308V
  • 5 0.159V
  • 7 0.090V
  • 9 0.0487V
  • 11 0.0253V
  • 13 0.0164V
  • 15 0.010V

The amplitudes for harmonics higher than the 15th and even-numbered harmonics are almost zero, so I did not include them in my calculation.  The net result is 0.118, or 11.8%.

A THD analyzer could automate the process of computing THD using the amplitudes and harmonics. The THD analyzer gives this signal a value of 11.8%. This confirms the accuracy of my manual method.

System THD: Importance

This article provides background information about THD and how it can be determined in real systems. It hasn’t discussed the types of systems in which THD can be an important measurement.

The THD is an important factor in many types of systems. Power systems have a lower THD, higher peak currents and greater efficiency. Audio systems have a lower THD, which means the audio signal reproduces more closely the original recording. Communication systems, on the other hand, have a lower THD and less interference from other devices.

 

THD FAQ’s:

 

Is Total Harmonic Distortion Important?

Most people who listen to music enjoy the sound of total harmonic distortion. This is why? THD levels on modern amps are almost non-existent to the human ear. Audiophiles can experience distortion at any stage. Poor mastering, file compression and headphone resonance can all lead to distortion. THD is one factor that can hinder the creation of a pure audio signal.

 

Do I Have to Worry about THD?

Most of the time, I would say no. You don’t have to worry about harmonic distortion because most audiophile components already pay attention to it minimization beyond human hearing. But that’s not what’s important. Audiophiles are a game of inches. The goal is to minimize distortion points as possible in the hopes of achieving noticeable improvements. While there are many benefits to improving other parts of your audio setup, it’s important to pay attention to THD and ensure it is as low possible.

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