Basics of Recording: What Is An EQ?

By Mitchell Sigman


In our “Basics of Recording” blog entries we’ll discuss the building blocks of creating pro recordings. We hope to help folks newer to music production, but hopefully we’ll provide some insights for experienced users too. In this installment we’ll talk about equalizers, usually shortened to simply “EQ.”

As most of us learned in school, the audible range of sounds is 20-20,000 Hz (or less if you’re a partially deafened rock ‘n’ roll curmudgeon like me). EQs are used to boost or cut audio frequencies in order to tailor sounds in a mix. Boosting frequencies can fatten thin tracks or brighten dull tones. Cutting frequencies is often used to fix issues such as boomy bass, unpleasant ringing, or harsh, cutting high frequencies. It’s important to understand that EQs almost never affect only a specific frequency, but instead boost or cut a region of frequencies around the selected frequency. Certain EQs feature a bandwidth control, allowing users to set just how wide a “swath” of frequencies are affected; more on this later.

One good way to think about EQs are as a frequency-specific volume control – in other words, when an EQ boosts or cuts a frequency range, it’s adjusting the volume of a certain region of the audio bandwidth. Let’s talk about the different kinds of EQs and exactly how they affect audio signals.

Lowpass and Highpass Filters – The simplest of EQs, with self-explanatory names. A highpass EQ passes sound above the selected frequency, whereas a lowpass EQ allows sound below the set frequency to pass through. Usually used for general correction, for example, highpass filters are handy for getting rid of low frequency rumbling in recordings. Lowpass filters might be good for reducing hiss in a kick drum track where extreme high frequencies aren’t needed. These are sometimes referred to as low- and high-cut filters.The graph below shows the effects of a highpass filter. Frequency is represented horizontally (note the frequency values displayed along the bottom), volume is represented vertically with zero (i.e. no boost or cut) in the middle. BTW, I’m using ToneBoosters Parametric EQ, included in Mixcraft Pro Studio 6)

high_pass_textAnd here’s what a lowpass filter looks like:


You may have heard the term “lowpass filter” in conjunction with classic analog synthesizers. This is because analog synths utilize a lowpass filter to shape the tone of harmonically rich basic waveforms by removing upper harmonics, commonly known as “subtractive synthesis” (we’ll get  into this deeper in another blog entry, oh yes we will :P).

Low- and High-Shelf  EQ – In their most basic form, these are the “one-knob” bass and treble knobs seen on home hi-fi or car stereos. Shelving EQs are called this because their effect on audio frequencies resembles a shelf on an EQ graph. Here’s what a low-shelf EQ looks like:

low_shelf_EQ_textSee how the curve rises around 126 Hz? In this case, 126 Hz is the initial or “corner” frequency, because all frequencies beneath it are boosted (or cut) nearly equally, hence the flat “shelf” profile. In a basic “one-knob” shelf EQ, the corner frequency is fixed and only the amount of boost or cut is user adjustable; most versions intended for pro audio (that’s us) let the user set the corner frequency.

Many low- and high-shelf EQs also let the user adjust the Q or “slope”, affecting how drastically the curve rises or falls. At extreme settings this puts tonally useful “bumps” near the corner frequency; vintage Pultec standalone EQ units are known for this behavior. Here’s what a high-shelf EQ with an extreme curve and “bump” looks like:

highpass_bump_textSee how the boost and cut profiles are far more pronounced near the corner frequency? This gives a little extra push to kick drums or bass guitar near the corner frequency.

Below is a high-shelf EQ. This works exactly like a low-shelf EQ, but the corner frequency is far higher, and the boost or cut happens above the corner frequency. In the graph below, I set the corner frequency around 4000 Hz.


Graphic EQ – A graphic EQ consists of a bank of slider controls, each corresponding to a frequency. Though each is labeled with a specific frequency, remember these are just the center frequencies and that nearby frequencies are also affected. They’re super easy (and fun) to use – just slide the sliders and make pretty pictures! But because of the fixed frequency points and preset bandwidth per slider, they’re less flexible than the parametric EQ described below. Graphic EQs with fewer bands are more useful for creative EQ, as opposed to “corrective” problem-solving EQ. (Shown below with Kjaerhus Classic EQ, included in all version of Mixcraft)


Parametric EQ –  A semi-parametric EQ lets the user select the exact center frequency for cut and boost, but the bandwidth is fixed. A fully parametric EQ adds a bandwidth control (often called “Q”, perhaps the inventor of modern EQ loved James Bond). This means the user can adjust frequency, amount of boost or cut, and how much neighboring frequencies are affected, making it the most powerful type of EQ (most virtual plugin parametrics include are fully parametric; semi-parametrics are typically found in budget hardware mixers).

Parametrics are sometimes called “bell-curve” EQs, because the profile of the boost or cut has the rounded appearance of soul legend Patti Labelle. Ok, I made that up – their profile resembles an actual bell. Sort-of-ish. I didn’t make up the name.


Practical EQ Tips
Full parametric EQ plugins usually include multiple bands with separate bands configurable as low cut, high cut, low- or high-shelf, or bell-curve parametric. All that EQ horsepower can be a little overwhelming, so I’ll offer some practical application tips.

Wide and narrow parametric bandwidth (aka “Q”) settings each have specific uses. Narrow-Q is almost always used for corrective purposes in order to attenuate a very specific frequency that’s sticking out, ringing, hurting one’s ears or otherwise causing trouble. These nasty “rings” can occur due to an acoustically resonant room, bad mic technique, poorly tuned drums, etc. Snare drums are a frequent offender. Regardless of the source, the easiest way to dial out a ringing frequency is to use a bell-curve parametric as follows (demo video below):

1. Solo the offending track and turn the master volume in the room way down, so you don’t go deaf, blow your tweeters, and blame me for the whole mess.

2. Set the parametric to a bell-curve with a very narrow Q and turn the gain way up (10 db should work). Should look something like this:

narrow_Q_text3. Hit the play button – I recommend looping a small region. While listening, move the frequency control up and down slowly until you find the offending frequency. You’ll know when you’ve found it, because the exaggerated setting will ring loudly (hence the reduced volume).

4. Now that you’ve locating the ringing frequency, turn the gain control way down to a negative value and set the master volume back to normal. Ideally you’ll want the EQ gain low enough to eradicate the ugly ring, but not low enough to negatively impact the remaining sound.

Here’s a video below. I actually dialed out two separate peaks to tame the snare drum, but it’s the same idea, just repeated twice. Notice that the higher frequency peak is almost exactly twice the first one, i.e. one octave apart. In the name of disclosure, I figured out the offending frequencies prior to making the video so you wouldn’t have to watch me poke around for four minutes –  don’t worry if it takes a bit to figure out the frequencies! Also notice that I click and unclick the “active” box at the end to let you A/B with and without the EQ.

One important thing to remember is that narrow Q EQ doesn’t usually sound pleasant or “warm”. Wide Q settings sound far nicer for more general tonal changes such as fattening guitars and snares in the 100-300 Hz range or kick drums around 100 Hz. Instruments can be brightened anywhere above 1000 Hz, but be careful and selective with boosts in the 2000-4000 Hz range, because this is the dreaded “icepick” area that can hurt the listener’s ear. I often dip around 3000 Hz if a female vocalist is particularly strident sounding.

Another important EQ tip is simply knowing where instruments sound good. For example, don’t bother boosting 80 Hz on a vocal, because humans don’t make any sound there – you’ll just be boosting low-end noise. Instead, think of “carving out” the important areas for vocals and instruments to sit and not clobber each other. I usually use low cut filters on almost every instrument in a mix with the exception of kick drums in order to prevent a buildup of low-end mud. Miked electric guitars can be especially nasty offenders here. Unless you’re recording down-tuned metal madness, there’s probably not much you want going on beneath 100 Hz. Electric guitars can also be particularly nasty in ear-scratching 2000 – 5000 Hz range, so don’t be afraid to apply a gentle, wide cut here (and don’t be afraid to tell your guitarist to shove it when he cries about it). This has the bonus effect of making vocals more audible.

If you find yourself using five bands of EQ on one instrument, it’s either an inherently awful sounding track, or you’re seriously overdoing it. I’ve seen some terrifying amateur parametric craziness… users will commonly boost a bunch of bands simultaneously (this happens with graphic EQ as well). Remember my “frequency-specific volume control” analogy? If you’re boosting a ton of bands at once, all you’re really doing is turning up the volume knob. Louder always sounds better, so don’t be fooled, EQ sparingly.

I’ll leave you with one last piece of EQ wisdom: “Cut more than boost, grasshopper” (pretty sure Mr. Miyagi said that, or Master Po, or maybe Master P, I get confused). A zillion tracks of boosted everything makes for a dense and congested mix. Some things don’t need any EQ to sit right. My most important EQ tips: 1) don’t let bass instruments become a thick, gucky mess, it’ll sound bad and the mastering guy will hate you, and 2) make room for the important stuff, like lead vocals, snare drums and solos by cutting frequencies in support instruments. Finally, mix and EQ in context, i.e. with everything playing, because the right EQ choices for instruments in a mix often sound funky (or thin) when soloed.