What Is Total Harmonic Distortion (THD)?

How is speaker accuracy measured?

Scan through the manual for an audio device and you're likely to find a specification called Total Harmonic Distortion (abbreviated as THD). This spec is found on speakers, headphones, media/MP3 players, amplifiers, preamplifiers, receivers, and more. Total Harmonic Distortion is important when considering equipment, but only to a certain point.

What Is Total Harmonic Distortion?

The specification for Total Harmonic Distortion compares the input and output of audio signals, with the difference in stages measured as a percentage. So you might see a THD listed as 0.02 percent with specified conditions of frequency and equivalent voltage in parenthesis (e.g. "1 kHz 1 Vrms").

There is a bit of math involved to calculate Total Harmonic Distortion, but all one needs to understand is that the percentage represents the harmonic distortion or deviation of the output signal. Lower percentages are better.

Remember, an output signal is a reproduction and never a perfect copy of the input, especially when multiple components are involved in an audio system. When comparing the two signals on a graph, you might notice the slight differences.

Fundamental vs. Harmonic Frequencies

Music is made of fundamental and harmonic frequencies. The combination of fundamental and harmonic frequencies gives musical instruments unique timbre and allows the human ear to distinguish between them.

For example, a violin playing a middle A note is producing a fundamental frequency of 440 Hz while also reproducing harmonics (multiples of the fundamental frequency) at 880 Hz, 1220 Hz, 1760 Hz, and so on. A cello playing the same middle A note as the violin still sounds like a cello because of its own particular fundamental and harmonic frequencies.

Why Total Harmonic Distortion Is (or Isn't) Important

Once the Total Harmonic Distortion has increased past a certain point, you can expect the accuracy of sound to be compromised. This happens when unwanted harmonic frequencies—ones not present in the original input signal—are generated and added to the output.

For example, a THD of 0.1 percent would mean that 0.1 percent of the output signal is false and contains unwanted distortion. Such gross alteration could lead to an experience where instruments sound unnatural.

In most cases, however, Total Harmonic Distortion is hardly perceptible, especially since manufacturers create products with THD specifications that are tiny fractions of a percent. If you can't consistently hear half a percent difference, then you're not likely to notice a THD rating of 0.001 percent (which can be difficult to accurately measure, too).

Moreover, the specification for Total Harmonic Distortion is an average; it doesn't take into account how even- and lower-order harmonics are harder for humans to hear than their odd- and higher-order counterparts. So music composition also plays a small role.

Every component adds some level of distortion, so it's prudent to assess numbers in order to maintain audio output purity. However, the percentage of Total Harmonic Distortion is not as important a specification when looking at the big picture, especially since most values are often less than 0.005 percent.

The small differences in THD from one brand of a component to another is often insignificant next to other considerations, such as quality audio sources, room acoustics, and selecting the right speakers.

FAQ
  • How much THD is acceptable?

    As long as THD is less than one percent, most listeners will not hear any distortion. Some musicians and audiophiles, however, may notice that level of distortion.

  • What causes high THD?

    High THD is indicative of an electrical problem with the audio equipment. A small amount of THD is inevitable, but it should be unnoticeable if all the wires and components are working correctly.

  • How do I calculate THD?

    Use a THD analyzer to automate the process. To determine THD, the analyzer divides the ratio of the equivalent root mean square (RMS) voltage of all harmonic frequencies by the RMS voltage of the fundamental frequency.

  • What is Intermodulation Distortion?

    Intermodulation Distortion (IMD) is a measure of the non-harmonic frequencies added to an input signal. Like THD, IMD is represented as a percentage of the total output signal, and lower numbers mean better performance.

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