Filters in plain English
A filter is an EQ band that only cuts. A bell or a shelf turns a range of frequencies up or down by an amount you set. A high-pass or low-pass filter draws a line at one frequency, the cutoff, and removes what's on one side of it, and the further past the line a frequency is, the more of it goes.
The names describe what gets through. A high-pass filter lets the high frequencies pass, so it removes the lows, and a low-pass filter lets the lows pass and removes the highs. That feels backwards, because you reach for a filter to get rid of something, so plenty of EQs name them by what they remove instead: low cut for a high-pass and high cut for a low-pass. Logic's Channel EQ, for example, calls its two filters Low Cut and High Cut.
A filter doesn't stop dead at the cutoff. At the cutoff itself a standard filter is already 3 dB down, and past it the level keeps falling at a steady rate called the slope, measured in decibels per octave. A 12 dB per octave high-pass at 100 Hz leaves 50 Hz 12 dB quieter and 25 Hz 24 dB quieter. Twelve decibels is a long way from silence, so a gentle filter thins the low end out more than it deletes it.
The EQ lesson covers every type of band together. This page stays with the two filters: where to set them on different sounds, what slope and resonance change, and the ways they go wrong.
Reading the lab
This is the EQ lab from the other lessons in this section with only two bands: band 1 is a high-pass filter and band 2 is a low-pass. Both start switched off, and the On switch under the band buttons brings the selected one in. Their numbered points sit on the middle line of the big screen, because a filter has no gain to set. Drag a point sideways, or turn the Freq control, to move its cutoff. The pink line is what the filter does at every frequency, and the teal shape is the spectrum of the sound coming out of it.
Under the screen, Slope sets how steeply the selected filter cuts, from 12 to 48 dB per octave. Q adds a bump at the cutoff, which is covered under resonance further down, and the Gain control stays greyed out for both bands.
The Listen to this band button is the one to use most on this page. On the high-pass it plays only what sits below the cutoff, and on the low-pass only what sits above it, so either way you hear the part you're about to throw away. The Right now panel says how much of the sound's energy lives there, measured on the loop. A/B switches the filters off and matches the level, so a filtered sound doesn't lose the comparison just because it came out quieter.
You can swap our loops for your own sound. Upload a file, or press Record and play or sing something for eight seconds, and it runs through the lab in place of ours. Nothing leaves your device.
What does a high-pass filter do?
It clears out the bottom of a sound. Nearly every recording has energy down there that nobody meant to put in: the hum of traffic and air conditioning, and the thumps and pops from a hand on the mic stand or a breath hitting the capsule. Directional microphones also pick up extra bass when the source gets close to them, called the proximity effect. Most of this sits below about 80 Hz, which is under the low E of a guitar at 82 Hz and under the pitch of most speaking voices.
Taking it out makes the sound cleaner on systems that can play deep bass, and it gives you back headroom. Low frequencies carry a lot of energy even when you can barely hear them, so rumble on a vocal moves the meters, and any compressor after it, for no benefit at all.
Across a whole mix the effect adds up. Every track with a little low end it doesn't need puts that energy in the same place as the kick and the bass, and twenty tracks of it is mud. High-passing the parts that don't carry the bottom of the song leaves the space to the two that do.
The risk is going too far. Raise the cutoff into the notes of a sound and it loses its body: a vocal filtered at 400 Hz sounds thin and small, and an acoustic guitar loses the wood of its body. Because that happens gradually, the safest way to set a high-pass is by ear while the whole mix plays, which the step-by-step below walks through.
What does a low-pass filter do?
A low-pass filter takes off the top. In mixing it's usually the smaller job of the two, because most sounds have far less energy up there to lose. It can soften a harsh synth, take the fizz off a distorted guitar, remove hiss from an old sample or stop a pad's top end crowding the air around a vocal.
It also changes how far away something sounds. Air absorbs high frequencies more than low ones over distance, and walls block highs more than lows, which is why music from a party next door arrives as mostly bass. Your ears have learned that a dull sound is a distant one, so a low-pass is a quick way to push a part behind the lead without turning it down.
In electronic music the low-pass is an instrument too. Most subtractive synths are built around one, with an envelope or an LFO moving the cutoff for every note, which the filter envelopes lesson covers. On a whole track, a low-pass that starts nearly closed and opens over eight or sixteen bars is a standard way to build into a drop. The lab can't automate the cutoff, but you can do the same thing by hand: switch band 2 on, set it to 200 Hz while the loop plays and drag its point slowly to the right.
Cutoff, slope and resonance
Cutoff frequency
The cutoff is where the filter starts working, and on most EQs it's the point where the level has dropped by 3 dB. So a high-pass at 100 Hz is already 3 dB down at 100 Hz, and it only gets back to full level about an octave above. If you want 100 Hz untouched, set the cutoff lower. With a 12 dB per octave high-pass at 65 Hz, 100 Hz is down by less than 1 dB.
That 3 dB makes the biggest difference on sounds whose lowest notes are close to the filter. A bass guitar's low E is 41 Hz, so a 12 dB per octave high-pass at 40 Hz, which sounds like a safe rumble filter, takes almost 3 dB off that note.
Slope
The slope is how fast the level falls past the cutoff, in dB per octave. It comes in steps of 6, because each stage of a filter, called a pole, adds 6 dB per octave: a one-pole filter falls at 6 dB per octave, a two-pole at 12 and a four-pole at 24. Mixing EQs offer different choices. Ableton's EQ Eight has 12 and 48 dB per octave filters, while FabFilter Pro-Q 3 runs from 6 all the way to 96.
Gentle slopes of 6 and 12 dB per octave sound natural and leave a little of the low end behind, which often suits vocals and acoustic instruments. Steep slopes of 24 to 48 clear a range out more completely and keep the cutoff precise, which suits making room under a bass line or removing rumble right up to a kick's weight. What a steep slope costs is more phase shift near the cutoff, and a slight ring at the cutoff frequency on sharp sounds like drum hits.
The filter slopes lesson goes further into choosing between them.
Resonance
On a high-pass or low-pass, the Q control sets resonance, a bump in level just before the cutoff. At a Q of 0.71 there's no bump; that's the Butterworth shape, as flat as a filter can be before it starts to cut. Raise the Q and a peak grows. In the lab, a 24 dB per octave low-pass at 800 Hz with a Q of 2 lifts the frequencies around 770 Hz by about 6 dB before it falls away.
That peak is the sound of a synth filter: sweep a resonant low-pass up and down and the peak is the whistling tone you hear moving. The Minimoog's filter is a four-pole ladder at 24 dB per octave, and its resonance control goes high enough to make the filter whistle on its own. In mixing, a small bump on a high-pass can add a little weight just above the cutoff while the rumble below it goes, but most of the time mixing filters stay at 0.71. The filter resonance lesson has more.
Low cut, high cut and the other names
Different EQs, synths and DJ mixers use different labels for the same few filters. This table lines them up.
| Filter | Removes | Keeps | Also called |
|---|---|---|---|
| High-pass | Everything below the cutoff | Everything above it | Low cut, HPF, bass roll-off |
| Low-pass | Everything above the cutoff | Everything below it | High cut, LPF |
| Band-pass | Everything above and below a band | The band around its centre | BPF |
| Low shelf, cutting | A fixed amount below the corner | Everything, with the lows quieter | The bass knob on a hi-fi |
A band-pass is what you get with a high-pass and a low-pass on together. Set them to about 300 Hz and 3.4 kHz and you hear roughly the band an old landline carried, the telephone effect that the frequency ranges lesson plays.
The low shelf is the one people mix up with a low cut. A cutting shelf turns everything below its corner down by an amount you set, say 6 dB, and stays at that amount however low you go. A high-pass keeps cutting further the lower you go, so at 20 Hz it has removed far more. The low cut vs low shelf lesson compares the two side by side, and EQ vs filters covers the wider difference.
Where to set a high-pass filter on each instrument
These are places to start. Every recording is different, so sweep around each number with the mix playing and let the A/B decide.
| Source | High-pass | Low-pass | The reason |
|---|---|---|---|
| Lead vocal | 80 – 100 Hz | Usually none | Rumble, pops and proximity bass sit below the voice |
| Backing vocals | 120 – 200 Hz | 8 – 12 kHz | Leaner and darker puts them behind the lead |
| Acoustic guitar | 80 – 120 Hz | Usually none | Close mics pick up a lot of body boom |
| Electric guitar | 80 – 100 Hz | 6 – 10 kHz | Leaves the bottom to the bass and takes fizz off distortion |
| Snare | 60 – 80 Hz | Usually none | Kick bleed and rumble sit underneath it |
| Hi-hats | 200 – 400 Hz | Usually none | There's nothing useful down there |
| Pads and chords | 100 – 200 Hz | 6 – 12 kHz if they crowd the vocal | Room for the bass at the bottom and the vocal at the top |
| Reverb returns | 150 – 300 Hz | 6 – 10 kHz | A leaner, darker reverb stays out of the way |
| Kick and bass | 20 – 40 Hz, or none | Usually none | Only clear what's below the lowest note |
A chord part played low on a piano may need its high-pass down at 80 Hz, and a hi-hat sample with a lot of body can take one at 500 Hz without losing anything you'd miss.
How to set a high-pass filter by ear
The same method works for a low-pass, coming down from the top instead of up from the bottom.
- Play the whole mixJudge the filter with everything playing, at a normal level, on speakers or headphones that can reproduce bass.
- Start low with a gentle slopeSwitch the filter on at 20 or 30 Hz with 12 or 24 dB per octave, where it does nothing you can hear.
- Raise the cutoff slowlySweep up until the sound starts to lose its body. A vocal goes thin and a snare loses its thud.
- Bring it back downLower the cutoff until the body returns, then a little further, so the filter sits safely under the point where you heard it.
- Listen to what you removedSolo the part below the cutoff, which is what the lab's Listen to this band does. Rumble and thumps mean the filter is right; if you can hear the notes of the part, it's too high.
- Compare at the same levelSwitch the filter off and on. On most sounds you should hear the low end get cleaner with the part itself unchanged.
Do filters cause phase shift?
Yes, a little. A standard filter delays the frequencies near its cutoff slightly compared with the rest, which is called phase shift, and steeper slopes shift more. On one track on its own you won't hear it. It can matter when two signals of the same sound are mixed together and only one of them is filtered, such as a kick with two microphones on it, or a kick sample layered with a sub. The filtered copy no longer lines up with the other near the cutoff, and the two can partly cancel there.
The fix is usually to filter both copies the same way, or to check the blend with the filter on and off. A linear-phase EQ keeps every frequency lined up in time, but it adds delay, and on steep settings it can put a faint smear before sharp hits.
Common filter mistakes
Copying one cutoff onto every track
A high-pass at 200 Hz on every track takes the warmth out of the whole mix, because each part's low end adds up to the body of the song. Set each filter by ear, and leave it off sounds that carry the bottom.
Setting filters on small speakers
Most laptop and phone speakers can't play the bottom octave or two of the spectrum, so you can't hear what a high-pass is taking away, or what it's leaving in. Use headphones or monitors for low filters, and the lab's Listen to this band if you're unsure.
Filtering in solo
A soloed vocal with its high-pass at 150 Hz can sound thin and wrong, then sit perfectly once the bass and guitars are playing under it. Use solo to find roughly where the filter goes, then set the final cutoff with the mix playing.
Forgetting the filter is already cutting at its cutoff
Because the level is 3 dB down at the cutoff, a filter set right on a note is already turning it down. Keep a high-pass a little under the lowest note you want to keep, and a low-pass a little above the highest detail you want.
Leaving resonance up on a mixing filter
Some synth-style filter plug-ins start with the resonance raised. On a mix filter that bump boosts a narrow band right at the cutoff, so a high-pass meant to remove rumble can make the low end boomier. Check the Q is at 0.71, or the plug-in's equivalent of no resonance.
Training your ears for filters
The quiz under the lab puts a 24 dB per octave high-pass on the sound and asks where its cutoff is: 50, 100, 200 or 400 Hz. The screens are covered and the level is matched, so you are listening for the point where the weight drops away. Your best score out of eight is saved in your browser. Between rounds, pick the vocal, sweep band 1's cutoff up slowly and stop the moment it thins, then read the frequency off the Freq control. Do it a few times and your guess will start landing close to the number.
Questions people ask
Is a low cut the same as a high-pass filter?
Yes. They are one filter named from opposite ends: low cut says what it removes and high-pass says what it lets through. In the same way, a high cut is a low-pass filter.
What frequency should I set a high-pass filter to?
Start just under the lowest note the part plays, then raise it until the sound starts to thin and back off a little. Rough starting points are 80 to 100 Hz on vocals and guitars, 100 to 200 Hz on pads, 200 to 400 Hz on hi-hats and 20 to 40 Hz on kick and bass.
What does 12 dB per octave mean?
It's the slope of the filter. For every octave past the cutoff, the level drops another 12 dB, so a 12 dB per octave high-pass at 100 Hz leaves 50 Hz 12 dB down and 25 Hz 24 dB down. At 24 dB per octave those become 24 and 48 dB.
When should I use a low-pass filter?
When the top end of a sound gets in the way: hiss on an old sample, fizz on a distorted guitar, a harsh synth, or a pad crowding the air around a vocal. A low-pass also pushes a sound further back in the mix, and swept slowly across a whole track it builds tension into a drop.
Should I high-pass every track?
No. Filter the parts that have low end they don't need, such as vocals, guitars, hi-hats and pads, and set each filter by ear. Leave the kick and the bass alone, or filter only the rumble below their lowest notes, because they carry the bottom of the song.
What is a band-pass filter?
A filter that keeps one band of frequencies and removes everything above and below it, which is what you get with a high-pass and a low-pass on together. A band from about 300 Hz to 3.4 kHz sounds like an old telephone line.