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.
1 compression An area of increased pressure. In longitudinal waves, the particles in areas of compression are closer together than on average.  electromagnetic radiation Energy travelling as waves in the form of changing electrical and magnetic fields.