Longitudinal vibrations
Vibrations in which particles move parallel to the direction the wave travels, as with sound in air.
What it means
Longitudinal vibrations are vibrations in which the particles of a medium move in the same direction as the wave itself travels. This creates alternating zones of higher and lower pressure, that is, compressions and rarefactions of the medium.
They contrast with transverse vibrations, in which particle motion is perpendicular to the direction of travel, as happens with the vibration of a plucked string.
Musical use
Sound travelling through air, water or other gases and liquids physically consists of longitudinal vibrations of the medium's molecules. When a string, an air column or a membrane vibrates, it sets the surrounding air into such longitudinal motion, which reaches the ear as sound waves.
Longitudinal vibrations should be distinguished from transverse vibrations, which occur for instance in the vibrating string itself or on a membrane surface before continuing as longitudinal waves through the air. In solid bodies, such as the wood of an instrument's body, both types of vibration can occur simultaneously.