In a previous chapter on vision, the description of light energy was placed upon a spectrum. At one end were the long wavelengths of infrared and red, and at the other end were the short wavelengths of violet and ultraviolet light. In the middle of this spectrum was human vision extending from 390 (red) to 700 (violet) nm. In a very similar fashion, sound can be placed along a scale, but with frequency instead of wavelength as the measurement. Sound waves are pressure waves and can be visualized as a slinky stretched out on a flat surface. If one were to just casually lay out a slinky on a desk or table, the rings on the slinky would likely be randomly spaced out along the toy. Some of the rings would be jammed closer together, and others would be pulled or spaced apart. The same type of distribution occurs with sound waves. When the rings of the slinky are closer together, this would represent what is called a compression in the sound wave. This is a higher pressure point on the sound wave. A rarefaction occurs where the slinky’s rings are spaced further apart and is associated with a lower pressure. In a different view of the slinky, the compression of rings can be considered the “crest” of a wave (despite the height of the slinky remaining constant), and the rarefaction is the “trough” of the wave. Just like the slinky, the sound wave is a series of crests and troughs traveling through space.