Explainer

EQs and Filtering

The equaliser is possibly the most important tool in the audio professional’s arsenal. By boosting or attenuating specific frequencies in a signal, you can solve a broad range of problems at every step of the music-making process, from initial sound correction issues at the recording stage right through to broad sweetening when mastering at the very end. And at the creative end of the spectrum, EQ can also assist with tone-shaping for sound design, either alone or with other processes.

With hundreds of processing options available in both hardware and software form, and the myriad ways you can apply them throughout every element in a mix, the art of EQ can be somewhat overwhelming. As every type of musical element varies in frequency content, each demands a slightly different approach. Furthermore, many producers go too far with EQ and introduce more problems than they solve. That’s why, in this month’s Explainer feature, we’re going back to basics with a look at this staple tool and its uses.

Sound vibrations are measured in cycles per second, or ‘Hertz’, and the frequency range of a typical human’s hearing spans from 20Hz at the lowest end of the spectrum right up to 20kHz. An equaliser device is the tool used to amplify (boost) and/or attenuate (cut) specific frequencies within a signal; in music production scenarios, the signals we work with often require some kind of frequency correction or enhancement in order to serve the collective perception of the mix as a whole.

An equaliser offers multiple bands that can be set up for customising frequency-specific boosting or attenuating. The simplest type of equaliser is the graphic EQ, which comprises a preset collection of sliders set to predetermined frequencies; but the most common – and flexible – type is the parametric EQ. Setting a band’s frequency value will, as expected, allow you to home in on your desired frequency area; the gain value is the amount of boost or cut applied to that frequency.

The Q (ie ‘quality’) or bandwidth parameter affects the range of surrounded frequencies affected by your boost or cut. In theory, the adjustment of only one specific frequency in isolation would sound completely unnatural to our ears, which is why a typical EQ boost or cut will also affect other surrounding frequencies to some extent.

Modern, flexible EQs allow you to change the type of each band to one of several shapes, each affecting the resulting sound in different ways. A peak or bell filter is the typical type used for surgical boosting and cutting, while high- and low-shelf filters raise or attenuate around their corner frequency point, and are useful for broad tilting of treble or bass. Then there are high- and low-pass filters, which we’ll look at on the adjacent page.