Here's why:

* Sound waves are longitudinal waves, meaning the particles in the medium (like air) vibrate parallel to the direction the wave travels.

* Compression: As the wave travels, the particles are pushed together, creating areas of high pressure known as compressions.

* Rarefaction: The spaces between these compressed areas have lower pressure, and these are called rarefactions.

So, the squishing together of particles in the air as a sound wave passes is known as compression.