Impulses fuse into a continuous tone at about 20 impulses per second

When a series of brief acoustic impulses is presented at increasing rates, the ear transitions from hearing separate events (fission) to hearing a continuous tone (fusion). The fusion threshold occurs at approximately 20 impulses per second, corresponding to the infrasonic/audio frequency boundary. Below this rate, forward masking between impulses is incomplete and each event is perceived individually. Above it, masking spans the entire gap between impulses and the ear integrates them into a continuous tone. This perceptual mechanism is the basis for granular synthesis: sufficiently dense grain streams produce pitched or continuous textures even when built from individually audible particles.

Examples

In synchronous granular synthesis, raising density from 10 to 30 grains/sec transforms a metrical rhythm into a continuous drone. The fusion threshold is the pivot.

Assessment

At what approximate grain density (grains per second) does granular synthesis transition from producing rhythm to producing a continuous tone? Explain the perceptual mechanism responsible.