Physical Acoustics, Volume XX: Ultrasonics of High-Tc and Other Unconventional Superconductors covers the many acoustic studies of the high-Tc superconductors. This book is composed of 10 chapters that include some unconventional superconducting systems, such as superfluid 3He, heavy Fermion superconductors, and magnetic re-entrant superconductors. The introductory chapter summarizes the results that have been observed in Bardeen, Cooper, and Schriefer superconductors as functions both of temperature and magnetic field. The subsequent chapters deal with the theoretical and experimental aspects of ultrasonic study of some unconventional superconductors. Considerable chapters are devoted to the measurements with sound waves on the sintered high-Tc superconducting systems. These chapters examine first the temperature and magnetic field dependence of the velocity and elastic constants in sintered high-Tc superconductors, as well as the sound absorption and dispersion measurements on single crystals of these superconductors. Discussions on the small-sample resonant ultrasound technique that uses thin piezoelectric films and the effect of oxygen on superconducting properties and the response of sound to these additions are also provided in these chapters. The concluding chapter presents a theoretical foundation for sound measurements in the superconducting state, emphasizing the effects of multigap structures and gas anisotropy on sound attenuation in the superconducting state of the cuprate superconductors. This volume will be of great benefit to researchers in the fields of electronics technology and in applied and engineering mechanics.