Raising a modulator from LFO rate into the audio range turns vibrato into a new timbre

Frequency modulation at a low modulator rate is heard as vibrato — a periodic pitch wobble (roughly 5–14 Hz), a cyclical squeezing and stretching of the whole waveform. As the modulator’s frequency rises toward and past the carrier frequency, the modulation stops bending the overall pitch and instead distorts the shape of the individual carrier cycles; beyond the hearing threshold the listener stops hearing pitch variation and starts hearing a new, complex timbre. This crossover is the whole conceptual basis of FM synthesis: the identical patch that gives vibrato at a few hertz generates a rich new spectrum once the modulator runs at audio rate, with no change except the modulator’s speed — ‘FM is simply very fast vibrato.’ Recognising this threshold is the key step from LFO modulation to FM synthesis.

Examples

A 5–6 Hz modulator on a carrier is heard as pitch vibrato. Push the same modulator up to a few hundred hertz (e.g. 110–220 Hz) and the wobble is replaced by a static, harmonically complex timbre — new spectral content, not pitch movement.

Assessment

Describe what you hear, perceptually and in the waveform, as a modulator on a fixed carrier is swept from ~6 Hz up past the carrier frequency, and explain why the high-rate result no longer sounds like vibrato.