Here's why:

* Sound waves are longitudinal waves: This means the particles of the medium (air in this case) vibrate parallel to the direction the wave travels.

* Compression: In a sound wave, the areas where the air particles are bunched together are called compressions. These areas have higher pressure and density than the surrounding air.

* Rarefaction: The areas where the particles are spread out are called rarefactions. These areas have lower pressure and density than the surrounding air.

So, the bunching up of air particles in a sound wave is called a compression.