Shelving EQs are most often seen on domestic hi-fi systems. They typically have a single control per band that boosts or cuts the gain at a pre-determined frequency, often labelled 'Treble' and 'Bass', or 'HF' and 'LF' (for 'high frequency' and 'low frequency'). All frequencies beyond the preset frequency are affected. Typically, low shelving EQ on a mixing desk will be fixed at around 80-100Hz, and high shelving at 10-12kHz. The maximum amount of cut or gain that can be applied is usually in the region of 12-15dB.
This type of filter allows you to cut or boost a limited band of frequencies in the middle of the spectrum, rather than all the frequencies above or below a specific point, as with shelving EQ. c As with shelving filters, there is a similar slope around the cutoff frequency, typically of around 6dB per octave, although in this case it falls off either side of the chosen centre frequency.
The frequency range covered by an EQ is traditionally broken up into specific groups - highs, mids and lows, and these are normally represented on a console by a series of knobs on each channel strip, with each frequency group having separate controls. For each frequency band, a standard 'sweepable' or 'semi-parametric' EQ will have one control for adjusting the centre frequency to be affected, scaled in Hertz, and another for adjusting gain, expressed in decibels. The mid frequency band will typically be adjustable from around 200Hz to 6kHz.
Parametric EQ is the most flexible type of EQ, and is essentially a peaking filter that incorporates a bandwidth control, sometimes known as 'Q' for 'quality factor', along with the usual frequency and gain controls. Bandwidth is scaled in octaves, ranging from narrow (fractions of an octave), to broad (around 3 octaves). As a rule of thumb, the broader the bandwidth the more natural the tone, as (especially) boosting narrow bands of frequencies (particularly in the midrange) tends to sound harsh, narrow bandwidths are normally reserved for the correction or cutting of audio. Most EQ circuits have a bypass facility, enabling you to quickly compare the original signal with the processed one.
Notch filters have a very narrow bandwidth and are often sweepable, but generally only offer cut rather than gain. They are most often used to remove problem frequencies such as mains hum, or in manual or automatic feedback suppression systems for live sound - once the frequency of the feedback has been found, it can be attenuated quickly.
Graphic equalisers get their name from the visual representation they give of their effect on the frequency spectrum. A series of sliders, each with its own frequency band, usually covers the full audible range from 20Hz to 20kHz,. A standard 31-band graphic equaliser spaces its controls a third of an octave apart. Each band consists of a fixed frequency peaking filter of narrow Q, applying boost or cut to that frequency range when the slider is moved up or down from it centre position, the exceptions being the highest and lowest which are usually shelving filters.
Graphic EQs are usually applied to complete mixes of sound, particularly live, to go some way towards correcting room response. For this reason they often come as dual-channel units, more sophisticated models with built-in spectrum analysers and a measurement microphone.