Contrasting a submarine on a sunny beach with a submarine in the deep, dark ocean, explain the sources of submarine noise and the various techniques for reducing it. Emphasize the importance of developing technologies to keep submarines stealthy.
People are having fun on a sunny beach. The sounds of children running around, adults chatting, and a loudspeaker playing music fill the space. The beachgoers are basking in the warm summer sun, playing ball in the sand or reading a book under a parasol. Far away from them, in the deep, dark waters, an object swims motionless. It darts around as if searching for something, or crouches still as if afraid of something approaching. It is a ghost of the sea, a submarine.
A submarine is literally a military vessel that can travel underwater, filling its tanks with seawater to adjust its weight, deflating and inflating as it dives and rises. Submarines are used to travel underwater to scout for enemies and attack targets, and were first used in combat in the American Revolutionary War in the 1700s, and have evolved rapidly through two world wars to reach their current state.
The most important characteristic a submarine needs to fulfill its role is stealth, or the ability to remain undetected by enemy ships. In the depths of the ocean, where objects cannot be judged visually, submarines rely on sound waves to detect their surroundings, and any noise generated by a submarine is immediately visible to nearby enemy ships. Therefore, in response to the development of sound detection technology called sonar, technologies to reduce noise have been developed, but there are many sources of noise, and even with modern technology, it is still a challenge to completely eliminate noise.
There are four main sources of noise in submarines: mechanical noise, compartment noise, hydrodynamic noise, and propeller noise. This can be easily understood by imagining a submarine in motion. In order for the submarine to move, a lot of machinery has to work, which generates mechanical noise, and the crew living inside the submarine also inevitably generates noise, which is compartment noise. As the submarine moves through the water, the water around the hull creates hydrodynamic noise due to the irregular motion of the water, and the propellers spinning to move forward also contribute noise.
But the most important source of noise is the spinning of the propellers. The boat is propelled forward by the reaction of the propeller pushing the still water at high speeds. This creates an empty space between the water and the propeller, and the pressure difference causes the air dissolved in the water to appear as bubbles. This phenomenon is called cavitation, and it can be seen in everyday carbonated drinks. When the lid is opened, the pressure difference between the inside and outside of the can causes the carbonated beverage and the empty space inside to release bubbles. When these bubbles are pressurized again, they burst and make noise, which is loud enough to be detected by nearby enemies. Also, when a propeller rotates, the frequency of the vortex around it increases, and when the frequency of the vortex is equal to the natural frequency of the propeller, the vibration and noise increase, and it is called ‘singing’. All objects have their own natural frequency and emit sound accordingly, and it is due to the principle of resonance, which increases energy when a force of the same frequency is applied from the outside. As such, propellers generate a lot of noise, but they are necessary for operation, so reducing the noise caused by their operation is an important issue.
Several technologies are being developed to solve this problem. The first way to eliminate propeller noise is to remove the propeller and install a water jet instead. A water jet is a propulsion device that sucks in water from an inlet at the bottom of the boat and pushes the boat forward by releasing water from a drain at the back of the boat. This is effective in reducing noise because there is no cavitation due to the lack of propellers, but it is still inefficient for large ships and more expensive than propulsion. The second is to change the shape of the propeller, the most common example used on ships is a high skew propeller. The idea is to change the shape of the blades as water flows through the propeller to suppress vibration and noise. However, you need to be careful not to change the blades too much, as this can cause noise. Another option is to apply a prairie system to the propeller. This is a technique that involves drilling small holes in the propeller blades and blowing air through them to create a film of air, which acts as a kind of wall that prevents the sound generated around the blades from spreading underwater. We are also developing and applying the Anti-Singing Edge method, which cuts off the tip of the wing and attaches another material to suppress noise generation to reduce the singing phenomenon.
In addition to noise reduction, other technologies are needed to maintain the submarine’s stealth. For example, the surface of the submarine can be coated with special sound-absorbing materials to prevent sound waves from being reflected. It is also important to design the submarine’s exterior to have a low radar-reflective area so that it is less exposed to radar detection.
The silent war between nations using submarines continues today. The U.S. Navy recently commissioned a robotic submarine to capture Russian-made submarines operating in U.S. territorial waters, and Russia is rushing to build a new generation of submarines equipped with the latest technology that will make them virtually silent. These submarines are constantly searching for and keeping tabs on each other, and their importance continues to grow. And noise reduction technology will continue to advance until they are completely silent.
