Sound occurs as a result of the back-and-forth vibration of sound waves travelling across a medium. A sound wave is sometimes referred to as a pressure wave because it contains repeating patterns of high-pressure and low-pressure regions.
The regions of high pressure in a sound wave are compressions, whereas the regions of low pressure are called rarefactions. A wavelength is a measurement of the distance compressions and rarefactions travel in one complete wave cycle. Sound waves are easily detectable by human ears and can be created using devices such as Fourier synthesizers. The detection of high- and low-pressure sound waves by the eardrum creates the perception of sound at varying frequencies.
The velocity of a sound wave decreases in liquids and increases in the presence of gas. When a sound wave encounters a surface, it is reflected and absorbed depending on the material of the surface. It is important to note that sound waves travelling through air are considered longitudinal waves as opposed to transverse waves. According to The Physics Room, the former occurs when motion of the particles is parallel and anti-parallel to the direction of the energy transport, while the latter refers to motion occurring perpendicular to the direction of energy transfer.