Subtractive Synthesis: Carving Sound Out of Noise

The sculptor's way round

Additive synthesis builds a sound up from single plain tones. Subtractive goes the other way. You begin with a full block and cut pieces off.

Wikipedia calls it "a method of sound synthesis in which overtones of an audio signal are attenuated by a filter to alter the timbre of the sound". The same page lists what you can start from: bright, buzzy waveforms, "or white and pink noise". Classic synthesisers mostly use the buzzy waveforms. I only use the noise half, so that is what this piece is about.

Noise is every pitch at once

White noise is the hiss of a radio between stations. All pitches are sounding together and none stands out. On its own it is harsh and tiring. There is more on the different kinds in the colours of noise.

For carving, that fullness is the useful part. Everything you could want is already in there. The work is deciding what to throw away.

A filter is a tone control

You have used a filter already if you've ever turned a bass or treble knob. I use two kinds.

• Low-pass. It lets the low part through and removes the top. Noise through a low-pass becomes a rumble, wind behind a wall, surf a long way off. The lower you set it, the heavier and more muffled the result.
• Band-pass. It keeps a slice in the middle and removes both sides. A wide slice sounds like air moving through leaves. A narrow slice begins to whistle.

MDN's page on the browser's filter describes the width setting this way: it "controls the width of the frequency band. The greater the Q value, the smaller the frequency band."

Movement is what makes it sound alive

This was the part I missed at first. Noise through a filter that stays still sounds like a machine: a fan, an air conditioner. Wind and water are never still.

So the filter has to move. In my app each texture has its filter sliding slowly up and down, far too slowly to hear as a wobble. Each one drifts at its own slightly different speed, so two of them never move in step. Once that was in, the same hiss began to sound like weather.

Where you can hear it

Circles are the texture layer, and every texture is noise through a filter.

• In Ocean there is a deep, heavily muffled current and a brighter ripple higher up.
• In Space there is static: noise through a band that wanders slowly up and down the dial, like a radio between stations. Noise has no pitch, so static can never clash with the notes around it.

A detail I like: the app makes a couple of seconds of random numbers once, when the sound starts. Every wind, current and burst of static after that is the same noise through a different filter.

What it can't do

Filtered noise gives you texture. It doesn't give you notes. I tried to push it that far and wrote up what went wrong in can filtered noise have a pitch.

Sources

• Subtractive synthesis, Wikipedia
• BiquadFilterNode, MDN