context: Waves that move in a direction perpendicular to the wave direction are called transverse waves, and waves that compress and expand the medium in the direction of wave propagation are called longitudinal waves. Acoustic waves are longitudinal waves in which air compression and relaxation are formed in the same direction as the wave direction. Like the water wave, which is a shear wave, there is no movement of the medium in the x-axis direction, and the movement (displacement) of the medium differs only in the x-axis direction.
question: What is the wave that compresses and expands the medium in the wave direction?
answers.text: ect