Synth Filters

The filter is the most expressive control on a synth. It turns a harsh, buzzy oscillator into a warm bass, a soft pad or a squelchy acid line. It's also the secret behind every DJ-style build-up. This lesson explains how filters work and how to use them.

What is a Filter?

A filter removes (or reduces) part of the frequency range of a sound and lets the rest pass through. In Synth Basics you learned that subtractive synthesis starts with a bright wave. The filter is the "subtract" part: it decides which harmonics stay.

Every filter has three main controls:

Type: which part of the spectrum it removes.
Cutoff: the frequency where it starts working.
Resonance: how much it boosts the frequencies right at the cutoff.

Filter Types

Type	Keeps	Removes	Sounds like
Low-pass (LP, LPF)	Lows	Highs above the cutoff	Darker, warmer, muffled; "through a wall"
High-pass (HP, HPF)	Highs	Lows below the cutoff	Thinner, lighter; "on a small radio"
Band-pass (BP, BPF)	A band around the cutoff	Lows and highs	Telephone, megaphone, nasal
Notch (band-reject)	Everything else	A narrow band at the cutoff	Subtle, hollow, phaser-like when moved

Filter shapes (frequency → , level ↑)

Low-pass ‾‾‾‾‾‾‾╲___ lets lows through, cuts above cutoff High-pass ___╱‾‾‾‾‾‾‾ cuts below cutoff, lets highs through Band-pass ___╱‾╲_____ keeps one band Notch ‾‾‾‾╲╱‾‾‾‾‾ removes one band

The low-pass is by far the most used filter in synthesis. Most synth patches have it as the default.

Cutoff

The cutoff frequency is the point where the filter starts reducing the signal. It's measured in Hz. Technically it's where the level has dropped by about 3 dB.

Low-pass, cutoff at 18 kHz: almost no change.
Low-pass, cutoff at 2 kHz: noticeably warmer, softer.
Low-pass, cutoff at 300 Hz: dark and muffled, only the body remains.

Resonance (Q)

Resonance boosts a narrow band of frequencies right at the cutoff point. On some synths it's labeled Q, emphasis or peak.

Low resonance: a smooth, natural filter.
Medium resonance: a vocal, "wah"-like peak, especially when the cutoff moves.
High resonance: a sharp whistle. On many analog-style filters, very high resonance makes the filter self-oscillate: it produces a sine-like tone at the cutoff frequency even with no input.

Protect your ears and speakers

High resonance can create sudden, very loud peaks, especially when sweeping through low frequencies. Turn your volume down before you push resonance, and keep a limiter on your master while experimenting.

Resonance plus a moving cutoff is the sound of acid house. The Roland TB-303's squelchy bass came from its resonant filter being swept by its envelope and accent controls.

Slope (12 dB vs 24 dB)

A filter doesn't cut everything above the cutoff instantly. It reduces the level gradually. The slope says how steep that reduction is, measured in decibels per octave (dB/oct).

Low-pass at 1 kHz: how much each slope cuts

Slope at 2 kHz (1 octave up) at 4 kHz (2 octaves up) 6 dB/oct −6 dB −12 dB (1 pole: very gentle) 12 dB/oct −12 dB −24 dB (2 pole: smooth, open) 24 dB/oct −24 dB −48 dB (4 pole: steep, focused)

These are approximate figures for frequencies well past the cutoff. Each "pole" adds about 6 dB/oct.

12 dB/oct (2-pole) filters sound gentler and brighter. The Oberheim SEM is a classic example.
24 dB/oct (4-pole) filters sound fatter and more dramatic. The Moog ladder filter in the Minimoog is the classic example.

Most soft synths let you pick the slope from a menu, often labeled "LP12" and "LP24". The filters in this page's demos are 12 dB/oct.

Filter Envelope and Key Tracking

Filter envelope

A static filter sounds static. To make each note move, connect an envelope to the cutoff. Every time you press a key, the envelope pushes the cutoff up and then lets it fall back down. The envelope amount (or depth) sets how far it moves.

Classic pluck: filter envelope settings

Filter: Low-pass 24 dB, cutoff 300 Hz, resonance low-medium Env amount: high (opens the filter by several kHz) Attack: 0 ms → filter opens instantly Decay: 150–300 ms → closes quickly Sustain: 0 → stays closed while the key is held Release: 100–200 ms

Each note starts bright and quickly turns dark: "pew". A slower decay of 800 ms or more gives a bass that "blooms". Learn the four stages in detail in Envelopes (ADSR).

You can also move the cutoff with an LFO. A tempo-synced LFO on a low-pass filter creates the "wub-wub" wobble bass of dubstep. See LFOs & Modulation.

Key tracking

Key tracking (also called keyboard tracking or key follow) makes the cutoff follow the note you play. Higher notes open the filter more, lower notes close it.

0%: cutoff stays fixed. High notes sound duller relative to low notes, because more of their harmonics are above the cutoff.
100%: the cutoff moves exactly with pitch (up one octave per octave played). The tone stays consistent across the keyboard.
50%: a common middle ground for leads and basses.

Key tracking is especially useful on self-oscillating filters: at 100% the filter's whistle plays in tune with the keyboard.

Filter Sweeps in EDM

A filter sweep is a slow, automated move of the cutoff. It's one of the most-used tools for building and releasing energy in dance music.

Low-pass build-up: start a full loop with the cutoff around 300–500 Hz and sweep it up to 20 kHz over 8 or 16 bars. The track sounds like it's "coming closer". Filter house and French house built whole tracks on this move.
High-pass build-up: sweep a high-pass up from 20 Hz to around 300–1000 Hz before a drop. The bass and kick disappear, the track feels like it's lifting off, and the drop hits harder when the full low end returns.
Breakdown filter: low-pass the drums or a whole mix to around 1 kHz to make a breakdown feel distant.

Add a little resonance (around 3–6) to a sweep and it becomes more noticeable and "whooshy". Draw sweeps with automation, covered in Automation, and see more build-up tricks in Transitions & FX.

Sound Design Filters vs Mixing Filters

The same filter types appear in two different jobs.

	Sound design	Mixing
Where	Inside the synth, on each voice	On the track, as part of an EQ
Goal	Create character and movement	Clean up and make space
Resonance	Often used on purpose	Usually low, to stay transparent
Movement	Envelopes, LFOs, sweeps	Usually static
Example	Resonant LP24 with an envelope for an acid bass	High-pass at 100–200 Hz on pads to remove mud

In mixing, high-pass and low-pass filters are called high-cut and low-cut in some EQs. Note the reversed names: a low-cut is a high-pass filter. Filtering out lows that an instrument doesn't need is one of the most effective mixing moves, as you'll learn in EQ.

Producer tip

When a synth sounds harsh, reach for the filter cutoff before you reach for an EQ. Lowering the cutoff a little on the synth itself often sounds more natural than cutting highs later.

Try it in your DAW

Load the saw patch you saved in Synth Basics, or any synth with a single sawtooth [Ableton: Analog or Wavetable; FL Studio: 3x Osc into a Fruity Filter, or FLEX; Logic: Retro Synth].
Choose a 24 dB/oct low-pass filter. Set the cutoff to about 400 Hz and resonance low.
Set the filter envelope: attack 0 ms, decay 250 ms, sustain 0, and raise the envelope amount until each note "plucks".
Play a short 16th-note pattern, then raise the resonance to about halfway. Listen to the squelch.
Set key tracking to 50% and play high and low notes to compare the brightness.
On a drum loop, add a high-pass filter effect [Ableton: Auto Filter; FL Studio: Fruity Filter; Logic: Channel EQ or AutoFilter] and automate the cutoff from 20 Hz to 600 Hz over 8 bars, then back to 20 Hz on the drop.

A filter that keeps the lows and removes the highs is called a filter.

A 4-pole filter has a slope of dB per octave.

Making the cutoff follow the note you play is called key .

1. Which filter would you use to remove the low rumble from a pad?

Low-pass
High-pass
Notch at 10 kHz
No filter; turn the pad up

A high-pass filter removes frequencies below the cutoff, so it cleans up unnecessary lows.

2. What does resonance do?

Moves the cutoff automatically
Changes the filter type
Adds reverb to the filter
Boosts frequencies right around the cutoff

Resonance creates a peak at the cutoff. Very high settings can make the filter whistle or self-oscillate.

3. A low-pass filter at 1 kHz has a 24 dB/oct slope. Roughly how much is 2 kHz reduced?

About 24 dB
About 6 dB
Not at all
About 48 dB

2 kHz is one octave above the cutoff, so a 24 dB/oct slope reduces it by about 24 dB.

4. Which filter envelope gives a classic "pluck"?

Long attack, full sustain
No envelope amount at all
Instant attack, short decay, zero sustain
Long release only

The filter opens instantly and then closes quickly, so each note starts bright and turns dark.

5. Why do producers sweep a high-pass filter up before a drop?

To make the track louder
To change the key
To add more bass
To remove the low end so the drop hits harder when it returns

Taking the kick and bass away builds tension; bringing them back at the drop releases it.