Synth Secrets
by Gordon Reid · Sound On Sound
Gordon Reid's 63-part Sound On Sound series (1999-2004) on the physics and practice of synthesis: harmonics, waveforms, filters, envelopes, modulation, FM and additive synthesis, and recreating real instruments such as strings, brass and drums. The most thorough free treatment of how synthesis works, from first principles to patch recipes. Nothing free matches its depth.
More resources on Synthesizers & Sound Synthesis
An Introduction to FM
FM is the hardest of the topic's synthesis methods to build intuition for. This is the authoritative free explanation from the institution where FM synthesis was invented, and it goes deeper than Synth Secrets' FM parts. Stanford CCRMA tutorial on frequency modulation synthesis: FM compared with phase modulation, Bessel-function sidebands, complex and cascade modulators, feedback FM, and FM voice and noise synthesis, with code examples.
The Theory and Technique of Electronic Music
The rigorous path for learners who want to understand and build synthesis algorithms, not just operate synths. University textbook on digital sound synthesis, free online with Pure Data example patches. Covers sinusoids, wavetables and sampling, modulation (ring, FM, waveshaping), envelopes, filters, delays and Fourier analysis, using trigonometry and complex numbers.
Patch & Tweak: Exploring Modular Synthesis
A 368-page illustrated guide to modular synthesis covering module types, signal flow, control voltage, patching techniques and tutorials, with 38 interviews with artists and designers and a foreword by Suzanne Ciani. Covers the modular part of the topic, which the other picks treat only lightly. Organized by function rather than by product, so it stays useful whatever system the reader owns.
Welsh's Synthesizer Cookbook
A 128-page patch reference that analyzes the harmonic spectra of real instruments and gives 102 synth-agnostic recipes for strings, brass, woodwinds, keys, percussion, basses, leads and pads, with chapters on oscillators, filters, envelopes and LFOs. Bridges theory and practice: explains why a given patch imitates an instrument by its harmonic content, and works on any subtractive synth.
Syntorial
Interactive ear-training course of 199 lessons that has you recreate patches by ear on a built-in subtractive synth. Video demos alternate with challenges; the first 22 lessons are free, and extra packs map skills to Serum and other synths.
Learning Synths
Interactive browser lessons from Ableton on subtractive synthesis, covering oscillators, waveforms, filters, envelopes, LFOs and modulation, each with a live playground for adjusting parameters by ear. Learners come away understanding how each building block shapes a sound and can start designing patches.