The physical phenomenon of sound is defined to be a disturbance of matter that is transmitted from its source outward. For example a vibrating string produces a sound wave. As it moves it transfers energy into the air ], that energy is mostly thermal created by turbulence. A small part of that energy goes into compression and expanding the surrounding air, creating slightly higher and lower pressures. These compressions move out as longitude pressure waves having the same frequency as the string. So these vibrations are creating a sound which is coming from a sound wave.
Sound waves in air and most fluids are longitudinal, because fluids have almost no shear strength. Sound waves can be both transverse and longitudinal. So lets go back to the vibrating sting. The vibrations of the string create enough sound waves that make the string have some sort of sound coming from it.
Another example would be an ear drum. An eardrum absorbs energy of the wave . The energy is converted to thermal energy due to the viscosity of the air. The heat transfer reduces the disturbance into random thermal motions. The compression and rarefaction significant depends on how far apart they are, so it depends on the wave length.
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