As shown in figure 15.6, a loudspeaker produces a sound wave traveling from left to right. When the speaker diaphragm bulges out, it pushes on the gas molecules in front of it, crowding them into their neighbors and producing a region of high molecular density and high pressure (a compression). The neighbor molecules, in turn, crowd their neighbors to the right, and so the high pressure region travels. Meanwhile, the speaker diaphragm flattens and then bulges inward, creating a low-pressure region (a rarefaction) that nearby air molecules rush into, moving right to left. Their places are taken by their neighbors, also moving right to left, and so on, so that the low-pressure region travels just as the high-pressure one does. A succession of alternating high- and low-pressure regions travels steadily from left to right. If you study the animation carefully, you will see that the dots representing individual air molecules vibrate back and forth (though they may appear to travel across the screen from left to right). Note that you can stop the animation and view it frame-by-frame using the controls at the bottom of the image.