General DescriptionAn equalizer is used to emphasize (boost) or de-emphasize (cut) specified portions of the content of an audio signal. It does this by increasing or decreasing the amplitude of the signal within certain bands, or frequency ranges. Equalizers are nearly always built into the channel input modules of mixing consoles, and can in this capacity range from rudimentary two-band tone controls with mostly fixed parameters, to four-band equalizers with adjustable parameters. In the capacity of outboard signal processor, an equalizer will be one of two basic types, namely, graphic or parametric.
Graphic EqualizerA graphic equalizer consists of a row of infinitely variable slider controls, ranging in number from three to thirty-one. The sliders affect potentiometers operating on active filters, facilitating the adjustment of amplitude of fixed bandwidths at fixed central frequencies. The range of the amplitude parameter is typically +/- 15 dB.
Graphic equalizers are so named because the row of sliders provides a visual representation of the adjustment to the frequency curve. They are most effective in tweaking and smoothing a mix, or in compensating for colorations of sound due to room acoustics or loudspeaker systems. For this reason graphic equalizers typically include two identical channels, each processing one channel of a stereo signal.
Parametric EqualizerA parametric equalizer consists of a number (typically not more than four) of groups of controls, referred to as bands. Each band will usually contain three controls, one each for the adjustment of frequency (Hz), bandwidth (Q), and amplitude (dB). This design allows precise selective control of several frequency components present in the audio signal.
The frequency parameter determines the central frequency affected. A different range of frequencies will be available for each band.
The bandwidth parameter determines the range of frequencies affected, extending upward and downward from the central frequency, variable from about 0.5 or less to more than 3 octaves.
The amplitude parameter typically provides for cutting up to 30 dB and boosting up to 15 dB.