Two-source circular interference
Equation: z = sin(sqrt((x - 4)^2 + y^2) - t * 3) + sin(sqrt((x + 4)^2 + y^2) - t * 3)
Each term is a water drop. Between the sources, the path difference creates amplitude stripes — interference fringes.
Drop the detail level on slower machines: a dense mesh shows fringes clearly but is costly while animating.