FM side bands need not be harmonically related to the Carrier, so FM can make inharmonic (bell, metallic) tones

Chowning found that FM, like AM, generates side bands, but these additional components are not necessarily harmonically related to either the Carrier or the Modulator frequency. This is the property that sets FM apart from subtractive synthesis, where the overtones you shape are harmonics of a fundamental. Because the side bands sit at Carrier ± n×Modulator, their harmonicity depends on how the carrier and modulator frequencies relate: some settings place the components in a harmonic (integer) relationship giving a pitched, unified tone, while others place them inharmonically, giving clangorous bell-, gong- or metal-like timbres. The ability to produce inharmonic spectra easily is FM’s signature capability. (This article states the fact; it defers the carrier-to-modulator-ratio rules to a later part.)

Examples

A modulator whose frequency is a simple whole-number relation to the carrier yields a pitched, organ-like tone; a modulator at an unrelated frequency yields a metallic, un-pitched clang — the classic FM bell.

Assessment

State why FM can synthesise bell and metallic sounds that subtractive synthesis cannot easily make, referring to the harmonic relationship (or lack of it) among the side bands.