Equalizers (EQ) or equalizers are used to compensate for frequency-dependent losses and to boost or cut frequencies in quadripoles, radio, transmission and audio systems. They usually form a functional unit together with amplifiers, since they influence the frequency response not only passively but also actively.
Equalizers for frequency response compensation basically reproduce an inverse function of the frequency response to be compensated. This means that if the frequency response of a system, a data cable or a transmission line drops sharply in certain frequency ranges, the frequency response of the equalizer in these frequency ranges is such that it raises the signal level. Thus, it always has a frequency behavior that is opposite to the frequency behavior of the system. This results in a frequency-independent transfer characteristic at the equalizer output.
On the other hand, the equalizer can also be used deliberately to raise or lower frequencies, as in the case of audio tone controls with which the sound image can be adjusted to suit one's own sensibilities.
Equalizers used in audio technology include versions with fixed filter banks, the fixed-frequency equalizers, and those with adjustable filter parameters, the parametric equalizers.
Fixed frequency equalizers or graphic equalizers work with fixed filters that can be recalled from a filter bank. The filter bank contains filters for octaves or thirds, or filters with a wide variety of filter curves and center frequencies. These include the shelving filters and bell filters frequently used in audio technology, but also Bessel filters, Butterworth filters and Chebyshev filters. In stereophonic systems, most graphic equalizers have two identical filter banks and amplifiers, one for each channel.
In parametric equalizers, all parameters such as the center frequency of the filters, their bandwidth and Q, their shape, attenuation and slope can be parameterized individually. They can be bandpass or bandstop filters, highpass or lowpass filters, notch filters for removing noise, or cow-tail or bell filters. Unlike fixed frequency equalizers, parametric equalizers have only between four and eight frequency bands.
Audio equalizers can be used to compensate for physical effects such as proximity effect or the damping of high tones with long audio cables, to attenuate vibrations and impact noise, and to influence the sound according to aesthetic sensibilities.