Definition
Additive synthesis builds a sound by summing sine waves (partials), each with its own frequency, amplitude and envelope. It is based on Fourier's theorem: every periodic sound is a sum of sine waves at multiples of the fundamental frequency. It is the opposite of subtractive: instead of removing harmonics from a rich wave, you add them one by one.
Waveforms in additive terms
- Sawtooth: all harmonics, amplitude 1/n (the second at half, the third at a third…).
- Square: odd harmonics only, amplitude 1/n.
- Triangle: odd only, amplitude 1/n² (falls off much faster): almost sine-like sound.
- Bells and metals: inharmonic partials (not integer multiples), each with a different decay.
Example: A2 = 110 Hz → harmonics at 220, 330, 440, 550 Hz… The fifth harmonic (550 Hz) is roughly a C#5: this is why in a rich timbre you can “hear” the major third.
The Hammond organ: mechanical additive
The Hammond's drawbars are an additive synthesizer: each drawbar adds a sine wave (tonewheel) at a fixed pitch: 16' (sub-octave), 5 1/3' (fifth above), 8' (fundamental), 4' (octave), 2 2/3' (twelfth), 2' (fifteenth), 1 3/5' (seventeenth, third), 1 1/3' (nineteenth, fifth), 1' (twenty-second). Registration “888000000” = full jazz/gospel sound; “800000888” = sharp. The B-3 V from the V Collection 9 emulates it.
Additive and resynthesis instruments
- Kawai K5 (1987) and Synclavier (partial timbre): commercial pioneers.
- Image-Line Harmor (in FL Studio): additive with resynthesis from images and audio; filters, phasers and “blur” applied to the partials.
- Native Instruments Razor, Arturia Pigments (harmonic engine), Loom (AIR).
- Resynthesis: you analyze a real sound (FFT), extract the partials and rebuild it, then being able to modify them (time-stretch without changing pitch, morphing between two sounds, “freezing” a spectrum).
Advantages and limits
- Total control of the spectrum; “impossible” filters (for example removing only the even harmonics, creating a comb).
- Spectral morphs, extreme stretching, crystalline pads without strange phase.
- Limit: for realistic sounds you need dozens or hundreds of partials, each with its own envelope: without macros or resynthesis it is hard to program.
- Transients and noise are the weak point: they are added with a noise generator or a sample.
Basic recipes
- Glass pad: harmonics 1, 2, 3, 5, 8 with decreasing amplitudes, slight random detune of the high partials, slow envelopes, long reverb.
- Organ: drawbars 888000000 + percussion on the third harmonic (2 2/3') with fast decay, Leslie.
- “Clean” bass for difficult mixes: only the first 4–6 harmonics, with the fundamental controlled: it is audible on small speakers without excessive sub.
- Bell: partials at ratios 1; 2.76; 5.40; 8.93 (typical of a metal bar, indicative values) with faster decay for the high ones.
Example in A minor
In Harmor, pad on Am(add9) – Fmaj7 – C(add9) – Gsus4: even partials attenuated by 6 dB (more “hollow” timbre), Harmor's filter slowly modulated, 3-voice unison, phaser on the partials. Organ bass (B-3 V) with 16' and 8' drawbars for the line A – F – C – G.
Exercise
- Build a sawtooth by hand by summing the first 8 harmonics with amplitudes 1/n in an additive synth (or in Sytrus): hear how it changes as you add each harmonic.
- Resynthesize in Harmor one of your own vocal notes and play it as a pad.
- Program three drawbar registrations on the B-3 V and describe the character of each.
Connections
“The harmonic series and timbre”, “Subtractive synthesis”, “FM synthesis”, “Granular synthesis”, “Pad design”.