Book excerpt: Loudness is the primary psychological correlate of intensity. When the intensity of a sound increases, loudness increases. However, there exists no simple one-to-one correspondence between loudness and intensity; loudness can be changed by modifying the frequency or the duration of the sound, or by adding background sounds. Loudness also changes with the listener’s cognitive state. Loudness provides a basic reference for graduate students, consultants, clinicians, and researchers with a focus on recent discoveries. The book begins with an overview of the conceptual thinking related to the study of loudness, addresses issues related to its measurement, and later discusses the physiological effects of loud sounds, reaction times and electrophysiological measures that correlate with loudness. Loudness in the laboratory, loudness of steady-state sounds and the loudness of time-varying sounds are also covered, as are hearing loss and models.