How Does a Bowl-Shaped Piezo Ceramic Focus Ultrasound?

Audience: engineers and technical readers who understand ultrasonics but want a clearer physical explanation.
Goal: explain focusing using geometry-based reasoning rather than heavy mathematics, with honest limitations.

The idea in one sentence

A custom spherical-cap piezo ceramic focuses ultrasound because it launches a curved wavefront whose timing is naturally “pre-aligned” by geometry. Many parts of the radiating surface send pressure cycles that arrive at a specific region in space close enough in phase that they add coherently. Outside that region, the arrivals are less aligned, so they add less efficiently and the field looks more spread.

That is the whole story. Everything else is the fine print that makes it accurate, and useful.