What is a shelf EQ?

Shelf EQ in plain English

A shelf turns a whole end of the spectrum up or down. You pick a frequency, called the corner, and a low shelf moves everything below it while a high shelf moves everything above it. Well past the corner every frequency gets exactly the same change. That flat stretch is the shelf the band is named after: on an EQ screen the curve looks like a step, with a level ledge running off to the edge of the screen.

If you've turned up the bass or the treble on a car stereo or a hi-fi amplifier, you've used a pair of shelves with the corners chosen for you by whoever designed it. A mixing EQ lets you set the corner yourself. Low shelf and high shelf are band types in nearly every parametric EQ, including Ableton Live's EQ Eight, Logic's Channel EQ and FabFilter Pro-Q.

Shelves are for broad changes in tone. A bell picks out one range and leaves both sides of it alone, which makes it the tool for a single problem; a shelf changes everything from its corner to the end of the range, which suits making a sound brighter or darker, heavier or lighter. The EQ lesson covers the other band types, and this page stays with shelves.

Reading the lab

The lab is the same EQ as the one on the main EQ page, set up with four bands for this lesson. Band 2 is a low shelf and band 4 is a high shelf, and those are the two you'll use most. Band 3 is a bell and band 1 is a high-pass filter. Both start switched off, so the later steps can put a shelf next to the two shapes it most often gets confused with.

The pink line is the EQ curve: above the middle line is a boost and below it is a cut. Behind it, the teal shape is the spectrum of the sound coming out of the EQ, laid over the pale line of the original. Drag band 2's point upwards and the whole left side of the curve lifts with it, all the way to 20 Hz. Hover anywhere on the screen and the lab tells you exactly how many decibels the EQ adds or takes away at that frequency, which is the quickest way to check any number on this page.

A/B switches the EQ off and matches the level of the original, so a low shelf boost can't win just by being louder. The Right now panel measures what share of the sound's energy sits in the part the selected band changes, and how much louder or quieter the whole loop comes out.

Low shelf and high shelf

Low shelf

A low shelf changes everything below its corner. Boost it and a sound gets heavier and warmer. Cut it and the sound gets lighter, without losing its low end altogether.

Say you set a low shelf to +6 dB at 100 Hz. Two octaves above the corner, at 400 Hz, it adds 0.03 dB, which nobody can hear. At 200 Hz it adds 0.4 dB, at 100 Hz exactly 3 dB and at 50 Hz 5.6 dB, and from about 25 Hz downwards it adds the full 6 dB. On the full loop in the lab, that's mostly the kick and the bass getting bigger, because they hold nearly all the energy down there.

Move the corner and the whole step moves with it. With the corner at 250 Hz, the same 6 dB shelf adds 5.8 dB at 100 Hz and 3 dB at 250 Hz. That puts the low mids under the shelf as well, the range where guitars, keys and pads all overlap, and a full mix turns boomy and thick very quickly.

It also adds a lot of level. The lab measures the full loop at 1.2 dB louder with the 6 dB shelf at 100 Hz, and 3.7 dB louder once the corner moves to 250 Hz, because more than half of the loop's energy sits below 250 Hz. That's why A/B matters so much with low shelf boosts: without the level matched, the boosted version nearly always sounds better for the first few seconds.

Low shelf cuts are one of the most useful moves in a busy arrangement. Chords and guitars carry some energy in the bass range that the song doesn't need from them, and a cut of 3 to 6 dB from about 150 to 250 Hz leaves the bottom of the mix to the kick and bass. The chords keep some body, which is the difference between this and filtering them.

High shelf

A high shelf changes everything above its corner. Boost it for brightness and for what engineers call air, the open, breathy top end above about 10 kHz. Cut it to darken a sound that's too sharp, or to push a part further back in the mix, since a distant sound reaches you with less of its top end than a close one.

A +3 dB high shelf at 10 kHz adds 0.1 dB at 5 kHz and 1.5 dB at 10 kHz, then 2.8 dB at 15 kHz and almost the full 3 dB at 20 kHz. On a vocal that lifts the breath and the top edge of the consonants while the body of the voice, down in the hundreds of hertz, stays exactly where it was.

Bring the corner down and the shelf starts to reach into the presence range from 2 to 6 kHz, where your ears are most sensitive. A 4 dB shelf at 3 kHz is already 2 dB up at 3 kHz and 3.8 dB up at 6 kHz, so every s and t gets louder along with the air. The vocal turns harsh fast.

The corner frequency

The frequency on a shelf band is called the corner, or sometimes the turnover frequency. In the lab, as in a lot of plug-ins, it's the point where the shelf has reached half its gain. A 6 dB shelf is 3 dB up at its corner and a 12 dB shelf is 6 dB up, whichever direction the shelf faces.

The climb from flat to full gain takes about two octaves, one on each side of the corner. An octave past the corner, out on the ledge, the shelf has done more than nine-tenths of its work; an octave the other way, it's down to less than a tenth. With a 12 dB low shelf at 100 Hz that's 11.1 dB at 50 Hz and 0.9 dB at 200 Hz.

EQs don't all agree on which point the number refers to. Some use the halfway point, as the lab does, and others mark a shelf by a different point on the same curve, so a shelf at 100 Hz in one plug-in can sit noticeably lower or higher than a shelf at 100 Hz in another.

Shelf vs bell vs low cut

There are three common ways to turn the low end down, and they leave very different amounts behind. The figure sets all three to the same frequency. Move the slider and watch the left-hand side, where they part company.

Shelf vs bell

A bell boosts or cuts a region around its centre and comes back to flat on both sides. A 6 dB bell at 100 Hz with a Q of 0.71 is the full 6 dB at 100 Hz but only 2.8 dB at 50 Hz and 0.8 dB at 25 Hz. The 6 dB low shelf at the same frequency is still adding the whole 6 dB at 25 Hz, and below it. The bell also spreads upwards by the same amount, so it's 2.8 dB up at 200 Hz, where the shelf adds 0.4 dB.

That makes a bell the better choice when a bass or kick needs more weight around its notes and nothing extra underneath them. A shelf boost keeps going below the lowest note, into sub and rumble that small speakers can't play but that still uses up headroom. A shelf is the better choice when the whole bottom end needs to move together, such as a full mix that's a little light, or a guitar part that sits too heavy all the way down.

Low shelf vs low cut

A low cut is another name for a high-pass filter. A low shelf cut of 6 dB at 200 Hz bottoms out at 6 dB down and stays there. A high-pass at 200 Hz with a 12 dB per octave slope is 3 dB down at 200 Hz, 12 dB down at 100 Hz and 24 dB down at 50 Hz, and it keeps falling from there.

Use the filter when the low end is something you want gone: rumble, a mic being bumped, the boom of an acoustic guitar that has no place in the song. Use the shelf when you want less of it and still want some, which is most of the time on a part that belongs in the low end at all. The low cut vs low shelf lesson compares the two on more sources.

Shelf Q, slope and other shapes

The lab is built on Web Audio, whose shelves have a single fixed shape, so its shelves have no Q control. Many plug-in EQs give shelves a Q as well. Raising it makes the step between flat and full gain steeper, and past a certain point the curve overshoots, with a small bump on the ledge side of the corner and a dip on the other side.

That overshoot can be useful. A boosting low shelf with a dip just above it adds weight and takes a little out of the low mids in one band, which is close to the curve people go after with the Pultec low-end trick. Push the Q too far and the bump starts to ring like a narrow bell, so keep an eye on the curve when you turn it up.

At the gentle end are very wide shelves. Peter Baxandall's tone control circuit, published in Wireless World in 1952, became the bass and treble controls in a great many hi-fi amplifiers, and its curves keep climbing across several octaves instead of flattening out quickly. EQ plug-ins sold as Baxandall designs copy that shape, and it suits small, broad changes to a whole mix.

Put a low shelf and a high shelf at the same corner, one cutting and one boosting by the same amount, and you have a tilt EQ, which tips the whole spectrum brighter or darker around a single point. The tilt EQ lesson covers that one.

How to add air to a vocal with a high shelf

This is the job a high shelf does most often. The same order works on acoustic guitars, overheads and whole mixes.

1. Make your cuts firstPut the high shelf after any high-pass filter and any cuts in the low mids, so you're judging the top end of a sound that's already balanced underneath.
2. Start at +2 dB with the corner at 10 kHzThat's enough to hear the shelf working on most vocals without it taking over.
3. Listen with the rest of the mix playingAir that sounds obvious on a soloed vocal can vanish once the cymbals come in, and a shelf that seems subtle on its own can be too much in context.
4. Sweep the corner between 8 and 16 kHzLower corners bring up more of the consonants; higher ones leave only the breath and shimmer. Stop where the voice sounds open without any one sound jumping out.
5. Check the s soundsIf they start to spit, raise the corner or lower the gain. If the voice only sounds right with the boost and the s sounds still spit, add a de-esser after the EQ, as the de-essing lesson shows.
6. Compare at the same levelSwitch the EQ off and on with the output gain matched. If the shelved version stops sounding better once the levels match, take a decibel off the shelf and listen again.

Starting shelf settings for common sounds

These are places to begin. Sweep the corner around each one and let the A/B decide where it ends up.

Common shelf EQ mistakes

Boosting a low shelf into the rumble

A low shelf boost has no floor. Put +4 dB on a kick at 80 Hz and any rumble at 15 Hz gets the same 4 dB, even though you'll never hear it on most speakers. A high-pass at 20 to 30 Hz alongside the shelf stops that, or you can use a wide bell at the kick's weight and leave the very bottom alone.

Setting the high shelf corner too low

A high shelf below about 5 kHz lifts the presence range along with the air, and that's where harshness and sibilance live. If a shelf boost makes a vocal tiring to listen to after a minute, the corner is usually the problem before the gain is.

Stacking low shelf boosts

Two or three decibels of low shelf on the kick, the bass, the keys and the guitar each sound fine one at a time. Together they pile up in the same octave and the mix gets muddy and quiet, because the low end eats the headroom. Boost the one or two parts that carry the low end and cut the rest.

Boosting what isn't there

A high shelf can only raise frequencies the recording already has. On a dull sample with nothing above 8 kHz, a 6 dB shelf at 10 kHz mostly brings up hiss. A lower corner around 4 to 6 kHz, or a brighter sample, does more.

Trusting the number between EQs

Copying "low shelf at 100 Hz" from one plug-in to another can give two different curves, because they don't all mark the corner at the same point. Copy the shape you see on the screen instead.

Train your ears

The quiz under the lab is the same one as on the main EQ page. It boosts one of five ranges by 9 dB with a bell, covers the screens, matches the level and asks you which range it was. Those ranges are the same ones your shelves move, and once you can tell 100 Hz from 350 Hz by ear you can set a shelf corner without looking at the screen. Your best score out of eight is saved in your browser.

Questions people ask

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