Not everyone has an audio engineering background, but thankfully, you don’t need one to reap the benefits of an equalizer (or EQ). An EQ is a tool for recording and mastering music, but anyone can use one to adjust the sound signature of their headphones or speakers via an app or physical controls. To get the best results from an EQ, though, you first need to understand how it works.

Understanding the Terms: Frequency and Response

Let’s start with one of the most important terms when dealing with EQ settings: Frequency. Frequency represents the number of cycles (or sonic vibrations) per second, and is measured in Hertz (Hz). Thus, a frequency of 1Hz is the equivalent of one vibration per second, and a frequency of 1kHz is equal to one thousand vibrations per second.

The range of human hearing spans roughly from 20Hz to 20kHz. Seriously low frequencies produce deep rumble, while the highest frequencies produce uncomfortably shrill sounds. That range is a general guideline, however, and the upper limit of audible frequencies typically declines as you age; infants can hear at frequencies slightly higher than 20kHz, while adults might be unable to hear anything above about 15kHz. Additionally, some of the very low frequencies might not be audible at all, but you might physically feel them if they’re loud enough.

Headphones and speakers are typically built around this range, with most able to output between 20Hz and 20kHz. They don’t treat all the frequencies in that range the same, though; a speaker or a pair of headphones might be able to output sound across the full frequency range, but the amount of power they put behind each frequency varies. The particular handling of frequencies is known as frequency response, and is typically represented as a graph that plots frequency (in Hz) against gain (power) in decibels (dB).

Taking a step back, we often use the phrase flat response in our reviews of audio products. If you’re not familiar with the term, it probably sounds like a negative descriptor—who wants to sound flat? But we’re neither discussing a singer whose pitch is off, nor someone whose joke fails to deliver. In this context, a flat response is a frequency response that is as close to the unaltered truth as possible. Audio engineers and some audiophiles prefer this type of response from headphones and speakers, because it denotes a sound signature that is precise, transparent, accurate, and free of coloration.

But, contrary to the name, frequency response graphs for even so-called flat response headphones aren’t usually flat at all, and typically have distinct curves and peaks. That’s hardly a bad thing though—if headphones and speakers didn’t account for the quirks of the human ear, a truly flat-lined frequency response would sound bizarre to most of us. There is no one pair of speakers or headphones that the world agrees is truly flat, but many engineers at least agree that certain studio monitors come relatively close.

So, think of a flat response as a technical ideal, but not necessarily something that most of us can obtain. It also isn’t a desirable quality for many people. Depending on your tastes and the quirks of your hearing, you might prefer additional bass depth or more crispness in the high-mids. The representation of sound gets even more complex once you introduce systems with bass and treble knobs or multi-band EQs. In these cases, you can alter the sound signature of these output devices quite significantly to taste.

What Is EQ?

Building on the above, an EQ is a tool to adjust the gain of various frequency ranges in the complete frequency range of a given output device. Before we get to consumer applications of EQ, we must establish a few more technical terms, even if you’re less likely to encounter full-scale versions of them.

A parametric EQ is a common studio tool that’s typically available either in a plug-in or on a console with knobs. These knobs generally allow you to select any frequency you wish and then boost or cut it as you see fit. You can also adjust the bandwidth (often displayed as Q on EQ knobs) of the range you’re boosting or cutting; a narrow bandwidth refers to a very precise frequency range, while a wider bandwidth includes a slowly tapered off range of frequencies around the selected one. In other words, parametric EQs enable you to get very granular; you can pinpoint problematic frequencies in a mix, or highlight frequencies that might benefit from a little boosting.

A shelving EQ (or shelf EQ) is another variation. It adjusts everything either above or below a frequency you select, depending on whether you use a high or low shelf. You can typically control both the cutoff (the frequency where the attenuation begins) and the amount of gain reduction that happens to the attenuated frequencies. Naturally, low-shelf EQs focus on lower frequencies, and high-shelf EQs deal with the highs.

You may not be aware, but you might have used both of these types of EQs—many consumer audio devices include vaguely worded bass and treble knobs that, depending on the implementation, function like parametric and shelving EQs. For instance, turning the bass knob might boost or cut everything below a certain frequency, with a gradual roll-off so the adjustment doesn’t sound strange. Or the change might affect a medium-to-wide bandwidth selection like with a parametric EQ. The same is true for a treble knob, though changes there affect the higher frequencies, of course.Bose offers an app-based EQ for a number of its headphones and earphones

But the most common type of consumer EQ is known as a graphic EQ. These types of EQs typically display sliders that correspond to different frequency bands. When we talk about a multi-band graphic EQ, each band (usually) represents a fixed frequency that you can raise or lower with a fader; the lowest frequency bands sit on the left and the highest ones are on the right. When you boost a slider or fader, you raise the gain. And when you dip it below zero, you cut it. To help you along, many apps with EQs offer built-in presets that provide different settings for each frequency to emphasize a specific genre of music or type of audio content. Rarely, you may even see a graph-like display in which the vertical axis represents gain (dB) and the horizontal axis represents frequency (Hz).

One note: The bandwidth feature described in the parametric EQ section is usually in play with a graphic EQ—you are most likely adjusting a range of frequencies with a graphic EQ here, not just a specific one. If the latter was the case, boosting or cutting it would result in a strange sound signature. Most graphic EQs don’t allow you to adjust the bandwidth of your selection or even see the bandwidth value, however. If yours does, you can play with this function to experience what we describe above—a wider bandwidth affects more frequencies around the selected one, and a tighter bandwidth has a more specific effect.

So, if you set all the EQ faders to zero, does that mean you can achieve a flat response sound signature? Likely not. Although it’s common to refer to this setting as flat or neutral, the only thing that is neutral here is the EQ. In other words, the frequency response of your earphones, even at a neutral EQ setting, is most likely not flat. If the drivers deliver thumping bass or really bright highs (or both) at these neutral EQ settings, we describe them as having a sculpted sound signature. Again though, plenty of listeners prefer this type of sound, and that’s why so many speakers and headphones either don’t strive for accuracy or otherwise ship with EQ settings that boost the low or high ranges.

How Can You Use an EQ Effectively?

So, now that you understand how EQ works, what are your plans for it? Go crazy and dial up the bass to 11? Probably not.

It’s important to remember that, in the recording studio, an EQ is first and foremost a tool for dealing with problematic frequencies that stand out in a recording. Secondly, it is a creative tool—there might not be a problem to fix, but adding some gain to the high-mids on a vocalist might give their voice a more gravelly delivery, while adding some highs to a guitar might give it a little more shimmer or airiness, for instance. Finally, you can use it to create distinct effects. Scooping out all of the low frequencies, dialing up the high-mids, and cutting out the highs on a vocal recording is one starting point for creating a low-fi radio or walkie-talkie-style sound. And DJs use EQ filters all the time in live settings—the slow sweep of a low-pass filter over a drumbeat or a synth loop is a common move in electronic music.You can often find the EQ in the companion app of your headphones or speakers alongside other adjustable settings

For non-professionals, you’re most likely to encounter an EQ in the companion app for your headphones or speaker. Not all headphones and speakers ship work with customizable in-app EQs, and some simply aren’t very useful (we evaluate this in each of our product reviews). If this is the case, you can download third-party EQ apps for both Android and iOS. There are also plenty of EQ programs for macOS and Windows if you listen to your music primarily via a laptop or desktop.

It’s best to think of an EQ as a way to subtly tweak the sound signature of your speakers or headphones. Just note that you won’t likely have as much flexibility to boost or cut frequencies (with the option to cut problematic frequencies out altogether) as audio engineers do in the studio.

Boosting or cutting is simply a matter of raising or lowering a fader (gradually) to taste. But finding a problematic frequency is a bit more difficult. Start by boosting a band and listening for changes. If the offending frequency range sounds louder and even more annoying, that’s the one to work with. Otherwise, return that band to zero and try another until you locate the band or bands closest to the culprit frequencies. Just make sure to slowly increase the gain instead of maxing it out at the start to avoid overloading your drivers.

Once you find the frequency you want, you can cut it back by (subtly) pulling its levels below zero. One issue you might encounter, however, is that, unlike a sound engineer, you can’t dial back specific instruments—instead, your only option is to dial back every part of the mix that exists in that frequency range. In other words, toning down some harsh vocals in the high-mids might also result in drums or guitar strums losing a bit of definition. If your EQ has bandwidth settings—which isn’t common with bundled apps—you can more precisely pinpoint the frequencies you wish to adjust.Shure offers the best in-app EQ we’ve tested

Regardless, subtle adjustments are usually the answer to getting the most out of EQ. If you feel like you need to make dramatic adjustments to the sound signature, you should likely just get headphones or speakers with a default sound signature more to your liking.

To help you use an EQ effectively and pinpoint which bands to work with, below is a description of each frequency range within the normal output range of headphones and speakers:

Sub-Bass and Bass: Rumble and Boom

On the low end, sub-bass (the deep rumble you are as likely to feel as hear) is any frequency between 20Hz and 60Hz, while bass (the booming element of music above the sub-bass) runs from 60Hz to 250Hz.

If you want to hear a little more bass depth, start by boosting the various low-frequency bands to see what each one does. The band farthest to the left might be low enough to tweak subwoofer-range frequencies; add a little bit more heft and body with a small bump, or opt for a thunderous response with a dramatic shift. Lows and low-mids have more of an impact on the richness of an electric bass or a baritone vocalist. Boosting these a bit results in a richer sound signature, while going wild with them makes elements of the mix sound muddy and undefined.

Midrange: Filling the Range

Midrange runs from around 250Hz to 4kHz. This is where the majority of sound sits, which is why it’s split into three sub-bands: low midrange (250Hz to 500Hz), midrange (500Hz to 2kHz), and high midrange (2kHz to 4kHz). The low-mids give brass, strings, and woodwinds a sense of resonance and fullness. The mids are where most instruments hit their higher notes, and if you boost them too much, they can sound overly horn-like. The high-mids cover a lot of vocals and are where the attack of percussion instruments sit. They can make tracks seem to jump out, but too much boosting in that range can lead to a harsh character.

Manufacturers rarely boost the mids and instead usually emphasize the lows and highs. This leads to what we call a scooped sound signature, in which there’s lots of bass and treble, but nothing in the middle to connect them and make the transition sound natural. If your headphones sound thin or nasally, try boosting the midrange.

The high-mid bands start to add subtle crispness to vocals, but can also increase sibilance and create a harsh-sounding response if you boost them too much. A slight extra emphasis on the highs generally adds a little bit of clarity, definition, and airiness, but past that point, you probably won’t get a pleasant sound. As mentioned, however, many manufacturers do just that, in combination with heavy boosting in the lows. In this scenario, the highs and lows balance each other out, leaving the defining aspect of the sound signature to be what extent the headphones or speakers emphasize the mids.

Highs: Crispness and Clarity

High frequencies run from around 4kHz to 20kHz, but a big chunk of that range isn’t audible by anyone older than a teenager. This range is known as presence at the lower end (4kHz to 6kHz), and brilliance at the higher end (6kHz to 20kHz). Presence adds clarity and crispness, and it is generally the range that a treble knob affects. Brilliance can make sound sparkle, and lends a sense of airiness. But boosting these frequencies too much can add hiss or sibilance to a track, and simply make the highs uncomfortable to your ear.

High-Res Sound and You

After you craft the sound signature of your headphones or speakers to your liking, it’s time to think about audio quality. We have everything you need to know about streaming high-res audio, as well as an in-depth guide to Bluetooth audio codecs.