When one part of a vibration pushes air particles together, they form a compression.

Here's why:

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

* Compressions: In a sound wave, the areas where the air particles are squeezed together are called compressions. This is where the pressure is higher than normal.

* Rarefactions: The areas where the air particles are spread apart are called rarefactions. This is where the pressure is lower than normal.

So, when a vibrating object pushes air particles together, it creates a region of higher pressure, or a compression.