v. perked, perkÂ·ing, perks. v.intr. 1. To stick up or jut out: dogs' ears that perk. 2. To carry oneself in a lively and jaunty manner.
However instead of crests and troughs, longitudinal waves have compressions and rarefactions. Definition 1: Compression. A compression is a region in a longitudinal wave where the particles are closest together. A rarefaction is a region in a longitudinal wave where the particles are furthest apart. As seen in Figure 1, there are regions where the medium is compressed and other regions where the medium is spread out in a longitudinal wave. The region where the medium is compressed is known as a compression and the region where the medium is spread out is known as a rarefaction.
An example of rarefaction is as a phase in a sound wave or phonon. Half of a sound wave is made up of the compression of the medium, and the other half is the decompression or rarefaction of the medium.