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.
Audible Sound. Usually sound is used to mean sound which can be perceived by the human ear, i.e., sound refers to audible sound unless otherwise classified. A reasonably standard definition of audible sound is that it is a pressure wave with frequency between 20 Hz and 20,000 Hz and with an intensity above the standard threshold of hearing.