Frequency State / Music Production Reference Library · Volume 1
Filters select or attenuate parts of the frequency spectrum. Cutoff frequency, slope and resonance work together, so a filter sweep is more than a simple change from bright to dark.
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:
Filters can reduce unwanted frequency ranges, change the character of an oscillator and create movement when their settings change over time. The musical effect depends as much on the source spectrum as on the filter itself.
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 marks the approximate boundary of a filter’s transition region. Attenuation is the reduction in level applied to parts of the spectrum, while the spectrum itself describes the frequencies present in the signal.
Low-pass Filter (LPF)
A low-pass filter allows low frequencies to pass through and reduces high frequencies.
How It Works
A low-pass filter generally allows frequencies below its cutoff to pass more readily while progressively attenuating frequencies above it. Real slopes are gradual rather than perfectly vertical.
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
As upper harmonics are reduced, a rich waveform can become warmer or darker. The result depends on the input and may be subtle if the original sound contains little high-frequency energy.
As you lower the cutoff:
👉 the sound becomes darker and more muted
Common Uses
Low-pass filtering can soften pads, tame harsh upper harmonics, shape bass patches or create a rising sweep by opening the cutoff during a phrase.
High-pass Filter (HPF)
A high-pass filter does the opposite of a low-pass filter.
How It Works
A high-pass filter does the reverse, reducing low frequencies while retaining more of the upper spectrum. It can remove unwanted rumble but also weaken the fundamental of a pitched instrument if set too high.
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
The result can sound thinner or less bass-heavy. Whether that is desirable depends on how much low-frequency information the musical part needs.
Common Uses
High-pass filtering is useful for managing rumble, cleaning up low-end buildup and helping instruments occupy appropriate spectral ranges in a mix. It should be applied by listening, not by assigning the same cutoff to every track.
Band-pass Filter (BPF)
A band-pass filter allows a specific range of frequencies to pass through.
How It Works
A band-pass filter passes a selected region between lower and upper attenuation areas. Its bandwidth and resonance influence how narrow or pronounced the remaining frequencies sound.
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
A narrow band-pass setting often produces an intentionally focused, telephone-like or resonant sound, while a wider band can retain much more of the original timbre.
Common Uses
Band-pass filtering is common in sound-design effects, moving synth textures and frequency-isolation exercises, particularly when cutoff is automated or modulated.
Cutoff Frequency in Practice
The cutoff frequency is the main control you’ll use.
Moving a low-pass cutoff down generally removes progressively more high-frequency information; opening it brings the harmonics back. The exact change depends on slope, resonance and the spectrum of the incoming sound.
Example
Start with a sawtooth wave:
Try a bright sawtooth wave with no filtering, then lower the cutoff gradually. Compare the difference with a sine wave: the sine offers very few upper harmonics to remove until the filter begins attenuating its fundamental.
Filter Slope (Briefly)
Filters don’t remove frequencies instantly.
They reduce them gradually, depending on the slope.
Common slopes:
A slope of 12 dB per octave attenuates out-of-band frequencies more gradually than a 24 dB-per-octave slope, though the measured behaviour depends on the particular filter. Neither is automatically superior; they create different responses.
A steeper slope = more aggressive filtering.
Why Filters Matter
Filters are one of the main tools for shaping sound.
They allow you to:
Understanding cutoff, slope and resonance lets you make deliberate decisions about tone, unwanted energy and movement rather than adjusting filter controls arbitrarily.
In synthesis:
👉 oscillators generate sound
👉 filters shape it
Common Mistakes
Too much filtering can remove musical fundamentals or essential texture. Recheck the filtered sound in the whole mix as well as in solo, and compare it with the bypassed signal at a similar listening level.
Put it into practice
For a practical test, choose a harmonically rich sawtooth oscillator and gradually lower a low-pass filter’s cutoff. Then add resonance and listen for emphasis near the cutoff. Compare the effect on a sine wave, which has no upper harmonics for the filter to remove until its fundamental enters the attenuation region. For the source material, read Oscillators Explained.
Related Topics
To continue, explore Oscillators, ADSR Envelopes and Modulation.
This article is part of the Music Production Reference Library, organised into volumes covering sound, theory, production, and release.