context: A wave in which the medium moves in a direction perpendicular to the wave direction is called a transverse wave, and a wave that compresses and expands the medium in the direction of wave movement is called a longitudinal wave. Acoustic waves are longitudinal waves in which air compression and relaxation are formed in the same direction as the wave direction. As with the transverse wave, there is no movement of the medium (material) in the x-axis direction, and the movement (displacement) of the medium is different only in the x-axis direction.
question: What is the wave that compresses and expands the medium in the wave direction?
answers.text: ect