2. Overall principle

The photoacoustic (PA) effect, in both gaseous and non-gaseous matter, was discovered in the 19th century and, in 1880, Bell briefly reported the accidental discovery of the PA effect in solids. During his experiments with the photophone, Bell discovered that it was possible to obtain a directly audible signal. He demonstrated that the PA effect in solids depended on the absorption of light, and that the strength of the acoustic signal depended on the power absorbed. Bell thus correctly deduced the intrinsic optical dependence of absorption on the PA effect. In addition to studying the PA effect in solids, Bell also worked on the PA effect in liquids and gases. He observed that...