EQ is a massive subject for audio engineers. It’s so fundamental to the process that the majority of your time mixing music is probably spent tweaking EQ plugins. For such a big topic, it’s important to have a firm grasp on the basics. EQ is a powerful tool and you can easily do more harm than good. In this article I’ll go over everything you need to know to start using EQ like a pro—from the ground up.

What is EQ in music?

EQ (or equalization) in music is the process of changing the balance of different frequency components in an audio signal. Our ears can detect a huge range of frequencies—roughly 20 Hz to 20 kHz. Each element of your mix has energy in different parts of that range.

EQ is the the tool you use to manipulate the frequency content of your mix so that everything is balanced and clear.
Here’s a rough break down of where the common mix areas sit on the spectrum.

Timbres and frequencies

Imagine a french horn and an electric guitar both playing the same A=440Hz note. What makes them sound different? Both instruments are playing a note with the same fundamental frequency, but each has its own unique timbre.

A sound’s timbre gives our brains a lot of information about what it represents in the real world. Unique, identifiable timbres are a property of all complex sounds. Complex just means any sound other than a basic sine wave. All complex sounds can be broken down into simple sine wave components. These basic components are called partials.

If the partials are related to the fundamental by a whole number ratio (ie, 2:1, 3:1, 4:1 etc.) they’re harmonic. If not, they’re inharmonic.

A highly harmonic sound like a bowed cello string is rich in evenly related partials, while a highly inharmonic sound like a cymbal crash is made up of only unrelated ones. When you use EQ to alter a sound, you’re really changing the volume of its partials relative to the rest. Keep this concept in mind as you move through the basics of EQ.

EQ in mixing

Going back to our example, let’s say we record the french horn player and the guitarist playing a unison melody. They each have distinct timbres, but when we record the performance with microphones, we notice the two sounds overlap.

In fact, they’re overlapping so much that it can be difficult to to hear them individually on the recording. This effect is called masking. EQ is used in mixing to help reduce the effect of masking so that each instrument can be heard clearly.
Remember: EQing doesn’t create new frequencies. Think of EQing as sculpting… You’re working with raw material—the existing frequencies of your sound.

Filters and Bands

EQ is essentially a special application of a filter. The qualities of the filter that your EQ uses for its cuts or boost determines a lot about its sound—and the best way to use it. If you break down the settings of an individual band of EQ, they’ll refer to basic characteristics of filters: type, slope, Q, and gain.
I’ll explain each one and how they affect your sound.

Type

The filter type determines the general shape of the EQ band. The most common types are high/low pass, bell, notch, and high/low shelf. High-pass (HPF) and low-pass (LPF) filters: These filters are named for the frequencies they leave unaffected, rather than the range they cut. A low cut is known as a high-pass filter and a high cut is known as low-pass filter.

Reach for these filter types when you need to clean up issues at the extreme ends of the spectrum. The steep drop-off around the corner frequency can easily tame boomy low-end or piercing ultra-high resonance.

Bell: Bell filters are your standard tools boosting and cutting. Their shape can be manipulated using the Q parameter. This is your go-to choice for sculpting and tone-shaping. Boost or cut at specific points and decide how broadly to affect neighboring frequencies with the Q control.

Shelf: Shelf or shelving filters boost or cut all frequencies above or below the corner frequency. This EQ type is effective for making broad tonal changes. Think of low and high shelves like the treble and bass controls on your stereo system.

Notch: Notch or “band-stop filters” are used to selectively eliminate extremely narrow frequency ranges. You can use these to eliminate offending room resonances that can arise from recording in untreated acoustic environments—but be careful, using many steep notches can create a comb-filtered, phase-shifted sound.

Slope

The slope of a filter refers to how aggressively the sound beyond its corner frequency is attenuated. Filter slope is usually associated with HPF and LPF and types, but some modern EQs allow you to choose the slope of bell or shelving bands as well.

Slope is measured in dB/octave. The higher the number, the steeper the drop off around the corner frequency of the filter. Slopes of 6db/octave to 24db/octave are common, although some advanced digital EQs can create slopes of up to 96db/octave.

Use less severe slopes like 6 or 12 db/octave to create gentle, transparent low end roll-off with a low-pass filter. Use sharper cut-offs like 24 and 48 db/octave for aggressive filtering at precise frequencies.

Q

Q is short for quality factor. You can think of it as the “bandwidth” of an EQ band. Q values of less than 1 will give you broader EQ curves, while values greater than 1 will give you tighter, more selective boosts or cuts.

Gain

Gain determines the amount of boost or cut you apply with your EQ. It’s measured in dB. Positive gain values indicate a boost, while negative ones make it a cut.
Hot tip: When you make adjustments with EQ, you also affect the overall gain of your signal. That means extreme EQ settings can change your levels a lot. Make sure to keep gain staging in mind so you don’t run out of headroom.

Frequency

The frequency is the centre of your EQ band’s action. This control determines the range where boosts or cuts will occur.

It’s important to remember that there’s no such thing as a perfect filter.No matter how tightly you set your Q or how steep you make your slope, an EQ band will always affect a range of frequencies around the target, so be careful.

Adding and subtracting

The first rule of EQ is less is more. Extreme tonal shifts can have a negative impact on your sound. The best way to use EQ is to use it as little as possible. With that in mind, there are two main ways to approach EQ.

• Additive EQ: boosting frequencies to achieve the results you want.
• Subtractive EQ: cutting offending frequencies out.

There’s plenty of debate about whether it’s better to do one or the other. But if your goal is to use the least EQ, you can simply choose whichever approach is the most direct route to your destination. For example, if you only need to cut one narrow range, using 3-4 bands to boost every other region of the spectrum is less transparent that using a single band to cut the main offender. On the other hand, if all you need to do to make a sound pop is boost one range, cutting everything else and applying makeup gain isn’t the quickest route.

This debate is sometimes framed in terms of “corrective” vs. “creative” EQ, but that can be misleading. No amount of EQ can fix a sound that wasn’t captured correctly at the source. As a recording engineer, your focus should be to get the sounds you need for your mix as early as possible in the process.

Equalize it

In the course of your mixing process, you’ll probably use EQ on every single track in your session.With such an important tool, it’s important to have a solid foundation for how to use it.

Knowing how EQ affects the timbre of your sounds is the key to getting the right parameters for your mix or master. Now that you have the fundamentals down, get back to your DAW and sculpt the perfect sound.