Longitudinal waves (sound waves):
A sound wave reaching the Ear,
that demonstrates the compression and rare fractions
Waves are vibratory disturbances travelling in a medium.
The key to understand the wave motion is that the phase is different for the different vibrating particles because they start vibrating at different period of time. The ones farther from the source osciallating later than the ones closer to the source. Longitudinal waves have two important features. Compressions are regions where particles are squished together. Rarefactions are regions where particles are spread apart.
Longitudinal waves hold all of the potential energy associated with the transverse waves, and they carry the forward momentum in the direction of propagation associated with transverse traveling waves. Longitudinal waves in a string typically travel an order of magnitude faster than transverse waves on the same string and are only weakly affected by changes in string tension.
Longitudinal waves are often neglected, e.g., in violin acoustics, because they couple inefficiently to the body through the bridge, and because they are “out of tune” anyway. However, there exist stringed instruments, such as the Finnish Kantele, in which longitudinal waves are too important to neglect. In the piano, longitudinal waves are quite audible.