This is unintuitive: the laser makes air heat up and expand. This is how the acoustic waves are created. But this alome is not enough for audible sound. Higher

This is unintuitive: the laser makes air heat up and expand. This is how the acoustic waves are created. But this alome is not enough for audible sound. Higher amplitude is required. They get it by "piling up" a wavefront (in colloquial terms) by moving the laser exactly as fast as the wave is moving away. This heats up more air and the additional expansion adds to the already traveling wavefront.