Sound occurs when vibrations travel through a medium. These vibrations, their propagation, and their interaction with the environment are dictated by laws of physics, and indirectly reflect underlying physical properties of things in the world. The sense of audition exists to measure sound and infer its causes in the world, so as to help organisms interact with the world around them. Audition is thus indirectly but intimately shaped by the physics of sound. This primer will review how physical principles influence sound, how these influences encode information about the world into sound signals, and how this information is used by the auditory system to infer the structure of the world.