How are compressions and rarefactions formed in a sound wave traveling through the air medium?
Particles in the air cluster together and spread out alternately
Sound in air is a longitudinal mechanical wave. Air particles oscillate back and forth about their mean positions along the direction in which the wave travels, without moving along with the wave as a whole.
When particles come close together, local density and pressure rise and a compression is formed. When the same particles move apart in the next half cycle, density and pressure fall and a rarefaction is formed. These regions alternate along the wave.
Spiral motion describes some transverse waves, permanent sticking would stop propagation, and equal spacing would mean no wave at all. Hence, the correct answer is Particles in the air cluster together and spread out alternately.
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