This book is a practical guide for researchers and advanced graduate students in biology and biophysics who need a quantitative understanding of acoustical systems such as hearing, sound production, and vibration detection in animals at the physiological level. It begins with an introduction to physical acoustics directed explicitly at their needs, covering the fundamental concepts and showing how they can be applied quantitatively to understand animals' auditory and sound-producing systems. Only after the relatively simple mechanical part of the system is explained does the author focus his attention on the underlying physiological processes.