Overview
Filters shape sound by controlling which frequencies are allowed through and which are reduced or removed.
In synthesis and audio production, filters are used to:
- remove unwanted frequencies
- shape the tone of a sound
- create movement and expression
Most filters work by defining a cutoff frequency, which determines where the filtering begins.
What Does a Filter Do?
A filter processes audio by reducing certain parts of the frequency spectrum.
Think of it like this:
👉 Instead of changing the source sound, a filter sculpts what you hear
Key Terms
- Cutoff Frequency
The point at which the filter begins to affect the signal - Frequency Spectrum
The range of low to high frequencies in a sound - Attenuation
The reduction of signal level
Low-pass Filter (LPF)
A low-pass filter allows low frequencies to pass through and reduces high frequencies.
How It Works
- Frequencies below the cutoff → pass through
- Frequencies above the cutoff → reduced
In the example below, a bright sawtooth wave is passed through a low-pass filter. Lower the cutoff to make the sound darker, then raise the resonance to hear the frequencies around the cutoff become more pronounced.
Notice that resonance does not simply make the whole sound louder. It boosts a narrow area around the cutoff frequency, which is why high resonance can make a filter sound more sharp or vocal.
How It Sounds
- Warmer
- Duller
- Less bright
As you lower the cutoff:
👉 the sound becomes darker and more muted
Common Uses
- Removing harsh high frequencies
- Shaping bass sounds
- Creating filter sweeps
- Smoothing pads and synths
High-pass Filter (HPF)
A high-pass filter does the opposite of a low-pass filter.
How It Works
- Frequencies above the cutoff → pass through
- Frequencies below the cutoff → reduced
A high-pass filter does the opposite of a low-pass filter. Instead of removing high frequencies, it reduces the lower part of the signal and lets higher frequencies pass through. Use the explorer below to hear how the sound becomes thinner and brighter as the cutoff rises.
How It Sounds
- Thinner
- Cleaner
- Less bass-heavy
Common Uses
- Removing rumble and low-end noise
- Cleaning up recordings
- Making space in a mix
- Tightening drums or vocals
Band-pass Filter (BPF)
A band-pass filter allows a specific range of frequencies to pass through.
How It Works
- Frequencies within a band → pass through
- Frequencies outside → reduced
A band-pass filter allows only a selected range of frequencies to pass through. This makes it useful for narrowing a sound, isolating a specific part of the spectrum, or creating more focused, nasal, or radio-like tones.
How It Sounds
- Focused
- Narrow
- Telephone / radio-like
Common Uses
- Special effects
- Sound design
- Isolating frequency ranges
- Creating movement in synth patches
Cutoff Frequency in Practice
The cutoff frequency is the main control you’ll use.
- Lower cutoff → fewer high frequencies (darker sound)
- Higher cutoff → more high frequencies (brighter sound)
Example
Start with a sawtooth wave:
- No filter → bright, full sound
- Low-pass filter applied → smoother, less harsh
- Sweep the cutoff → evolving tone
Filter Slope (Briefly)
Filters don’t remove frequencies instantly.
They reduce them gradually, depending on the slope.
Common slopes:
- 12 dB per octave
- 24 dB per octave
A steeper slope = more aggressive filtering.
Why Filters Matter
Filters are one of the main tools for shaping sound.
They allow you to:
- control tone
- remove unwanted frequencies
- create movement over time
In synthesis:
👉 oscillators generate sound
👉 filters shape it
Common Mistakes
- Over-filtering and removing too much frequency content
- Using filters without listening carefully
- Ignoring how filters affect the balance of a mix
Related Topics
- Oscillators (Sine, Saw, Square, Noise)
- Frequency, Amplitude and Waveforms
- What is a Synthesizer?
- Signal Flow Explained
- Envelopes (ADSR)
This article is part of the Music Production Reference Library, organised into volumes covering sound, theory, production, and release.
