Audio engineers use equalization in order to boost or cut specific frequencies of a particular sound in order to make it stand apart from other elements in a mix. The frequencies which humans are capable of hearing fall roughly between 20hz at the low end, and 20,000hz at the high end. Frequencies outside of this range are often present in a mix and these extreme frequencies also need to be addressed usually during the mastering phase, in order to assure that a mix is not wasting too much energy on inaudible sounds during playback. This shaping of extreme frequencies, particularly at the low end of the spectrum, allow tracks to be made louder without sacrificing sonic quality.
Given the importance of equalization to the mixing and mastering processes, there are many different types of equalizers available on the market. Though some equalizers may allow more precise control and fine tuning of boosts and cuts being made, they ultimately all work in more or less the same way, in that they allow the engineer to manipulate the gain of a particular frequency range with precision.
Most equalizers feature a combination of high-pass filters, low-pass filters, shelving filters, and bell filters. High-pass filters attenuate audio frequencies below the specified cutoff setting, whilst low-pass filters do just the opposite, attenuating frequencies only above the cutoff setting. Shelving filters apply fixed boost or cuts starting at a designated cut-off setting. They are much like high-pass and low-pass filters, with the exception being that gain is applied in a fixed rather than progressive manner. For example, a high-pass filter at 200hz will cause attenuation of -6 db at 150hz, -12 db at 100hz, and -18 db at 50z, a -6db low-shelving filter set at 200hz will cause attenuation of -6db at 150hz, -6 db at 100hz, and -6 db at 50hz. The same applies for high-shelving filters.
High-pass and low-pass filters often come in many different varieties. Many equalizers will offer 12dB/octave or 6dB/octave slopes, which refer to the amount of attenuation applied per octave. The greater the slope, the steeper the cutoff. While extreme slopes may be available on a given equalizer, they are best used with care, as they can cause phase problems. For the purposes of mastering, 12db/octave and 6db/octave slopes should get the job done.
The third filter typically featured on an equalizer is the bell filter. Bell filters are meant to boost or cut specific frequency bands without affecting those frequencies above and below the frequency setting. These filters get their name from the bell-shaped curve they produce, and are extremely useful for making precise adjustments to particular frequencies in an audio signal.
The bandwidth adjust feature, or “Q value” as it is often called, is useful for adjusting the resonance of all of the aforementioned filter types, in that it adjusts severity if the initial slope of the attenuation. High-resonance settings can create very interesting sounds when used creatively, such as the sounds of the Roland TB-303 featured on many acid tracks (synthesizers often feature filters just like equalizers due, though they are often limited to high-pass and low-pass varieties.)
When performing EQ techniques it is important to note that there are many ways to determine what needs to be done. While most engineers will tell you to use your ears and not your eyes, there are times when spectrum analysis, a visual display of frequency response, can be particularly helpful, as some of the most problematic frequencies in the extreme low-end cannot be heard or played back on most studio monitors.
Many newer software plugin equalizers will include a visual spectrum graph for the purposes of analysis. Often these plugins also allow the user to switch between viewing the incoming and outgoing signals, in order to see how the adjustments they make effect the sound. On more expensive hardware units, spectrum analysis is typically performed on a separate and dedicated unit in the signal processing chain, if at all. Though this is one reason that many mastering engineers will tell producers to rely on their ears and not their eyes, that does not mean that this method of performing equalization is only useful when working with hardware.
As many industry veterans will tell you, an experienced mastering engineer has put in enough hours performing equalization techniques to be able to discern boosts and cuts as small as .5db or even less. While that may seem hard to believe, it is not uncommon for mastering engineers to spend time making boosts and cuts under 1db, as this is their area of expertise. As an experiment, see if you can play back some sounds in your favorite DAW and a/b test boosts and cuts of 3db or less. Many novice producers can’t even hear the difference in just 3db, which is a huge change to the trained ear of the average professional mastering engineer.
Learning to properly use an equalizer is an art and for those who are just now becoming familiar with the technique, it is recommended that engineers rely on their ears and not their eyes. Visual representations displayed on graphic equalizers are useful but not always accurate. Ultimately, you should keep in mind that the final product was meant to be heard not watched. I cannot emphasize this point enough.