Rhythm and pitch are the same phenomenon at different time scales

Infrasonic pulsations (below ~20 Hz) are perceived as rhythm; audio-frequency repetitions (above ~20 Hz) are perceived as pitch. These are the same physical phenomenon - a repeating waveform - experienced at different repetition rates. Cowell observed this in 1930; Stockhausen formalized it in his unified time theory (1957). In granular synthesis, this continuum is directly exploitable: increasing grain density continuously morphs a metrical rhythm into a pitched drone without any discontinuity in the signal. Conversely, a descending frequency glissando that crosses the 20 Hz threshold transitions from a pitched tone into a rhythmic pulse sequence.

Examples

In SuperCollider: Impulse.ar(freq) - sweep freq from 2 to 200 Hz to hear the rhythm-to-pitch transition in real time.

Assessment

Describe what you hear as grain density in a synchronous granular synthesizer increases from 2 grains/sec to 100 grains/sec. At what density does the percept cross from rhythm to pitch, and why?