:This page is about the physical process. For the ecological technique, see Rarefaction (ecology) Rarefaction is the reduction of a medium's density, or the opposite of compression. A natural example of this 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. Another natural example of rarefaction is in the layers of our atmosphere. Because what constitutes our atmosphere… (More on Rarefaction)