In this guide
A low-pass filter lets lower frequencies through while progressively attenuating higher frequencies. A high-pass filter does the opposite. Neither filter automatically separates a source from a mix.
A low-pass filter in audio allows lower frequencies to pass while progressively reducing higher frequencies. It is also called a high-cut filter. Moving the cutoff lower reduces more high-frequency content; moving it higher leaves more of the upper range. The change is gradual, not a perfectly sharp wall.
A low-pass filter can shape a sound or reduce some high-frequency hiss, but it also affects wanted detail in that range. If a voice becomes muffled or consonants disappear, compare a higher cutoff or a different approach.
Low-pass vs. high-pass filter
| Filter | What passes more readily | What is attenuated | Example |
|---|---|---|---|
| Low-pass / high-cut | Lower frequencies | Frequencies increasingly above the cutoff | Darken a sound or test whether high-frequency hiss can be reduced. |
| High-pass / low-cut | Higher frequencies | Frequencies increasingly below the cutoff | Test whether low rumble can be reduced without thinning speech. |
Remember the name by what the filter passes: low-pass lets lows through; high-pass lets highs through. Shure’s audio-systems guide describes both as progressive cuts and explains slope.
Cutoff frequency and slope
The cutoff frequency, measured in hertz (Hz), is a reference point around which the filter transitions from passing to attenuating. Its precise definition and response depend on the filter design. A common convention uses the frequency at which the response has fallen by 3 dB relative to the passband; do not assume every plug-in uses the same convention.
The slope, often shown in decibels per octave (dB/octave), describes how rapidly attenuation increases as frequency moves beyond the transition. A 24 dB/octave slope generally rolls off more steeply than a 12 dB/octave slope, but exact response near cutoff depends on the filter. An octave doubles or halves frequency: 4 kHz to 8 kHz is one octave.
For example, if a low-pass filter is set around 4 kHz, listen to both the voice and the background above that region. A steeper slope can reduce more high-frequency energy, but may also remove useful consonants, cymbals or ambience. The cutoff is not an on/off switch, and this example is not a recommended universal setting.
When might you use a low-pass filter?
| Goal | What to try | What to check |
|---|---|---|
| Reduce high-frequency hiss | Compare a gentle high-cut on a representative passage. | Hiss may remain, while consonants and air can become dull. |
| Create a darker effect | Lower the cutoff and adjust the transition in an editor with a low-pass filter. | Bypass the effect and check whether the sound still serves the scene or mix. |
| Route low frequencies to a subwoofer | Use the crossover controls designed for the speaker system. | Follow the device manuals and listen at the listening position; crossover settings depend on the system. |
| Reduce low rumble | Consider a high-pass filter instead. | Listen for lost voice body or instrument fundamentals. |
A low-pass filter and a high shelf are not identical. A shelf boosts or cuts a broad region and levels off; a low-pass filter continues to attenuate more strongly beyond its transition. The equalizer guide compares filter shapes and EQ controls.
How to test a low-pass filter
- Keep the original. Select a short passage with both the unwanted sound and important speech or music.
- Choose a low-pass or high-cut filter in an editor that offers one. Start with a high cutoff and a moderate slope, then move the cutoff while listening to the actual recording.
- Compare bypassed and processed audio. Match playback level as closely as practical; listen for missing consonants, instrument attacks, brightness and background hiss.
- Check the whole passage. A setting that helps a pause can damage a word or cymbal hit. Adjust or undo if the tradeoff is worse than the original.
- Export and listen again. Keep a working copy and check the finished file, especially if it must stay aligned with a video.
To reduce low rumble instead, the local high-pass filter can test a low-frequency cut on a supported short MP3 or WAV. It is not a low-pass filter. The separate four-band equalizer changes fixed frequency regions and is not a configurable low-pass filter.
Can a low-pass filter remove noise or a voice?
It can reduce energy in high frequencies, including some hiss, but cannot identify which source produced that energy. Speech and music span overlapping ranges; removing enough high end to change a voice can also make instruments or the voice itself dull. A filter will not reliably separate a singer from a finished song.
Choose the method based on the actual problem: a stable tone may suit a narrow notch, steady hiss may be worth a noise-profile test, and a distinct competing source may warrant a source-isolation comparison. The cleanup-method guide explains these tradeoffs and the limits of each method.
Common questions
Does a low-pass filter remove bass?
No. It passes lower frequencies and attenuates higher ones. To reduce low-frequency rumble, compare a high-pass filter.
Is low-pass the same as high-cut?
These names commonly describe the same filter direction: lower frequencies pass and higher frequencies are progressively attenuated.
What is the best cutoff frequency?
There is no universal setting. The source, unwanted sound, filter slope and intended effect all matter. Listen to a representative passage and check what wanted detail is lost.
Can filtering fix clipped audio?
It can change the tone of distortion, but it does not reliably restore peaks already clipped during capture. Compare a clean alternate take when available.