It seems when measuring distortion in audio circuits we look for four possible types of distortion:
- Harmonic distortion - unwanted components are multiples of each tone in the signal
- Intermodulation distortion - unwanted components are multiples of the difference between each pair of tones in the signal
- Noise - unwanted components are spread out across a wide band of frequencies
- Interference (e.g. power supply hum) - unwanted components are specific frequencies, independent of the signal