A spotlight with an adjustable beam width sweeps along the shore. Its light represents long ocean waves that are born when short waves break at the coast. If the spotlight is narrow, it shines far and clear. If it is wide, its light smears, and the fastest oscillations cancel each other out.

The surf zone is not a point but a strip, and it moves. When a wave rolls onto the shore, the source of long waves shifts toward the beach. When it retreats, the source moves back. This motion compresses waves heading to the shore and stretches those going out to sea, much like an ambulance siren changes pitch as it approaches and recedes.

Numerical experiments on beaches with different slopes showed that the wider the surf zone, the stronger the damping of fast oscillations. This knowledge helps improve flood forecasts and protect harbor structures from resonance.