The particles in air that spread farther apart after the compression of a wave passes by are called rarefactions.

Here's why:

* Sound Waves: Sound travels through air as a series of compressions and rarefactions.

* Compressions: When a sound wave passes through air, the air molecules are pushed together, creating an area of higher pressure called a compression.

* Rarefactions: As the compression moves on, the air molecules that were pushed together have room to spread out again, creating an area of lower pressure called a rarefaction.

So, the rarefaction is the region of lower density and pressure that follows a compression in a sound wave.