While you can call any alteration to the waveform of a signal between two points of a system distortion, harmonic distortion refers to the unintended generation of harmonics (frequencies that are integer multiples of a fundamental frequency). It’s worth keeping in mind that distortion in general isn’t necessarily a negative, and can even be considered a positive when applied in an artistic manner (e.g., equalization and electric guitar pedals).
In sound reproduction, however, harmonic distortion is considered a flaw since it indicates the presence of frequencies that weren’t included in the original content. This can color the sound and make the music sound impure, harsh, or muddy. Still, distortion is very difficult to hear unless headphone drivers are pushed in volume far beyond what’s considered safe for human enjoyment. Harmonic distortion shouldn’t be a critical part of your buying decision, but if you’re curious about all the fine details, keep reading.
We perform harmonic distortion tests at 94 dB SPL and at 104 dB SPL. In addition to graphing the total harmonic distortion (THD) ratio and second and third order harmonics from these results, we calculate the headphones’ weighted average THD for the mentioned sound pressure levels.