Caustics are caused by refraction or reflection, not diffraction. There are many analogies between simulating light and sound, but you take different shortcuts

Caustics are caused by refraction or reflection, not diffraction. There are many analogies between simulating light and sound, but you take different shortcuts to make those simulations fast. E.g. you don't simulate diffraction when simulating light (it's effect is neglible) and you discard small objects when simulating sound. Those differences exist mostly because visible light spectrum and hearable sound spectrum are so far apart.