You've probably used it before, but what is EQ? We present a mini-guide to this incredibly useful process and the various ways in which it's employed...

It’s on your stereo, iPod, pedal-board, and in your DAW – but what is EQ?

EQ is a feature of musical equipment of all kinds. From your car stereo to your iPod, your guitar effects pedal-board to your DAW software package to your mixer, all feature EQ (short for equalisation) to add some final tweaks to an overall sound.

An effect that is so commonly used is clearly pretty universally useful. And, if you’ve used it to tweak your sound, you’ll understand why.

Even if you’re familiar with the effect, however, you still might wonder exactly what is EQ?

Here’s a mini guide to the effect…

Complex frequencies

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Sound, put simply, is the human perception of vibrating air particles. The sound generated by musical instruments, with few exceptions, is very complex stuff.

In a pitched musical instrument (i.e. one that you play notes and melodies on, like a piano or guitar, or a tuned drum, for example) the sound will consist of a fundamental frequency, which dictates the pitched ‘note’ of the instrument, and many other harmonic overtone frequencies, which create its unique sound.

In atonal, non-pitched sounds, whilst there may seem to be no obvious fundamental frequency, their tone still consists of many different harmonics, of differing frequencies.

An EQ works by allowing the user to boost or cut particular frequencies as they desire, thus shaping the character of the sound they are applying it to.

What Is EQ?

So, if you have a kick drum that lacks ‘weight’, you can boost the very low-end to dial in some of that ‘chest-thumping’ quality. If a guitar is just too bright and scratchy, you can take the edge away by cutting some of the upper frequencies.

It should be stated, however, that EQ should only be used to make a good sound better. You should always aim to get the sound right at the recording stage- don’t aim to ‘fix it in the mix’, folks…

Parametric EQ controls

What Is EQ? - Parametric EQ Controls

One of the easiest ways to understand how EQ is used is to take a look at a parametric EQ. A parametric EQ has three main control knobs for each ‘band’ of EQ it offers: frequency, gain boost/cut and Q.

Frequency dictates the frequency that will be boosted or cut. Gain boost/cut will increase or decrease the level of the frequency set. The Q control sets how narrow or how wide the curve is around the effected frequency.

A very useful way of getting to know the harmonics present in a sound is to boost the frequency gain of a parametric EQ by a few dB, set the Q to a narrow setting then slowly sweep the frequency control throughout its range. By doing this, you will be able to hear easily where the most prominent harmonics are within any given sound, and enable you to easily find any ‘problem’ frequencies.

What Is EQ? - Parametric EQ Bell curve boost

The boost/cut curves are typically a bell curve type. This means that the frequency is boosted or cut at the set point, then its effect falls away either side of this point, so that when looked at on a graph, it looks a bit like a bell. Basically, if affects a very narrow frequency band.

What Is EQ? - Parametric EQ Shelving Curve Cut

Often, parametric EQs can feature shelving bands. Here, the gain control will boost or cut all frequencies below a set frequency (low shelf), or everything above a set frequency (high shelf).

What Is EQ? - Parametric EQ Shelving Curve Cut

Though these might seem less precise than bell-curves, shelves are very effective as ‘fixers’. A low shelf cut can be great to remove low-end rumbles, or ‘boominess’ on acoustic guitars without drastically affecting tone, for example. Judicious use of a high shelf can similarly remove noises such as hiss.

Semi-parametric EQs feature sweepable frequencies and gain boost/cut, but with fixed Q (as in the Vibe EQ pictured above).

Multi-band Graphic EQ

What Is EQ? - Multi-band Graphic EQ

Graphic EQs are the type most commonly seen on guitar effects pedals (such as Boss’s GE-7) and PA system master outputs (and eighties stereo systems).

Here, the sweepable frequency control is swapped for a series of sliders. These are set to particular frequency bands. Moving a slider upwards will boost that frequency band, moving it down will cut it. All frequency bands are fixed, and there are no Q controls.

What Is EQ? - Multi-band Graphic EQ

Though these lack the flexibility and precision of a parametric EQ, they allow for incredibly quick curves to be set-up, and many bands of control can be crammed into a small space- perfect for effects pedals.

What Is EQ? - Boss GE-7 Guitar EQ

In the case of a PA rig, graphic EQ controls make it easy to identify problem feedback frequencies and turn them down.

Time spent exploring the EQ harmonics of a given sound source is never wasted. You can find the ‘sweet spots’ of your guitar tone, or learn how to smooth out the rough edges of your drum sound. All it takes is a bit of time and patience…

A point to note is that when mixing, it is often better to cut frequencies than to boost. Boosting frequencies will make a sound louder, meaning that you’ll have to pull the channel fader down. But as will all things in music, there is no right or wrong method – if it sounds right, it is right.