Look through the manufacturer’s manual — or perhaps even the retail packaging of the audio device — and you’ll probably read a data sheet called Total Harmonic Distortion (THD for short). You can find it in Speakers, Headphones, Media/MP3 Players list, amplifiers, preamplifiers, receivers and much more. Basically, when it comes to sound and music playback, this specification will (should) be available. Total harmonic distortion is important when considering equipment, but only up to a point.
What is total harmonic distortion?
The specification for total harmonic distortion compares input and output audio signals with a step difference, measured as a percentage. So you can see THD listed as 0.02 percent with frequency terms and equivalent voltage in brackets after it (eg 1 kHz, 1 Vrms). It does take a bit of math to calculate total harmonic distortion, but all you need to understand is that a percentage represents the harmonic distortion or deviation of the output signal — the lower the percentage, the better. Remember that the output signal is a reproduction, not a perfect copy of the input, especially when multiple components are involved in the audio system. When comparing two signals on a chart, you may notice slight differences.
Music consists 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 the middle note A generates a fundamental frequency of 440 Hz, and also produces harmonics (multiples of the fundamental frequency) at 880 Hz, 1220 Hz, 1760 Hz, and so on. A cello playing the same middle note as a violin still sounds like a cello due to its own fundamental and harmonic frequencies.
Why Total Harmonic Distortion Matters
After the total harmonic distortion exceeds a certain point, you can expect the sound fidelity to be compromised. This happens when unwanted harmonic frequencies — which are not present in the original input signal — are generated and added to the output. Thus, a THD of 0.1 percent would mean that 0.1 percent of the output signal is false and contains unwanted distortion. Such a drastic change can result in instruments sounding unnatural and not how they should.
But in reality, total harmonic distortion is hardly perceptible to most human ears, especially since manufacturers create products with THD specifications that are tiny fractions of a percent. If you can’t consistently hear a half-percent difference, then you’re unlikely to notice a THD rating of 0.001 percent (which can also be difficult to accurately measure). Not only that, but the specification for total harmonic distortion is an average that does not take into account how much more harmonious it is for humans to hear even and lower harmonics compared to their odd and higher counterparts. So the musical composition also plays a small role.