Sound describes a set of longitudinal mechanical waves. Mechanical waves must travel through a medium that can be deformed by the wave. Sound can therefore travel through solids, liquids, and gases, but not through a vacuum, such as interstellar space. The wave is transmitted as particles in one deformed area of the medium strike other particles in a neighboring area, causing it to deform as well. If the first particles can strike lots of other particles, the wave will travel faster. Since solids are more dense than liquids, sound travels slower in liquids than in solids and even slower in gases.
Human hearing can detect waves that fall in the audible range, which includes frequencies from 20 to 20000Hz. Frequencies slower than 20Hz are known as infrasonic, while those above 20000Hz are called ultrasonic. The concept of pitch also describes frequency. Although pitch does not have a precise mathematical relationship to frequency, a high pitch corresponds to a high frequency, while a low pitch corresponds to a low frequency.
Sound can be produced either through the striking of a solid object, causing a vibration, or by a vibration of a column of air. This latter production of sound is also called an acoustic vibration.
