Compression — When a vibrating object advances, it pushes the particles of air in layers in front of it. So the particles of air in these layers gets closer to each other i.e., air of these layers gets compressed.
Rarefaction — When a vibrating object, moves left, it pushes back the layers of air near it towards it’s left and thus produces a space near it towards it’s right side and thus produces a layer of very low pressure on right side. The air layers on the right side of the strip expand in this region thus forming the rarefied air layers. This region of low pressure is called the rarefaction R.
