Harmonic effects devices may also be known as a harmonic or aural exciter. This type of audio signal processing can enhance dynamic equalization (EQ), manipulate phase, add higher frequencies in the harmonic series, apply harmonic synthesis and create harmonic distortion.
Dynamic EQ will depend on the capacity and characteristics of the device being used, with performance coming down to time and circuitry specifications. The extent its application can either enhance or exaggerate the harmonics of the track being processed, depending on the desired effect.
Harmonic synthesis involves creating higher order harmonics from the fundamental frequencies by processing the recorded track. As ‘noise’ is often more prominent at higher frequencies, harmonic effects units aim to derive these higher order harmonics from purer frequency bands so that a cleaner sound is produced.
Harmonic effects units can also be used to synthesize harmonics of lower frequencies to assist in stimulating bass and deep bass tones in smaller speakers, or units with limited audio capabilities.
Harmonic effects units were original made using valve/tube transformers. Digital units have been around for many years, emulating their counterparts but offer more control of the effects processing. Digital and analog signal processing affects audio signals differently so units will vary in tone as a result.