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 (for example, 1 kHz 1 Vrms). There is a bit of math involved to calculate Total Harmonic Distortion. Still, all you need to understand is that the percentage represents the harmonic distortion or deviation of the output signal. Lower percentages are better. 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 a unique timbre and allows the human ear to distinguish between them. For example, a violin playing a middle A note produces 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 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, 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, you’re not likely to notice a THD rating of 0.001 percent (which can be challenging to measure accurately). 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 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 are often insignificant next to other considerations, such as quality audio sources, room acoustics, and selecting the right speakers.
