Propagation of sound in air happens, when a sound source, usually a vibrating object mechanically affected, makes the air molecules next to its surface oscillate around their equilibrium position in the stagnant air.
Due to the elasticity of the air the sound is propagated through the air by successive oscillations of the neighbouring elastic air particles, and we have a steadily progressing sound wave. If the sound source is vibrating harmonically, the fluctuations of the air particles will be harmonic too, and this in any arbitrary point. In a sound wave in air the particles will oscillate in the same direction as the wave front propagates (=longitudinal wave), and this creates areas with higher respectively lower pressure compared to the atmospheric pressure. Therefore sound waves can be treated as pressure waves.

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