When mixing and mastering, EQ is one of the most common processes that you will be working with. EQ or equilization allows you to adjust individual frequencies of a given track. This can allow you to either boost pleasant frequencies, reduce unpleasant ones, or completely cut unnecessary frequencies (say below 100Hz on an acoustic guitar tracks). Proper use of an EQ will allow each instrument to have its own space in the mix and eliminate any competing frequencies that create a "muddy" mix. Below is a diagram of the EQ Eight plug-in that comes with Ableton Live 11. In this article I will explain the main elements of an EQ and what they are used for.
Frequency Selector - allows you to select the target frequency that you would like to EQ. Frequencies from 10Hz to 22kHz are selectable on the EQ Eight. Some EQs may have frequency bands which will have a range of either selectable or preset frequencies to be adjusted.
Gain Boost/Reduction - allows for the boost or reduction of the current selected frequencies. Boosting and reducing should be done carefully. For every 10dB of gain boost, the sound doubles in loudness.
Q Control - allows you to select the curve of the boost or reduction. A high Q value will create a very sharp curve only affecting the frequencies close to the selected frequency. A High Q is generally good for reduction and cutting. A low Q value produces a gentle curve and will effect more of the frequencies surrounding the slected frequency. A low Q is generally good for boosting.
EQ Bands - These are the available bands of frequencies that you can control. In the picture above, there are 8 selectable frequencies. On this EQ you can select the affected frequency as well as choose whether the point will act as a cut, boost/reduction, or a shelf. Shelfs will boost all frequencies higher than or lower than the selected frequency by the amount set with the Gain control. A cut off will cut all frequencies higher than (low pass) or lower than (high pass) the selected frequency. A low pass filter of 5-6kHZ would generally be used on bass instruments. A high pass filter of around 80-120Hz is used on most other instruments. A boost or reduction will either raise or lower the level of the selected frequency by the amount of the gain control. The Q will change how many of the frequencies adjacent to the selected frequency will be affected and by how much.
Understanding the basics of how an EQ works and what it is used for is essential to creating a balanced and vibrant mix. EQ will be used on almost every track that you as a producer, mixing or mastering engineer will touch, and it can be a powerful tool in the arsenal of any home studio recording artist.