Additive synthesis is inherently inexpressive because changing one partial parameter has little perceptible effect

In FM or subtractive synthesis, a single parameter (modulation index, filter cutoff) globally reshapes the timbre and is musically responsive. In additive synthesis the perceptible timbre is determined by the combined interaction of dozens of partial amplitudes, phases, and frequencies; changing one partial slightly produces a minimal, largely inaudible effect. To achieve a musically meaningful timbral change, many parameters must be changed simultaneously. This makes real-time performance control of additive synthesis very difficult — the performer cannot intuitively steer the sound with a few knobs.

Examples

Turning a filter cutoff on a subtractive patch sweeps the entire spectrum; turning one partial amplitude on an additive patch of equivalent complexity changes just a sliver of the spectrum. Only moving all partials coherently (e.g. via morphing) yields a comparable expressive result.

Assessment

Compare the expressive controllability of an additive synth patch with an FM patch using the same number of controls, and explain why the additive synth requires a ‘morph’ paradigm to be performable.