cat size · room temp · cat mood
A real cat's purr comes from the larynx snapping open and shut 20-30 times a second on both the in- and out-breath — that repetition is the sound. This is a live model of it: a low tone gets re-triggered on a precise schedule at that rate, with a breath-noise layer riding along. The three sliders below bend every part of that live, while it's playing — no presets, no samples.
Size sets the purr's register — bigger cat, lower chest resonance and rumble pitch, and a slightly slower purr rate. Room temp sets comfort: it peaks in a warm, cozy band (cats like it warmer than humans do), and falls off toward either extreme — cold adds a shivery jitter to the purr's timing, heat swaps a chunk of it for open-mouth breath noise (panting). Mood runs from a grumpy, irregular, distorted growl undertone, through a steady textbook purr, to a loud, richly articulated purr with the occasional rising trilling chirp layered on top.
Under the hood there's no audio-rate LFO — each purr pulse is scheduled individually a fraction of a second ahead, the same lookahead-scheduler trick synths use for tempo, just running at a rate (20-30 pulses a second) fast enough that you feel it as texture instead of counting it as rhythm.