Subtractive Synthesis Explained: How Synthesisers Really Work

Every synth looks different.

Different panels, different names on the knobs, different amounts of extras.

But underneath, nearly all of them are the same three boxes in a row.

I used to teach this by sitting people in front of a Roland Juno-106. If you want to understand subtractive synthesis, it’s one of the best places to start.

Once you see the signal path, you can walk up to almost any subtractive synth – hardware or plugin – and know roughly where everything is before you’ve touched it.

The path is this:

Tone generation > tone shaping > volume shaping

Or in synth language: oscillator > filter > amplifier.

That's it.

That's the whole machine.

Everything else is decoration.

Start With Too Much

The oscillator is where the sound starts.

It generates a raw waveform – a repeating shape – and each shape has its own character, decided by its harmonic content.

Sine – just the fundamental. Pure, round, almost flute-like. Nothing to remove.

Triangle – a sine with a few harmonics sprinkled on. Slightly less polite.

Square – odd harmonics only. Hollow, woody, a bit clarinet.

Sawtooth – every harmonic. Rich, brassy, aggressive. The classic synth rasp.

Noise – every frequency at once. Wind, snares, hats.

If you've read the post on even and odd harmonics, this is the same idea. The waveform is just a recipe for harmonics.

Most oscillators also have a range control marked in footage – 16', 8', 4' – borrowed from organ pipe lengths.

It's an octave switch.

Sine wave at 16’ gives you a sub bass. The same sine at 4’ is heading towards a flute.

The oscillator deliberately gives you more harmonics than you need.

A raw saw wave is not a finished sound. It’s raw material.

Carve, Don't Add

The filter is where the sound becomes your own.

It works like an EQ with attitude.

Two controls do most of the work: cutoff and resonance.

Cutoff decides where the filter starts removing frequencies. On a low-pass filter – by far the most common – everything above the cutoff gets shaved away. Pull it down and the sound goes dull and dark. Open it up and the brightness comes back in.

Resonance boosts the frequencies right around the cutoff point. Push it and the filter starts to sing – that whistling, vocal edge. Push it further on the right filter and it howls.

If you want to hear resonance at full stretch, listen to a TB-303. That squelch that built acid house is the resonance being pushed while the cutoff moves.

There are other flavours – high-pass, band-pass, notch – but they all do the same job from different directions: they take harmonics away.

And that's why this whole method is called subtractive synthesis.

You start with a wave that has too many harmonics and you chip bits off until what's left is the sound you wanted.

Other forms of synthesis create sound in different ways.

Subtractive sculpts by removal.

Start rich. Remove. Listen.

Shape How It Lives and Dies

So far we have a tone with a colour. But it just sits there, droning.

The third stage – the amplifier, driven by an envelope – decides how the sound behaves over time. How it arrives, how it settles, how it leaves.

Think about real instruments for a second.

Hit a woodblock: instant full volume, gone almost as fast.

Hold an organ key: instant full volume, stays there until you let go.

Strike a piano key: instant start, then a long slow fade even while you hold it.

All of those are volume envelopes, and a four-stage ADSR generator can fake nearly all of them.

I've written a full post on ADSR, so here's just the map:

attack is how long the sound takes to reach full volume, decay is the fall to a resting level, sustain is that level – a level, not a time, the thing that confuses everyone – and release is the fade after you let go.

Percussive sound? Fast attack, fast decay, no sustain.

Pad? Slow attack, high sustain, long release.

The Trick That Makes It Come Alive

An envelope doesn't just have to control volume.

Route it to the filter cutoff instead, and the filter opens and closes in the shape of the envelope – brighter as the note blooms, darker as it dies.

That's the classic filter sweep. That’s why a good synth bass sounds like it’s moving rather than standing still.

Many subtractive synths give you two envelopes for exactly this reason – one for volume, one for the filter – so the loudness and brightness of a note can live separate lives.

Learn One Synth Properly

Back then, you saved up for one keyboard, and because it cost you everything, you learned every fader on it. Every sound got replaced with your own.

Now you can download fifty soft synths in an evening – and know none of them.

Pick one. Choose a straightforward subtractive synth: one oscillator, one filter, a couple of envelopes, nothing hidden.

Turn every knob and try to make ten sounds you’d actually use.

The signal path you learn on one synth is the same path on all of them.

Final Thought

Subtractive synthesis isn't fifty modules and a wall of cables.

It's three decisions, made in order.

What's the raw material? What do I take away? How does it live and die?

Oscillator. Filter. Amplifier.

Everything else is decoration.