Sound waves can be thought of as fluctuations in the pressure of a medium through which they propagate. Since the pressure also makes the medium's particles vibrate along its direction of motion, the waves can be modeled as the displacement of the medium's particles from their mean position.
Sound waves are longitudinal in most fluids because fluids cannot sustain any lateral pressure. In solids, however, shear forces help in propagating the disturbance in the lateral direction as well. Hence, in solids, sound waves are both longitudinal and transverse.
The medium absorbs a fraction of the energy propagated through the sound wave due to its viscosity. In each compression, a fraction of the energy is converted as heat, and in each rarefaction, a smaller amount of this returns to the wave. Hence, with time, a wave propagating through a medium loses its energy as random thermal energy to the surrounding medium.
The human ears collect sound waves in the air and channel them to the brain, which creates the perception of hearing. They are sensitive to a gamut of sound wave frequencies, from about 20 Hz to 20,000 Hz. This range is called the audible range.
This text is adapted from Openstax, University Physics Volume 1, Section 17.1: Sound Waves.
