This book presents information on the general theory of critical sound propagation, which takes into account important nonasymptotic effects. Unsolved questions and future prospects in this field are outlined. The electronic and magnetic structure of pure and doped manganese clusters from density functional theory are also discussed using generalized gradient approximation for the exchange-correlation energy. Furthermore, a critical review of the applicability of the resonant ultrasound spectroscopy for determination of all independent elastic coefficients of anisotropic solids is presented. Grating and hybrid integration techniques and several types of hybrid integrated lasers are explained as well, with a variety of silica waveguide gratings on a Si-substrate. Finally, systems of free particles in a quantum theory bases on a Galois field are explored in detail. Aspects of the thermodynamics of black holes are also reviewed; the quantum entanglement between the degrees of freedom of a scalar field, traced inside the horizon, is particularly taken into account as the origin of black hole entropy.