An Equaliser is a device that changes the amplitude of a given part of the audible frequency spectrum. Historically, an equaliser was so-called as it was used by engineers to 'equalise' the sound from what was recorded to what it 'should' sound like. Today, equalisers have both creative and practical uses.
In a studio environment, equalisers allow engineers and producers to build sounds and fit them together within the sound scape of a mix. In a live environment this is also true but equalisers are also useful for removing room nodes which 'ring' and cause feedback, this allows the engineer more gain before feedback.
Equalisers come in different types and forms, mainly as hardware devices and software algorithms changing the sound data. Equalisers or EQs range from a one knob tone controls right up to fully digital channel EQs. Some types of equalisers can include
Regarding graphic vs. parametric equalisers:
The graphic EQ is the unit you've probably seen pictures of showing a box with 31 "faders" in a row. Each fader controls a specific frequency and allows you to cut or increase the sound centred on that frequency but, like any filter, actually spanning a small range. A graphic EQ is not always the most accurate way to cut out a certain frequency but has the advantage of ease of use--you just grab a fader for the frequency you want and tweak it up or down.
A parametric EQ allows more flexibility in control. For each frequency you can adjust, the parametric gives you 3 controls: one adjusting the centre frequency you want to adjust, one to take that up and down and a third called the "Q" of the filter. Basically the "Q" controls how wide a span of frequencies you're adjusting around the centre frequency you've selected. On a good parametric, the narrowest "Q" will allow you to notch out a very narrow band indeed without affecting the surrounding frequencies. The extra narrow Q, along with the ability to select ANY centre frequency, not just the 31 presets, makes a parametric EQ a very precise tool for feedback control but the drawback is the fiddly nature of the controls. It takes longer in a live situation to find the exact frequency, adjust the Q then adjust the gain for that frequency. FYI a typical parametric will have the controls to process 4 or 5 frequencies. Automatic versions of parametric EQ tend to be found in feedback killers.
Typically separate rackmount units, these devices have a set number of faders that can be used to 'notch' in or out specific bands. On these devices, notching one frequency range out will result in a steep dip, while drawing a curve with faders will result in a gradual dip. Units designed for live work often have LEDs to signal which frequency ranges are most active, this is particularly useful for removing feedback-causing room nodes. Typical band mid-points for a 10 octave equaliser are 31.5Hz, 63Hz, 125Hz, 250Hz, 500Hz, 1kHz, 2kHz, 4kHz, 8kHz, 16kHz. High quality equalisers divide the octave spectrum up by 1/3 resulting in 31 bands.
Typically incorporated into channel controls on Mixing Desks) typically include low and high frequency shelves and cuts as well as mid-range sweep and amplitude controls. A parametric EQ allows one to sweep to find a specific frequency band while using the amplitude controls to change the gain of the selected frequency.
/\| | a | m | p |\ /\ ___WF___ | \ / \ / \ | \___/ \__/ \____ \/|_______________________________ < sweep range >
In this diagram you can see that the x axis represent frequency while the y axis represent amplitude. If the curve WF is an EQ curve, turning the sweep control will move it along the sweep range. Turning the amplitude control will raise or lower the curve, creating a steeper drop/slope.
Some equalisers, often digital ones include a Q parameter for each EQ curve inserted (excluded low/high shelves/cuts which have dB/Octave slope controls). The Q parameter is derived from mathematics used to define the gradient of a curve. A Q parameter with a low value will result in a tight peak such as those found on a graphic EQ while a high value makes a very slight slope. Many Digital desks will have Q control on their channel and output EQ. this is often classed as X band fully parametric EQ, where X is the number of separate bands of EQ available per channel, commonly 4. Larger analogue desks like the Allen and Heath ML5000 have bands of Fully parametric EQ. Some smaller desks will have a switch to alter between a wide and narrow Q on some or all of the channel EQ bands, this gives a bit more control but can often do more harm than good.