Ships create noise from their propellers, motors and gears. 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). The rotation of the propellers creates bands of noise at more or less constant frequencies that are proportional to the rate of rotation of the propeller. The noise created by these rotations, called “blade-rate lines,” can help distinguish between different size ships and even particular ships. Low frequency noise generated by ships contributes significantly to the amount of low-frequency ambient noise in the ocean, particularly in regions with heavy ship traffic. In fact, because of the increase in propeller-driven vessels, low-frequency ambient noise has increased 10-15 dB in the past 50 years.Tugs with barges typically produce less near-surface sound than other ships. This is not because they are quieter, but rather the propellers of a typical tug boat are recessed to reduce propeller cavitation and to protect the propeller from damage in case of grounding. With the propeller in this position, the sound rays from the propellers are blocked by the ship’s hull. Thus, propeller noise cannot be heard ahead of the tug and barge. This effect is called acoustical shadowing.