MOTORS-The noise created by the motor is continuous and caused by the mini-explosions that occur as fuel burns rapidly inside the engine cylinders and by the rotating gears and shafts. Sound is also created by bubbles formed by the rotating propellers and, to a lesser extent, by the wake of waves produced by the movement of the ship. As the ship moves and the propellers rotate, many bubbles form in the water. The formation of these bubbles is known as cavitation. The breaking of these bubbles creates a loud acoustic sound. Cavitation noise is directly related to the speed of the boat. The faster the propeller rotates, the more cavitation. The breaking bubbles produce sound over a range of frequencies, and at high speeds, these frequencies can be as high as 20,000 Hz. On the other extreme, a large ship with slowly turning propellers can generate very low frequencies (below 10 Hz).