Ocean Source (3/3): Submarines & Sonar

I’m a little embarrassed to say this, but my knowledge about submarines is informed by the internet, and largely premised on the 2018 Hunter Killer movie, an American thriller in which one of the settings is a submarine. The synopsis of the film goes like this: An American submarine captain teams up with the U.S. Navy Seals and enters enemy waters to rescue the kidnapped Russian president from a rogue general (Rotten Tomatoes, 2018). From what I remember, submarines are stealthy under the ocean.

Indeed, submarines are closely intertwined with war and military action, seeing as they are warcraft used in naval warfare. According to the National Museum of the U.S. Navy (2018), the first American submarine was designed before the Revolutionary War of 1775 with a torpedo to attack enemy ships. This was later adapted to use against the British during the war, and developments in submarine technology have made submarines an essential part of the U.S. Navy today.

Submarines and noise

The quieter the submarine, the stealthier it is, which is critical for exploration and navigation, and in the military, for observing the enemy. This is because a submarine’s source of noise can reveal its location, speed and allow others to identify it (Miasnikov, 1996).

Hull vibration, power plant equipment, propellers, and hydrodynamic noise contribute to noise production by submarines (Miasnikov, 1996; Makichuk, 2020). Similar to ships, the movement of propellors results in cavitation which generates noise. In the words of naval expert H. I. Sutton, manufacturers seek to make submarines “as quiet as the ambient sea noise”, which is around 90 dB (Makichuk, 2020). In 2020, it was reported that China was working on a new generation of submarines that would likely incorporate newer technology to minimise noise generation.

Quantitative data on the actual noise level of combat submarines are highly classified, while information on the noise level of nuclear submarines is difficult to find in open literature (Miasnikov, 1996).

SONAR

SONAR (Sound Navigation and Ranging) systems is a technique that utilises wave propagation for navigation, measuring distances, communicating with or detecting objects on or under the surface of the water. They operate based on the propagation of waves between a target and a receiver (Hodges, 2010). Sonar systems may be passive or active – the former picks up noise made by others, while the latter picks up echoes of its own transmitted sound waves as shown in the diagram below.

For ships or submarines, the pulse of sound that bounces off an object and returns as an echo gives an indication of where the object is (National Geographic Society, 2019). Sonar sounds can be as loud as 235 dB and travel hundreds of kilometers underwater. This is concerning as there is evidence that suggests that mid-frequency sonar may contribute to marine mammal mortality (Reynolds, 2007).

Unfortunately, I do not have any personal experiences with submarines nor sonar. I do imagine that it must be very thrilling to be underwater, and have no one else know that you are there. I wonder if people inside submarines can hear the movement of the vessel through the water. If you have any clue, please leave a comment below!

 

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