Equalization, or EQ, is one of the most rudimentary parts of the mixing process. EQ can be used for a multitude of things, notably including finding spot in the mix, frequency wise, for a given track, and cleaning up any unwanted resonances and removing unnecessary frequencies that may cause problems in a mix.
There are several parameters that can typically be found on any given EQ, but as with compressors not all plugins/devices have the same parameters, however, this isn't necessarily a bad thing.
All EQs have a few different values that a user can tweak. They have two possible effects, it either effects spectral content, or it changes the cut/boost of the given frequency. However, different EQs take these simple ideas and apply them radically differently. Also some EQs have options for filters both shelving and passing, a detailed account of which can be found on the filters page.
Clearly, the most important choice of an EQ is what frequency range you wish to effect. Some EQs have a fully sweepable frequency selection (Parametric) and some have designated frequency bands (Graphic). There will be a section later in this page on the differences between the two.
That being said, the first thing you have to determine is what area of the frequency spectrum you wish to change in some way, either with a cut or a boost.
When speaking about the frequency spectrum, people tend to divide it into 4 bands. While there are no hard and fast rules on where the divisions between the bands are, generally they lie in the following regions
0-250Hz is considered the Low Band
250-1.5kHz Is generally termed the Low-Mids
1.5k - 6kHz - Are the Hi-Mids
6k - 20kHz - Are the Highs
Again, these are not set values, and different people may have different meanings when talking about the bands.
Bandwidth (Or Q)Edit
The Q (or Quality Factor) is the width of the boost/cut that the EQ is doing. The Q is determined by the Center Frequency divided by two cutoff frequencies (CF/Fh-Fl) and range from 0.1 (very wide) to whatever maximum value the manufacturer decides. Thus, the higher the Q number, the narrower the range being affected becomes.
Proportional Q vs. Constant QEdit
There are two different operation principles that a Q could possibly rely on. There is Proportional Q and Constant Q.
Proportional Q's bandwidth narrows as the cut or boost is given more gain/attenuation. This is a given option due to the belief that if you are affecting a range of frequencies by that much, there must be a specific frequency that you either wish to highlight or remove.
Constant Q is just the opposite, the bandwidth stays the same regardless on gain. So if you boost/cut more, you'll be boosting/cutting more frequencies, the more gain you apply.Some manufactuers allow you to choose either proportional or constant, while some simply allow one.
This is an example of a proportional Q in Logic. Note the "Frequency Gain/Slope" section in the bottom left part of the plugin.
This is an example of a constant Q band in Logic. Notice how the Q number and boost is the same as in the previous example, but the affected area is much wider. For Logic specifically, unchecking the small blue box in the bottom left corner changes it from proportional to constant Q. However, it should be noted that when you do this, Logic will automatically compensate and change your Q value.
Graphic EQs operate a bit differently from parametric EQs. Instead of having a sweepable center frequency, the spectrum is divided into bands (4, 8, 16, or 32 are the typical number of selectable bands depending on the EQ unit/plugin)
This is an example of a 16 band graphic EQ. They can often be found in live-sound rigs and on home stereo equipment
Gain is as simple as it sounds. All that it means is how much boost or cut is being applied to the given frequency spectrum. The idea for mixing, in fact the definition of it, is to have every instrument occupy its own unique space, and to avoid clashes sonically, unless desired. This can be achieved in part through EQ'ing tracks differently, and also by use of panning.
One very useful tips for beginners to bear in mind is the idea of "cut before boost" or, if you are having trouble with EQ on a track, say you want to boost 2kHz for the snare because your specific snare has the perfect "crack" right at that range. Instead of boosting that frequency range into oblivion, try dipping that particular band on a few other tracks that the snare is fighting to share space with (panning wise), and then if you find that you still want a little more of that frequency, apply the boost.
A notch EQ or a notch filter is simply a cutting EQ with a very, very high Q value and a very low cut. This is used to remove unwanted frequencies in a track (for example, an unwanted ring in a snare track). The purpose of these is to zero in on a specific frequency, or small range of frequencies, and fully remove them.
This is Logic's stock EQ shown with a notch at 500Hz