If the vibrating body is moving in backward direction, it forms a low-pressure zone known as:

The particles in a gas become more spread out as rarefaction occurs. This term may be used in the context of sound waves. A sound wave travelling through air is made up of zones of higher and lower density that alternate. Rarefactions are places with a lower density. 

Rarefaction is the opposite of compression. Rarefaction means the reduction of density of the object.

When a vibrating object goes backwards, it forms a low-pressure area known as rarefaction.