In physics, sound is any phenomenon that involves the propagation of mechanical waves (whether audible or not), through a medium (fluid or solid) that is generating the vibratory movement of a body.

Humanly audible sound (between 20 and 20,000 hertz) consists of sound waves and acoustic waves that are produced when oscillations in air pressure are converted into mechanical waves in the human ear and perceived by the brain. Sound propagation is similar in fluids, where sound takes the form of pressure fluctuations. In solid bodies, sound propagation involves variations in the tensional state of the medium.

Sound propagation involves energy transport without matter transport, in the form of mechanical waves that propagate through an elastic solid, liquid or gaseous medium. Among the most common are air and water. Sound, sound waves, do not propagate in a vacuum, unlike electromagnetic waves. If the vibrations occur in the same direction in which the sound propagates, it is a longitudinal wave and if the vibrations are perpendicular to the direction of propagation, it is a transverse wave.

Acoustic phonetics focuses its interest especially on the sounds of speech: how they are generated, how they are perceived, and how they can be described graphically or quantitatively.