Book excerpt: Noise is usually defined as unwanted sound: loud music from a neighbor, the honk of a taxicab, the roar of a supersonic jet. But as Garret Keizer illustrates in this probing examination, noise is as much about what we want as about what we seek to avoid. It has been a byproduct of human striving since ancient times even as it has become a significant cause of disease in our own. At heart, noise provides a key for understanding some of our most pressing issues, from social inequality to climate change. In a journey that leads us from the Tanzanian veldt to the streets of New York, Keizer deftly explores the political ramifications of noise, America's central role in a loud world, and the environmental sustainability of a quieter one. The result is a deeply satisfying book -- one guaranteed to change how we hear the world, and how we measure our own personal volume within it.