Model: driven damped Klein-Gordon / sine-Gordon equation utt = c²∇²u - w0²u - βg(u) - γut on a 128x128 grid (leapfrog, absorbing sponge or reflecting walls). Waves below the cutoff sqrt(w0²+β) are evanescent. In a nonlinear medium two tones generate harmonics and sum/difference products (intermodulation). Frequencies in cycles per time step, model units.