equalisers

An equaliser, or equalizer, is an audio signal processing device or software that adjusts the amplitude of specific frequency components within an audio signal. It shapes the overall tonal balance by boosting or cutting selected bands while leaving others relatively unchanged.

Common types include the graphic equaliser, which provides a fixed set of frequency bands with adjustable gain, and the parametric equaliser, which offers control over the center frequency, gain, and bandwidth, or quality factor (Q), of each band. Shelving equalisers affect all frequencies above or below a chosen cutoff with a gradual slope.

Dynamic equalisers add a level-dependent component, applying selective processing only when the signal exceeds a threshold. Equalisers may be implemented as analog circuits using calibrated filters or as digital algorithms running in audio workstations, hardware processors, or embedded devices.

Applications include musical production, live sound reinforcement, mastering, broadcasting, and consumer audio equipment. Proper use aims to balance tonal color, reduce problematic frequencies such as boom or hiss, and adapt to the acoustic environment or listening context.

Historically, simple passive tone controls preceded modern equalisation; graphic equalisers became common in the 1970s and 1980s, followed by parametric and digital variants with advances in digital signal processing. Contemporary systems often combine multiple types and integrate with mixers and digital audio workstations.

See also tone control, DSP, and loudness.