The investigation of the effects of the laws of quantum mechanics on the propagation and absorption of sound. At the present stage of development of physical science, quantum mechanics is the most fundamental theory of physical phenomena. However, for many applications in the everyday world, a sufficiently accurate description of nature is provided by classical mechanics. Quantum acoustics refers to acoustic experiments that are carried out under conditions such that the results can be understood only in terms of quantum theory. As a general tendency, quantum effects become more important in acoustic experiments that are performed with higher-frequency sound waves or that are carried out at lower temperatures.