Eq, or equalisation, is the most common effect applied to audio during mixing. Here is our guide on how parametric eq plugins works, and how to use them effectively.
Parametric Eq Overview
Equalisation is used to balance an instrument in the mix, to remove unwanted sounds and to make something just sound nicer – anything from a vocalist to a master fader. Parametric eq uses a number of settings to choose a range of frequencies and control how much their gain is increased or reduced. This is different to graphical equalisers, which have a large number of fixed frequency bands that can be increased or decreased independently. Graphical eq can be useful for tuning PA systems but seldom for actual mixing where you need smoother and more precise control.
Most parametric equalisers have three or more independent sets of eq controls, these are called eq bands. Some plugins will limit the range of each frequency band, so each has it’s own area of use within the entire frequency range, with some overlap. Fortunately this restriction is becoming less common but keep an eye out for is, as it can be extremely limiting if you need two or more eq adjustments very close to each other.
Bell / Shelving Eq
There are two shapes that parametric eq can use, the first is a bell shaped; literally a bell shaped boost to the frequencies either side of the ‘center frequency’. Most plugins have a graphical display where you can see the shape of this eq curve, as pictured above. The bell shape means that the frequencies closest to the center frequency are affected more by the change in gain than those further away.
Shelving eq is used to boost or reduce all the frequencies on one side of the ‘center frequency’. The gain change slopes up on one side, and then levels off to like like a horizontal shelf. This basically means you can boost or reduce all of the top or low end frequencies by the same amount in one go.
This is sometimes just labelled ‘Frequency’ and simply sets where the middle of the eq band sits for bell shaped eq, or for shelving eq where the slope is.
The gain value for bell shaped eq bands will increase the center frequency by that exact amount. For shelving eq, the gain value sets the level of the actual shelf – where the gain is the same across all frequencies.
The ‘Q’ setting determines how far either side of the center frequency the eq band has an effect. A high Q means that only a very small range of frequencies are changed, and a low Q affects a very broad range of frequencies with a gentle slope either side of the center frequency. For shelving eq bands there often is no Q setting available, but if there is one it will set how steep the slope up to the shelf is.
A notch filter is a special kind of eq which has a very high Q value and a lot of gain reduction. They are used to eliminate specific frequencies which need to be removed, such as the ring on a snare drum or the vibration of a mic stand when it is knocked.
High and Low Pass Filters
These are a special type of shelving eq which are commonly included in eq plugins. They completely remove all frequencies beyond the center frequency. You can usually change how steep the slope is to make them more or less obvious or effective. High pass filters remove low frequencies (they let High frequencies Pass), and low pass filters remove high frequencies (and let Low frequencies Pass).