Acoustic shaping of imperfect shells

Thin shells are exceptionally stiff for their weight but lose much of their strength to small geometric imperfections, still handled with empirical knockdown factors. Most experiments impose a single dimple, whereas real imperfections are distributed across the surface. We fabricate shells with realistic, distributed imperfections by casting liquid silicone on a compliant mold mounted on a speaker: a driven standing-wave mode redistributes the silicone, which gathers at the antinodes through acoustic streaming and crosslinks into a permanent thickness pattern. Frequency selects the number of bumps and amplitude sets their severity, giving a continuously tunable handle on imperfection sensitivity, and the method more broadly patterns soft materials by treating thickness as a design variable.