Linear wave equation on a 2D membrane, first-order form:
∂v/∂t = ∂s_x/∂x + ∂s_y/∂y (Newton's 2nd Law,
transverse displacement velocity v with membrane stresses s_x, s_y)
∂s_x/∂t = ∂v/∂x, ∂s_y/∂t = ∂v/∂y (stress
rate vs strain velocity)
Double-slit IC: two narrow strips of v = A on the right edge
(j ∈ [80, 85) and [120, 125)) at the moment Reset is pressed. The two
wavefronts propagate leftward and superpose, generating the
characteristic interference pattern.
Toggle periodic source to drive the slits at frequency ω
(v_slit = A·sin(ωt)) — produces a sustained interference pattern
rather than a one-shot impulse.
Tiny damping factor (0.9999 per substep) keeps the wave bounded
inside the box. dt = 0.10·h is well within the CFL bound dt ≤ h/√2
for 2D wave propagation; slide down for sharper fronts, slide up
for faster propagation (with risk of CFL instability).