Equalisers are used to adjust the balance between the different frequency components of an electronic signal, or more simply, the tone. They do this by boosting or attenuating particular frequencies or groups of frequencies (band). Equalisers vary in complexity from simple 3-band equalisers, to complex 31-band graphic and multi-band parametric equalisers. They can analog or digital, and can be designed to add broad sweeping colour to a sound, or to be a surgical tool and hone in on very specific problem frequencies. Equalisers are used are used by engineers and musicians in both studio and live sound applications.