Surface density maps log | A₂/A₀ | (left panels, red = high, blue = low amplitude); Re A₂/A₀ (right panels, red = positive, blue = negative amplitude at the x-axis) for B-0, B-1, B-1m, and B-1-110 runs (top to bottom), reflecting the interference between the bar-like perturbation in the centre, random perturbations, and tidally induced waves. The time period is 0.6 <t< 1.8 Gyr.