Perceptual quality depends on the group character of spectral evolution, not the precise amplitude curve of each partial

Chowning’s conclusion addresses a potential concern: FM gives the composer control over global spectral bandwidth (via the index envelope) but not precise individual partial amplitudes, which are governed by Bessel function curves. His finding is that this limitation does not matter perceptually. The ‘precise amplitude curve for each frequency component in a complex dynamic spectrum is not nearly as important, perceptually, as the general character of evolution of the components as a group.’ This is a deep perceptual principle with broad implications: synthesis methods that control gross spectral trajectory are perceptually adequate even without per-partial precision. It also justifies the economy of FM — two oscillators plus an envelope produce perceptually convincing timbres that would require dozens of envelope generators in additive synthesis.

Examples

A brass simulation whose individual harmonics each follow individual Bessel curves sounds convincingly brass-like, even though no individual harmonic follows a ‘correct’ acoustic trumpet partial envelope.

Assessment

An additive synthesist argues that FM cannot accurately reproduce a trumpet because it doesn’t control each partial independently. Refute this using Chowning’s perceptual evidence.