The audible range of the human ear is said to be from 20 Hz to 20 kHz. Audio distortion can be considered to affect this audible range. To explain this in simple terms, let’s use a 1 kHz sound as an example. When there is no audio distortion, a sine wave can be observed over the time domain as shown in Figure 2, while the frequency spectrum shows only one fundamental frequency.
However, the shape of the sine wave is altered when distortion occurs in the audio. Looking at the frequency spectrum, harmonic spectrum lines appear in addition to the fundamental frequency.
Expressing the measurement as an audio parameter gives us THD + N (Total Harmonic Distortion + Noise), which indicates the harmonic ratio that is produced. For a noise filter, it is important that harmonic distortion does not occur when an audio signal flows.