Here's why:
* Compressional waves are waves where the particles in the medium vibrate parallel to the direction the wave travels.
* Compression is the region where the particles are squeezed together, creating high pressure.
* Rarefaction is the region where the particles are spread out, creating low pressure.
Think of it like a slinky: when you push a section of the slinky together (compression), you create a region where the coils are close together. When you pull the slinky apart (rarefaction), you create a region where the coils are spread out.
Sound waves are a great example of compressional waves. The compressions create the high pressure regions of a sound wave, and the rarefactions create the low pressure regions.
