Use of Equalization in Home Recording Mixing
The most useful tool that you have for mixing is equalization (EQ). Equalizers allow you to adjust the various frequencies of your instruments so that you have enough room for each of them in your stereo tracks. Four types of equalizers are used in a recording studio — graphic, shelf, filter, and parametric.
The graphic EQ has a prescribed number of frequencies that you can adjust. Graphic EQs generally have between 5 and 31 frequency bands, each affecting a small range of frequencies. (The manufacturer determines the range, which can’t be adjusted.) Graphic EQs are useful for eliminating an offending frequency from the signal or for making other adjustments to the tonal quality of the source signal.
You probably won’t use a graphic EQ much in the mixing process because the parametric EQ can do what the graphic EQ can do — and a whole lot more.
High- and low-shelf
A shelf equalizer affects a range of frequencies above (high-shelf) or below (low-shelf) the target frequency. Shelf EQs are generally used to roll off the top or bottom end of the frequency spectrum.
For example, you can set a shelf EQ to roll off the frequencies below 250 hertz (Hz) to reduce the amount of rumble (low-frequency noise) in a recording. You generally use the shelf EQ for the lowest and highest frequencies and the parametric EQ for any in-between frequencies when you mix.
High- and low-pass filters
Sometimes your track just sounds better if you eliminate a few carefully chosen frequencies. You just need to know which ones to target. That’s where another type of EQ can help with the needed audio acrobatics: low-pass (ducking the high frequencies that you don’t want) and high-pass (jumping over the low frequencies that you don’t want).
This type of EQ is called a filter because, um, it filters out frequencies either higher (low-pass) or lower (high-pass) than the target frequency. A low-pass filter eliminates unwanted high frequencies, and a high-pass filter gets rid of unwanted low frequencies.
In Pro Tools, the low- and high-pass filters are designated with the icons seen here in the margin. The low-pass filter is the downward slope (top), and the high-pass filter is the upward slope (bottom) art in the margin.
The parametric equalizer allows you to choose the frequency that you want to change as well as the range of frequencies around that frequency. With a parametric EQ, you dial in the frequency that you want to change and then you set the range (referred to as the Q) you want to affect.
The Q is a number that signifies the number of octaves that the EQ affects. Generally, you can adjust the Q setting to affect frequencies between 1/2 and 2 octaves wide. Not all parametric EQs use the same reference numbers for their Q settings. Some have ranges from 0.7 (2 octaves) to 2.8 (1/2 octave), while others, such as Pro Tools, use numbers from 0.33 to 12 without indicating what the numbers relate to in terms of octaves. The one constant among parametric EQs is that lower numbers affect larger ranges of frequencies than the higher numbers do.
The fact that each brand of parametric EQ uses slightly different numbers to reference its Q settings shouldn’t matter much to you, because you choose your Q setting based on what you hear in the mix. Just as you can experiment with different frequencies to adjust in the mix, you can also try different Q settings to find the best possible frequency range to use.