Equalisers are digital audio processors used to make adjustments to the frequency content of sonic material. They allow producers and sound engineers to shape the tone and colour of their audio recordings. There are two main types of equalisers; graphic and parametric. Graphic equalisers allow user selectable fixed frequencies to be reduced or increased in level. Parametric equalisers allow user selectable frequencies and associated radio bands to be adjusted. Equalisers can be used to reduce or enhance certain frequencies, create tonal quality to instruments or vocal tracks, or to generally shape the desired sound of a recording. Equalisers are essential tools in any mixing engineer's toolbox.