Book excerpt: This book describes modern techniques for reducing the level of airborne noise through the introduction of sound radiated by additional secondary sources, bringing together the results of contemporary research in this area. It is the combination of the physical properties of sound fields and modern digital signal processing technology that has made the active control of sound a practical proposition in a number of important applications. The book covers both these aspects of the subject, initially at a fundamental level, and then in detail in later chapters. The structure of the book is such that it should be suitable for both those seeking a basic understanding of the subject and as a reference for researchers in the field. A key feature of the work is the unified presentation of material from the two disciplines of acoustics and signal processing.