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Learn Music and Audio Production | iZotope Tips and Tutorials

Principles of Equalization: How to EQ Your Mix

by Nick Messitte, iZotope Contributor October 25, 2021
Learn what EQ is and what it can do for you in your mixes.
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In the beginning, there was me, my DAW (Logic), and an EQ. I had no idea what an EQ did when mixing music, so I sort of figured it out by feel: I’d move a knob, look at the corresponding frequency analyzer, listen with my ears, and by and by, I got the hang of it.

Before we learn what EQ is and what an EQ can do for you in your mixes, take a moment and play some music and listen to how it sounds. Listen to how it makes you feel. Listen to how the bass hits you in your gut, and to how the cymbals draw your attention from one side to another. Notice how the reverb extends and elongates notes. What words do you use to describe what you hear? Bright? Full? Harsh? Smooth?

Active listening helps us to define our own vocabulary in music production. As this article takes you on a somewhat technical journey through EQ concepts and terminology, referring back to this listening experience will ultimately let you make good use of the information that follows.

If you don’t know the first thing about EQ (the shorthand of “equalization,”) this one’s for you. 

In this piece you’ll learn:

Interested in trying out the different EQs discussed in this article? Start your free trial of Music Production Suite Pro to access EQ modules in Neutron Pro, Nectar Pro and Ozone Pro

What is EQ in music? 

EQ (or equalization) in music is the process of adjusting the level or amplitude of specific frequencies to achieve a clear and balanced mix. 

To dig even deeper, music is a combination of sound waves vibrating at various frequencies. All sound can be described in terms of its frequency. Frequencies (indicated in Hertz) determine a note’s pitch; a frequency vibrating at 440 Hz is the note “A” in modern western musical tuning, as heard through the speaker of many annoying tuners and metronomes:

A 440 Hz in Insight Pro
A 440 Hz in Insight Pro

That’s A 440 Hz measured on a frequency analyzer in Insight Pro. Frequencies we use are measured in Hertz (Hz) or Kilohertz (kHz). 

Of course, music is much more complicated than pure sine tones. If I pluck an open A-string on my guitar and run the result through a frequency analyzer, you’ll see more than a thin needle spiking at one specific frequency. You’ll see a multitude of frequencies. 

Frequency spectrum of a guitar string pluck in Insight Pro
Frequency spectrum of a guitar string pluck in Insight Pro

This has to do with many factors. The construction of every instrument contributes to its particular harmonic content—the way the wood is bent changes the amplitude of the frequency content and the harmonics (timbre). 

If I played a note on one guitar, it would not have the same timbral make-up as another. In the case of the screenshot above, the room also contributed a harmonic coloration all its own. It threw its own unique signature into the mix. 

That’s just one note on one guitar. What happens when we run music under a frequency analyzer?

Frequency spectrum of music in Insight Pro
Frequency spectrum of music in Insight Pro

Yeah, that’s a lot of frequencies, all happening at once! And now we can get to what an EQ is, in simple language: 

You know the volume controller on your phone? The buttons that make the music louder or softer? An EQ is basically a level controller for individual frequencies. 

That’s a little too basic, so I’ll pull out a bit: 

EQs allow you to focus on a select group of frequencies, giving you the ability to choose how this group is affected. You can grab a bunch of frequencies and turn them up (“boost,”) or select only a few frequencies, and turn them down (“cut.”) 

When to use EQ

A good starting point to decide when to use an eq when mixing is to turn all the eq off. Try to balance the levels between instruments with faders and get the mix as close as you can. Once through with that, if you have trouble hearing what you want to hear….time to start thinking about EQ. 

EQs can be used for a variety of different ways to correct problems or as a sound design tool in your mix. Typically, EQs are used for the following: 

  • EQs address frequency masking issues. Frequency masking is the “muddiness” that occurs when instruments with overlapping frequencies clash with each other, making it difficult to clearly perceive simultaneous sounds. Do you have problems hearing the kick drum over the bass guitar? This is a job for an EQ.

  • EQs also identify and fix problematic frequencies: sometimes you may find annoying resonances or harsh sounds that were captured during recording or processing. Do you hear one ringing resonance on a snare drum that you find annoying? An EQ is the right tool to use to remedy this. 

EQs can be used to sculpt an audio signal’s timbre, acting as a creative sound design tool to achieve a desired sound effect.

EQ Parameters

The parameters in an EQ are used to adjust the character of filters that will boost or cut frequencies from an audio signal. Common parameters you’ll find include: 

  • Frequency: selects the frequency you want to boost or cut (attenuate). 

  • Q: (center frequency divided by the bandwidth) controls the bandwidth - how wide or narrow the boost or cut will be. In other words, you can determine the range of frequencies that will be affected by the equalizer. Higher Q values will give you a narrower bandwidth, while lower Q values will allow you to boost or cut a wider range of frequencies. 

  • Gain: determines how much of the selected frequencies are cut or boosted. Think of it as a level knob for selected frequencies.

  • Filter type: allows you to choose the shape of the filter for the selected frequency band. 

  • Filter slope: Sets the steepness of the filter for a high-pass or low-pass filter. 

We’ll get into more detail with these parameters in action below. 

How to use different types of EQ

When equalizing, you will have to make small adjustments every time you add tracks to the mix, since the frequencies and respective ranges of the other instruments affect the way an instrument sounds. The most important concept here is to be able to emphasize the characteristic frequencies of the track you are working on and eliminate frequencies that do not enhance its sonic features in any particular way.

We classify a group of frequencies as a “band.” We can refer to frequency bands generally: “bass,” “low-mid,” “mid-range,” “high-mid,” and “high.” We can also pinpoint bands as specific ranges: “between 300 and 500 Hz”, “between 1 and 2 kHz.” 

In choosing how frequency bands are manipulated, we talk of filter shapes. We say “high-pass filter,” “bell,” “shelf,” and other terms. We qualify the width of the filter, or speak of its slope. We also refer to its amplitude. 

You’ll use different filter types depending on what you want to accomplish. These are the main types of filters you’ll find in an equalizer: 

Before we dive into what these look like and what they’re used for, let’s listen to an example piece of audio, unaltered:

Unaltered Audio Example

Parametric, bell, and peak filters

Parametric, bell, or peak EQ is a common type of EQ and is extremely versatile. It can be used to pinpoint and cut/boost a very small range of frequencies or it can be used in a broader way to adjust tonal character. It is usually utilized in the middle of the frequency range. It is delineated from other types of EQ by its bell-like shape.

Example of a bell filter in Neutron Pro
Example of a bell filter in Neutron Pro

This particular bell filter sounds like this:

Bell Filter Example

When we talk about making some frequencies louder or softer—when we say we’re going to “cut” or “boost”—we’re speaking about amplitude. 

As we said earlier, the number of frequencies we can make louder or quieter is dictated by the Q or “Quality Factor” control. The Q point determines the “shape of the bell” or how wide the area around the center frequency is going to be. At low or “wide” Q values, the filter can grab more frequencies, like so:

Example of a wide bell filter in Neutron Pro
Example of a wide bell filter in Neutron Pro

It sounds like this:

Wide Bell Filter Example

At high or “narrow” Q values, the filter is much more specific, grabbing fewer frequencies in the process.

Example of a narrow bell filter in Neutron Pro
Example of a narrow bell filter in Neutron Pro

Narrow Bell Filter Example

In general, we use wide parametric EQs to enact broader, sweeping changes. Narrow parametric EQs, on the other hand, accomplish more specific tasks, such as resonance control—they’re a great tool for taking out excessive ringing from a snare drum, for example. 

High pass and low pass filters

High pass and low pass filters are fantastic for cutting out whole swaths of frequencies that you flat-out don’t want in your tracks—particularly in a harmonic, melodic, or otherwise a-rhythmic recording. 

For instance, many engineers say there’s nothing of note for a commonly-tuned guitar below 90 Hz, so if you notice signal below 90 Hz in your frequency analyzer, you might try cutting out with a high-pass filter, as it could interfere with other instruments that do make use of that range. This isn’t always true, of course. 

High- and low-pass filters often have slope controls, which can be similar to the Q controls on parametric EQs. The slope dictates how steeply or gradually the filter will cut off all extraneous sound. 

High Pass Filter

High pass filters cut all the frequencies below a selected cutoff point. It is often used to cut very low rumble noises below 60 Hz. 

Example of a high pass filter in Neutron Pro
Example of a high pass filter in Neutron Pro

High Pass Filter Example

High-pass filters are often called low-cut filters to avoid confusion. Or you can think of them as being called high pass filters because they only allow high frequencies to “pass”, while they cut or filter out the lower frequencies. 

Low Pass Filter

Conversely, a low pass filter cuts all the frequencies above the cutoff point. A low pass filter is often used to cut very high hiss noises above 18 kHz. Use with caution in order to avoid cutting too much high end of the track. In addition, low-pass filters can be called high-cut filters.

Example of a low pass filter in Neutron Pro
Example of a low pass filter in Neutron Pro

Low Pass Filter Example

High Shelf and Low Shelf Filters

Shelf filters are great for strengthening the low end or for adding some high-end sparkle to the treble when necessary. 

Low Shelf Filter

Example of a low shelf filter in Neutron Pro
Example of a low shelf filter in Neutron Pro

Low Shelf Filter Example

A low shelf is usually used in the low-mid and low range of the audible spectrum to reduce some of the rumble noise caused by microphone stands and other low end sources, including percussion instruments, like so:

Example of a low shelf cut in Neutron Pro
Example of a low shelf cut in Neutron Pro

Low Shelf Cut Example

High Shelf Filter

Conversely, a high shelf filter boosts or cuts the high end. It can be effectively used to brighten up a track by using a positive gain of 3 or 4 dB and a cutoff frequency of 10 kHz and higher (be careful because this setting can increase the overall noisiness of the track). 

High Shelf Filter Example

It can also be used to reduce the noise of a track by reducing by 3 or 4 dB frequencies around 15 kHz and higher. 

Use EQ in your audio mixing and mastering process

iZotope Music Production Suite Pro offers plug-ins all with various EQ modules, including Nectar Pro for vocals, Neutron Pro for instruments, and Ozone Pro for the master bus. 

All three products provide shelves, parametric bells, and pass filters. They differ, however, what situations you should use them in. These articles go into more detail about which type of EQ is best for your production stage and how to use EQ in mixing and mastering. 

The aim of this article is to show you, in simple terms, what an EQ can do for you. I hope you feel you’ve gotten a good grounding in the subject. If you have, I encourage you to read more on the topic, and explore the EQ options available to you in a free trial of Music Production Suite Pro

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