Subtractive synthesis makes a sound by starting with a bright, harmonically rich waveform and then removing parts of it until what is left sounds the way you want. It is the most common and the most classic way a synth makes sound, the method most people picture when they think of a synthesizer. Learn its short chain, an oscillator into a filter into an amplifier, and most synths stop looking like a wall of knobs.
The ideas here apply to any subtractive synth, in any DAW, or a browser instrument. Names and layouts differ, but the oscillator, the filter, the amplifier and the envelopes mean the same thing wherever you meet them.
Subtractive synthesis is a way of making sound where you begin with a waveform that is already full of harmonics and then take some of those harmonics away with a filter. The name comes from that removing: you subtract from a rich sound rather than build one up from nothing. It is the classic analog approach and by far the most common form of synthesis, the sound most people have in mind when they think of a synthesizer.
The raw waveform on its own is harsh and fairly static, with more in it than you need. What makes subtractive synthesis musical is everything that happens after the oscillator, as a filter carves the tone down and a set of controls shape it over the life of each note. If you are still getting a feel for how a synth is put together, our guide on what a synthesizer is walks through the same chain in more general terms.
Start rich, then carve down. The oscillator hands you a waveform with more harmonics than the finished sound needs. A filter takes the excess away, an amplifier decides how loud it is, and envelopes decide when all of that happens. Almost every control on a subtractive synth is really a question of which harmonics survive, and when.
Subtractive synthesis works by passing a raw waveform through a short, fixed chain of stages, each one shaping the sound in turn. The signal flows from an oscillator into a filter into an amplifier, while envelopes and an LFO move those stages over time. The classic analog version of this path is often written as VCO into VCF into VCA, short for voltage-controlled oscillator, filter and amplifier.
It starts at the oscillator, which generates the raw, harmonically rich waveform at the pitch of the note you play. Subtractive synthesis leans on the brighter waveforms, the sawtooth, the square and the pulse, because they contain plenty of harmonics for the filter to work on. A pure sine has almost nothing to subtract, so it is rarely the starting point here. For more on the shapes and how they are generated, see our guide on what an oscillator is.
Next the waveform passes through the filter, and this is the stage that gives the method its name. Most often it is a low-pass filter, which lets the low frequencies through and turns down everything above a point called the cutoff. Lowering the cutoff removes the bright upper harmonics and darkens the tone; raising it lets them back and opens the sound up. A resonance control emphasises a narrow band right at the cutoff for extra bite. Because the filter is where subtractive synthesis does most of its shaping, it has its own guide on what a filter is.
After the filter, the amplifier sets how loud the sound is. On its own it would only switch on and off, so an envelope shapes the level across the life of a note. The usual envelope has four stages known together as ADSR, for attack, decay, sustain and release, which decide how the sound fades in, settles, holds and fades out. The same envelope is often routed to the filter cutoff as well, so the tone brightens and darkens as a note plays. An LFO, a wave too slow to hear as a pitch, adds slower repeating movement to pitch, filter or volume. Together the envelopes and the LFO are what keep the sound from feeling frozen.
| Stage | What it does |
|---|---|
| Oscillator (VCO) | Generates the raw, harmonically rich waveform, a sawtooth, square or pulse, at the pitch of the note you play. This is the starting material, with more harmonics than the final sound needs. |
| Filter (VCF) | Subtracts harmonics from the raw wave, usually with a low-pass set by a cutoff and a resonance. This is the stage that gives subtractive synthesis its name. |
| Amplifier (VCA) | Sets how loud the sound is and shapes that level over the life of a note, so it can fade in, hold and fade out rather than only switch on. |
| Envelopes and LFO | Move the other stages over time. An ADSR envelope shapes the filter and the volume across a note; an LFO adds slower, repeating movement to pitch, filter or volume. |
The filter is the heart of subtractive synthesis, the stage that removes harmonics from the raw wave and, in doing so, gives the whole method its name. Everything before it makes a sound that is deliberately too bright and too full; the filter is what shapes that excess into something musical.
The most common filter is a low-pass, and its two main controls are the cutoff and the resonance. The cutoff sets the line above which harmonics are turned down, so sweeping it up and down is the single most recognisable gesture in synthesis: the sound opens and closes, brightens and darkens, as the cutoff moves. Resonance boosts a narrow band right at the cutoff, adding a vocal or whistling emphasis that grows more pronounced, and eventually squelchy, as you push it. Because the filter is usually being driven by an envelope, the brightness of a note can evolve on its own, opening sharply on the attack and closing again as the note decays.
Low-pass is the default, but it is not the only choice. A high-pass filter does the opposite, removing the lows and leaving the top, and a band-pass keeps only a slice in the middle. Reaching for a different filter type is common, but the low-pass sweep is the sound most people mean when they talk about a subtractive synth.
The name is literal. A subtractive synth starts with more sound than it needs and takes parts away, mostly with the filter. That is the opposite of additive synthesis, which starts from nothing and adds pure tones one at a time. If you remember only one thing from this page, remember that the filter is where the subtracting happens.
Subtractive synthesis is different from the other main methods in the direction it works: it starts with a full, harmonically rich sound and takes parts away, where the others build a sound up or create their harmonics in another way. Here is how it sits against the methods you are most likely to meet:
In practice these labels are not strict boxes. Most modern software synths mix several methods in one instrument, and a great many of them, whatever generates the raw tone, still finish with a filter. That last point is why subtractive is the best place to start: its oscillator, filter and envelope turn up in almost every synth, so learning it once carries over almost everywhere.
Subtractive synthesis clicks into place faster once you have heard a filter open and close while a sound is playing. Reading the theory gets you the map; hearing a change happen is what makes it stick.
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The oscillator, filter and envelope ideas on this page work by hand on any synth, which is rather the point of writing them down.
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