How are sounds made?
Sound is created by a vibrating object. For example, when a drum is struck, the flexible skin (sometimes called a membrane) of the drum vibrates. The compression and expansion of the air on either side of the vibrating membrane produces differences in air pressure. The pressure differences generate a sound wave that propagates outward from the drum surface.
The large, round, drum-shaped object about to be lowered into the ocean in the picture to the right is a low-frequency sound source. There is a second, drum-like surface on the side facing away from the camera. Pistons located inside the source push on the centers of the drum-like surfaces. Sound is generated by the in-and-out motion of the surfaces at the frequency at which the pistons push in-and-out.