As the source vibrates, it periodically presses against and pulls away from the adjacent medium. The results in regions of particle compression and expansion (rarefaction) in the medium. 

Compressions are regions of increased particle density. Rarefactions are regions of decreased particle density. 

The diagram illustrates compressions and rarefactions at 1 instant of time. In reality, the pattern moves quickly to the right as the sound travels through the medium. 

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