What is a notch filter?

Notch filters in plain English

Most EQ bands turn a range of frequencies up or down by an amount you choose. A notch takes out a thin slice of the spectrum as completely as it can and leaves the rest alone. Drawn on an EQ screen it looks like a narrow V cut into a flat line, which is where the name comes from.

You reach for one when something in a recording sits at a single fixed frequency and shouldn't be there at all. Mains hum is the commonest. A whine from a power supply and a stage monitor starting to feed back are others, and a drum that rings at one note is where people often reach for a notch when a bell cut would do better.

A notch has two settings, frequency and Q. There's no gain control, because the depth is fixed at as deep as it can go. The EQ lesson covers every band type together and the Q and bandwidth lesson covers width in general; this page stays on notches: how deep and narrow they are, and when a bell cut does the job better.

Reading the lab

The lab starts on a 110 Hz sine wave, a pure tone with nothing else in it, because that's the one sound a notch can remove completely. The big screen shows the spectrum of the original as a pale line and what comes out of the EQ as the teal shape. On the sine, that's a single spike.

Band 1 is a notch and band 2 is a bell, there so you can compare the two. Drag a numbered point sideways to move its band, or use the Freq and Q controls under the screen. The pink line is the EQ curve; a notch is the thin spike pointing down. The Right now panel measures what your settings do to the sound, including how far the level of the whole loop moved.

Listen to this band plays only the slice of the sound around the selected band, turned up so you can hear it. With the notch selected, that's the slice the notch would take away. A/B switches the EQ off and matches the level, so you hear the change in tone without the change in volume.

How does a notch filter work?

A notch passes every frequency at full level except a narrow band around its centre, where the level drops steeply, and at the centre itself the output falls to nothing. A digital notch like the one in this lab really does reach zero at that one frequency. Analogue notches get close, though tiny differences between their parts always let a little through.

Q sets the width. On a notch it's measured between the two points where the level is 3 dB down, so the width in hertz is the centre frequency divided by the Q, which gives some very small numbers.

Step 2 in the lab puts a notch with a Q of 10 right on the 110 Hz sine. It's 11 Hz wide, from about 105 Hz to 116 Hz, and the tone disappears.

Move the same notch up to 120 Hz and the tone comes back almost at full level, only about 1 dB down. Missing by 10 Hz is as good as missing completely. Lower the Q to 2 and the notch spreads to 60 Hz wide, which reaches 110 Hz again from 120 Hz and takes about 10 dB off the tone. A wider notch is easier to aim, and the price is that it takes more of whatever sits nearby.

Narrow notches are slow

A filter that's very narrow in frequency is slow to react in time. When a tone starts suddenly, a notch lets the start of it through and closes in over the next fraction of a second. Measured in the lab on the 110 Hz sine, a notch with a Q of 10 pulls the tone 60 dB down in about 0.2 seconds, and at a Q of 30 it takes about 0.6 seconds. The time grows with the Q and shrinks as the frequency goes up, so a 50 Hz notch at a Q of 30 needs more than a second.

Hum never stops, so for hum this doesn't matter. On a short drum ring or a single note it does, because a very narrow notch can miss the start of the sound it's meant to remove. So on a drum or a note, don't make a notch any narrower than it needs to be.

Notch filter vs a narrow bell cut

A bell cut is the other way to take a narrow slice out, and on a screen the two look alike. The difference is depth. A bell stops at the gain you set, so a −12 dB bell never takes more than 12 dB off anything, while a notch keeps getting deeper towards its centre with no floor. Drag the Q in the figure and both shapes narrow together, but only the notch reaches the bottom of the chart.

Use a notch when you want a frequency gone, which mostly means noise that was never part of the music: hum, a whine from a power supply, a whistle. Use a narrow bell cut when you want a frequency quieter. That covers nearly every problem that comes from an instrument, since the frequency you're cutting is usually part of its tone as well.

Steps 5 to 7 show this on a drum. Its ring at 297 Hz is the tallest narrow peak in the loop, and Listen to this band lets you hear it by itself. A notch at a Q of 8 takes about 14 dB off the ring, and the drum's pitch goes with it, because that ring is the drum's note. A 6 dB bell cut at the same Q takes about 6 dB off and keeps the note, which is what you'll want most of the time.

Some EQs blur the line. A bell with a cut of 30 dB or more at a high Q behaves almost exactly like a notch, and some plug-ins let the gain go that low. Ableton Live's EQ Eight and FabFilter Pro-Q both have a separate notch shape too, which is quicker when you know you want the frequency gone.

How to remove hum with a notch filter

Mains hum is the electricity supply leaking into a recording, through a ground loop, a badly shielded cable or a guitar's single-coil pickups. It sits at the mains frequency: 50 Hz in the UK, Europe and Australia, and 60 Hz in North America, while Japan uses both, depending on the region. It rarely stays at one frequency, though. Hum usually brings harmonics, extra tones at whole-number multiples of it, so 50 Hz hum can also have energy at 100 Hz and every 50 Hz above it. The buzzier it sounds, the more of them there are.

1. Find a gap where only the hum playsThe start of a take, before the first note, is usually best. Loop it so you can hear the hum without the music on top.
2. Put a notch at the mains frequencyStart with a Q of about 10 at 50 or 60 Hz. If the hum drops a lot, you've found its fundamental, the lowest pitch in it.
3. Narrow it as far as it'll go and still workRaise the Q towards 20 or 30, stopping before the hum creeps back. Mains frequency drifts slightly, so a notch that's too narrow can miss it on a long recording.
4. Add notches at the harmonicsListen to what's left and put more notches at 2, 3 and 4 times the frequency, at the same Q. Stop when you can't hear the hum in the gap any more.
5. Check it with the music playingSwitch the notches in and out over a busy part of the song. If the bass sounds thinner, the lowest notch is too wide.

Bass notes sit in the same range, so don't skip that last check. G1, the G three frets up the low E string of a bass, is 49 Hz, just 1 Hz away from 50 Hz hum. A notch at 50 Hz with a Q of 10 cuts that note by about 8.5 dB, and at a Q of 30 the cut on the note shrinks to about 2 dB, so hum notches are kept very narrow.

When the hum has a lot of harmonics, a stack of notches gets fiddly. Dedicated de-hum plug-ins, such as the De-hum module in iZotope RX, place a whole row of notches at the hum and its harmonics from one frequency setting. If you can record the part again, look for the cause first. A ground lift switch on a DI box often cures ground-loop hum, and single-coil pickups hum less when the guitarist turns to face a different way.

Taming a ringing drum or resonance

A resonance is one frequency that sounds louder and longer than the rest, like the ring of a snare head, the boom of an acoustic guitar body or the tone of a small room. You hear it as a pitch that hangs on after the hit, or a note that jumps out every time the player lands on it.

Finding it is the hard part. Put a band on the sound and turn on Listen to this band while you move the frequency, and the ring gets loud when the slice lands on it. The other common method is to sweep a big, narrow bell boost until the ring jumps out, which the Q and bandwidth lesson walks through. With hum or a constant whine you can also sweep the notch itself and stop where the sound vanishes.

Then decide how much to take. If the ring belongs to the drum and is just too strong, a bell cut of 4 to 8 dB at a Q of 6 to 10 is plenty. If it's a tone with nothing to do with the instrument, a notch can remove it completely. Start with the bell and switch to a notch only if the ring is still there.

A notch fixed at one frequency only helps when the problem stays at that frequency. A boom that follows the notes, like one loud note on a bass line, needs a cut that acts only when that note plays, and the dynamic EQ lesson covers how. And if you can hear the same ring on several tracks, fix it on the track it comes from; on the mix bus, every part would lose that frequency.

Other places you'll find notch filters

Feedback on live microphones

Feedback happens when a microphone picks up its own sound from a speaker and the loop builds at one frequency until it howls. Live engineers pull that frequency down with a cut on a graphic EQ or a narrow parametric band, or let an automatic feedback suppressor do it: a unit that listens for a frequency starting to feed back and drops a notch on it.

Synth filters

Many synths have a notch mode next to low-pass and high-pass, and so do filter plug-ins like Ableton Live's Auto Filter. It cuts a band around the cutoff and passes the rest, so the sound goes hollow without getting darker. Sweep the cutoff with an LFO and the hole moves through the sound. It's a single-notch cousin of a phaser.

Phasers, flangers and comb filtering

Those effects are built out of notches. A phaser mixes the sound with a copy whose phase has been shifted, and the frequencies where the two cancel become a handful of moving notches. A flanger mixes in a delayed copy, which cuts a long row of evenly spaced notches called a comb. The same row turns up by accident when two mics pick up one source from different distances, as the comb filtering lesson explains. Step 8 in the lab gives you a feel for a single wide notch on a full mix, the static version of that hollow sound.

Notch filter starting settings

Set the frequency first, then adjust the Q while you listen.

Drum tuning varies so much that the ranges for rings are only a rough guide. The ring in the lab's drum loop sits at 297 Hz, and a big floor tom can ring below 100 Hz.

Common notch filter mistakes

Notching what only needed turning down

A notch at full depth takes the frequency out of the instrument along with the problem. If the result sounds thinner, or the drum loses its pitch as it does in step 6, swap the notch for a bell cut of a few decibels.

Missing by a few hertz

At a Q of 10 or more, a notch 10 Hz off frequency does almost nothing, as step 3 shows. If a notch doesn't seem to work, sweep it slowly across the problem, or widen it until it catches and then narrow it back down.

Going too wide

A notch with a Q of 1 or 2 takes out a whole range, and on music you hear that as a hollow, phasey hole. Once a notch has to be that wide the problem is a broad one, and a gentle bell cut will sound better.

Forgetting the harmonics

One notch at 50 Hz often leaves a buzz behind, because the rest of the hum sits at 100 Hz and above. Listen to what's left in a quiet gap and notch the harmonics too.

Stacking notches by habit

Each narrow cut is hard to hear on its own, yet six of them on one track can add up to a sound that's lost its body. Only notch what you could hear as a problem before you started looking.

Training your ears for notches

A notch is easiest to place when you can hear roughly where a problem sits before you touch the EQ. The quiz under the lab cuts a notch with a Q of 2 out of one of five frequency ranges and asks which, with the screens covered and the level matched. Pick the full mix or the pink noise before you start, because a pure tone has nothing around the notch to hear it against. Narrower notches are much harder to hear than this one. To hear a narrow notch by itself, switch the lab to pink noise, give band 1 a Q of 10 and drag it slowly across the screen. You'll hear a faint gap move through the hiss, and lowering the Q makes the gap easier to hear. Your best score out of eight is saved in your browser.

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