High-pass and low-pass filters

By Dan Murtagh · Updated

Short answer: a high-pass filter lets the highs through and takes out everything below a frequency you choose. A low-pass filter does the opposite: it keeps the lows and takes out the top. In mixing, the high-pass is the one you’ll use most, to clear rumble and low end a part doesn’t need.

The names trip everyone up. A high-pass filter is also called a low cut, and a low-pass filter a high cut: same filters, named for what they remove instead of what they let through.

A full mix through a high-pass or a low-pass filter at 12 dB per octave. Flip Filter off and Filter on, and slide the frequency to sweep it. Both sides play at the same loudness, so you hear what’s taken away rather than the drop in level.

What each one does

  • High-pass (low cut): everything above the frequency passes; below it, the sound falls away more and more the lower you go. Use it to clear rumble, handling noise, and low end that isn’t the part’s job.
  • Low-pass (high cut): everything below the frequency passes; above it, the sound falls away. Use it to take hiss, fizz or string noise off a part, or to push something back: less top sounds further away.

Two numbers set a filter. The frequency (or cutoff) is where it starts to bite: at that point it has already taken about 3 dB off. The slope is how fast it falls away past that point, in dB per octave. At 12 dB per octave, one octave below a 100 Hz high-pass (50 Hz) is about 12 dB down. At 24 dB per octave it’s about 24 dB down. Gentle slopes sound more natural; steep ones clear a range out more firmly.

A high-pass filter on vocals

A voice has little useful sound in the lowest octaves. What’s down there is often rumble: traffic, air conditioning, a mic stand being bumped, or the boom a directional mic adds when the singer is close to it (the proximity effect). A high-pass clears it without touching the voice.

A common way to set it: solo the vocal, raise the frequency until the voice starts to thin out, then back it off a little. Then check it with the band playing. Where it lands depends on the singer, the mic and the song, and it often ends up somewhere around 80 to 150 Hz. Set it by ear, not by the number.

A vocal on its own, then with the band. This vocal has almost nothing below 100 Hz, so a filter there changes nothing you can hear, and many clean vocal recordings are like that. Raise it to 250 Hz and 500 Hz and listen for the voice getting smaller. Then press With the band: the same move is often harder to hear in the mix.

Should you high-pass everything?

Some engineers filter almost every track except the kick and bass; others filter only what needs it. Both work. Two things to watch: filters stacked on many tracks can leave the whole mix thin, and every filter shifts the timing (phase) of the sound around its frequency a little. Filter what has low end it doesn’t need, and listen to the mix, not just the track.

Where a low-pass helps

  • Taking the edge off: the fizz on a distorted guitar, the click and string noise on a bass.
  • Depth: high frequencies fade over distance, so taking some top off a part pushes it back in the mix.
  • Effects: a low-pass on a reverb or delay return keeps the tail from competing with the dry sound.

Questions

What’s the difference between a high-pass and a low-pass filter?

A high-pass filter keeps the highs and takes out the lows below its frequency. A low-pass filter keeps the lows and takes out the highs above its frequency. Use both on one track and you have a band-pass: only the middle gets through.

Is a low cut the same as a high-pass filter?

Yes. A low cut and a high-pass filter are the same thing, and a high cut is a low-pass filter. The first name says what it removes, the second what it lets through.

Where should I set a high-pass filter on vocals?

By ear: solo the vocal, raise the frequency until the voice starts to thin, then back it off a little and check it in the mix. It often ends up around 80 to 150 Hz, but it depends on the singer, the mic and the song.

What slope should I use?

12 dB per octave is gentle and sounds natural. 18 or 24 dB per octave clears a range more firmly. If you can hear the filter itself, try a gentler slope or a lower frequency.

Does a high-pass filter change the sound above its frequency?

Near its frequency, yes: at the cutoff it has already taken about 3 dB off, and it shifts the phase a little around there. Well above it, the sound is untouched.

Train it

Take away what’s in the way.

Cuts Only trains the instinct behind a filter. The sound is dull, thin or buried and you want to boost, but you only get a low cut and a high cut, both shelves. Cut what’s left over until it sounds like the original, at matched levels.

Train it in Cuts Only

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