How Does a Synthesizer Work? Every Knob Explained

A synthesizer builds a sound from scratch with circuits or code. An oscillator makes a raw, buzzy wave, a filter trims its brightness, and an amplifier sets how loud it is from moment to moment, while envelopes and an LFO turn those knobs for you as you play. Learn that one chain and nearly any synth, hardware or app, stops looking like a wall of knobs.

How do synthesizers make sound? The signal path

Sound is air vibrating back and forth. The number of vibrations per second sets the pitch (A4, the A above middle C, vibrates 440 times a second, or 440 Hz), and the shape of each vibration sets the tone. A synthesizer creates that motion as an electrical voltage or a stream of numbers, and a speaker turns it back into moving air.

The classic design, called subtractive synthesis, is a chain. You start with a wave full of overtones, carve some of them away, then shape the volume:

When you press a key, the synth gets two signals: which note, and a “gate” that stays on while the key is down. The note sets the oscillator’s pitch. The gate starts and stops the envelopes, which is how the synth knows when a note begins and ends.

Two more parts carry no audio at all. Envelopes and LFOs are modulators: they move a knob on the stages above for you, faster or more precisely than your hand could. A modular synth connects them with patch cables; most synths and apps come prewired, and you set how much each one does.

FM and wavetable synths swap in a different oscillator. In FM, one oscillator wobbles the pitch of another hundreds of times a second, which gives the metallic tones of bells and electric pianos. A wavetable oscillator sweeps through a set of stored wave shapes. Many of these synths still feed a filter, amp, envelopes and effects, so learn this chain first. FM vs subtractive vs wavetable synthesis compares the methods.

Synth controls explained: oscillator, filter and amp

Oscillator

The oscillator makes the raw tone, and its main control picks the wave shape. Each shape has a different set of overtones, the quieter pitches stacked above the note you play:

• Sine: no overtones at all. Pure and round, good for sub bass and whistles.
• Triangle: a few soft odd overtones. Flute-like and gentle.
• Sawtooth: every overtone, bright and buzzy. The starting point for strings, brass and big leads.
• Square: only the odd overtones, hollow and woody. Think clarinet or an old game console.
• Pulse: a square with a narrower top. It gets thinner and more nasal as you narrow it.

Start with a sawtooth when you are learning. It has the most overtones, so the filter has something to remove. The synth waveforms guide compares the shapes in more detail.

The other oscillator controls set pitch. Octave jumps by twelve semitones; older synths label it in feet, borrowed from pipe organs, where 8’ is normal pitch, 16’ is an octave down and 4’ an octave up. Detune works in cents, and 100 cents make one semitone. Set a second oscillator 5 to 10 cents sharp of the first and the two drift in and out of phase, which is the thick, shimmering sound behind many “big” synth patches. Glide (also called portamento) makes the pitch slide from one note to the next instead of jumping; around 100 ms gives the classic sliding lead.

Most synths also have a noise source, a hiss with every frequency in it at once, used for breath, wind, snare rattle and the scratch at the start of a pluck. A mixer sets the level of each oscillator and of the noise.

Filter

A low-pass filter lets the low frequencies through and turns down everything above its cutoff point. Sweep Cutoff from the top down and the sound goes from bright to muffled, like music heard through a closing door.

Resonance boosts the frequencies right at the cutoff. A little adds a vocal “wow” when you sweep. A lot makes the filter whistle, and on many synths it will eventually ring on its own as a sine tone. Slope, if your synth offers it, sets how steeply the filter cuts: 12 dB per octave is gentle, 24 dB per octave is steeper and darker. Drive pushes the signal harder into the filter for grit.

High-pass is the mirror image: it cuts the lows, which is handy for thinning a pad so it stops fighting the bass. Band-pass keeps only a middle band and sounds like a telephone or a nasal horn. Many synths also have key tracking, which opens the filter as you play higher so top notes do not turn dull. The synth filter guide goes further into cutoff, resonance and slopes.

Amp

The amplifier sets the volume. On analog gear it is a VCA (voltage-controlled amplifier), so other parts of the synth can turn it up and down. The envelope driving it decides whether a note plucks or swells; without one, the sound would switch on and off with the key, often with a click.

What moves the sound: envelopes and the LFO

Envelopes

An envelope is a shape that plays out once each time you press a key. Almost every synth uses the four-stage ADSR:

• Attack: how long it takes to reach full level after the key goes down.
• Decay: how long it takes to fall from the peak to the sustain level.
• Sustain: the level held for as long as the key is down. This one is a level, so it has no time.
• Release: how long the sound takes to fade after you let go.

These starting points get you close to the classic shapes:

Some synths give the filter its own envelope; on others, one envelope drives both the amp and the filter. To get the “bwow” of a synth bass, set the cutoff low (around 20%), turn the filter envelope amount up, and give it a 300 ms decay with sustain at zero. Every note snaps open and closes again. Slow the filter envelope’s attack to around 100 ms and raise its sustain, and the same trick becomes a brassy swell.

The LFO

An LFO (low-frequency oscillator) is an oscillator running too slowly to hear as a pitch, usually somewhere between 0.1 Hz and 20 Hz. You hear its motion instead. Point it at a target, then set Rate (speed), Amount (depth) and Shape:

• Pitch: vibrato. Around 5 to 6 Hz with a small amount sounds like a singer.
• Filter cutoff: a wah or a wobble. Sync the rate to your tempo at eighth notes for a bass that pulses in time.
• Volume: tremolo, the pulsing sound of an old amp or a vibraphone.
• Pulse width: slow movement here gives the shifting, chorus-like shimmer of classic analog string pads.

A sine LFO sweeps smoothly. A square one jumps between two values, which turns vibrato into a trill. A random shape (often called sample and hold) jumps to a new value on every cycle, which on pitch gives the stepped bleeps of a sci-fi computer.

Effects: chorus, echo and reverb

Effects sit at the end of the chain, after the amp, so an echo or a reverb tail keeps ringing after the note itself has stopped.

Chorus copies the sound, delays the copy by around 20 milliseconds, and wobbles that delay with a slow LFO so the copy drifts slightly out of tune with the original. The copy makes the sound wider and thicker; many lush string pads and electric pianos owe their size to it. If a patch feels thin, try chorus before adding another oscillator.

Delay, or echo, repeats the sound. Time sets the gap between repeats, feedback sets how many repeats you hear, and mix sets how loud they are. At 120 BPM a dotted-eighth delay is 375 ms, a setting that makes a simple pluck line sound busy and rhythmic.

Reverb imitates a room: thousands of reflections blurring into a tail. A decay under a second sounds like a small room, around 2 seconds like a concert hall, and 5 seconds or more turns a pad into an ambient wash. For most parts a mix of 15 to 25% is plenty. Set it last, once the sound already works dry.

How to learn a synth, one stage at a time

The same dozen or so knobs repeat on almost every synth. The skill that takes time is hearing which one a sound needs. Stay with one synth while you learn; the skills transfer. On an iPhone, Gelato Synth lays the chain out one page per part (Osc A, Osc B, FM, Noise, Env, Filter, LFO and Space), with a live oscilloscope of the wave you hear and a blank Init sound to start from. Its pads only play notes from the scale you choose, so your attention can stay on the knobs. Then work in this order:

1. Start from an init patch. Most synths have one: a single sawtooth, filter wide open, sustain at 100%. Change one control at a time and say out loud what changed.
2. Learn the filter next. Sweep the cutoff with resonance at zero, then again at halfway. Then add the filter envelope from the bass recipe above.
3. Build three sounds from scratch. A pluck, a pad and a bass, using the ADSR table as a starting point. Save each one.
4. Add one LFO. Make a gentle vibrato, then a tempo-synced filter wobble.
5. Take presets apart. Load a preset you like and switch pieces off one at a time: reverb to zero, LFO amount to zero, filter wide open. Each step shows you what that part was adding.
6. Train your ear. Try to rebuild a sound you heard, by ear. Syntorial, from Audible Genius, is a paid course built around exactly this, with a free demo of its first 22 lessons. Ableton’s Learning Synths site lets you play a synth in your browser while it explains each part.