Propagation of SoundWhen sound propagates through a medium, such as air, and reaches our ears, we perceive sound. Sound is physical as well as perceptual. The physical aspect of sound is vibration. There is also the perceptual aspect of sound, including loudness, pitch, and timber. A sound propagates as a compression wave and it contains both dense and rarefied regions of air. In the dense regions, the air particles are compressed, while in the rarefied regions, the air particles are expanded.
The following is a schematic figure of the propagation of a wave.
In this figure, each particle moves back and forth within the short distance around the equilibrium. Whatever the disturbance, the wave propagates towards the right. When the direction of the moving particles corresponds to the direction of the propagation, the wave is called longitudinal. When the direction of the moving particles is perpendicular to the direction of the propagation, the wave is called transverse.
Speaks, C. E., Introduction to Sound: Acoustics for the Hearing and Speech Sciences, Singular Publishing, San Diego, CA, 1999.