In the movies, you have seen submarines where to emit a ‘ping’ sound when traversing waters. This is fictional because, in reality, no such sound originates from the submarine. In fact, these pings won’t be able to travel far enough or hold enough frequencies to be useful. The sound being played is called ‘ping’ but doesn’t sound like ‘ping.’ For surface ships, the sonar array makes an array of loud ‘whoop’ sounds that will vary based on the settings being used. The sound of a submarine’s sonar, on the other hand, is very complex yet quite beautiful. It consists of a series of beeps and whoops that are aimed at maximizing the coverage of optimal frequencies. How often can you hear it is what brings us to the next point;
Even if the active sonar did sound anything like the pinging sound that you usually hear in movies, you wouldn’t be able to hear it constantly as the submarine moves along the ocean floor. That is because active sonar also broadcasts the submarine’s position. The surface ship is already loud enough that it doesn’t care about giving away its position. However, submarines are supposed to be stealthy, and therefore almost everything is done passively, not actively. That is to say that instead of actively pinging contacts, they listen to contacts.
You will usually see the following kind of picture every time you see a submarine in the movie.
Right? This is a radar display and not a sonar display. Radar arrays spin, that is how they collect information; a straight line of sight is moving around the ship’s position. Since sonar arrays do not spin and sound is way slower than light; the sonar displays are updated by the distance from the ship. The information closest to the submarine is updated first, and the circle expands outwards. The passive sonar display is known as ‘waterfall display.’ The horizontal axis of the display shows the bearing/frequency and time is shown on the vertical axis.
Submarines are also experts at implying unique and fascinating techniques for hiding from sonar. They can hide inside eddies, in the ‘shadow zone’ that is created because of different temperatures between the surface layer and ocean depths, behind oceanographic features such as ravines. One of the ways that the submarines hide is by expelling the waste tanks at the right time. This helps them attract big schools of fish that serve as biological noise to hide the submarine’s signature.
You’d be surprised by the degree to which submarines are able to decipher even the minute variation in sound underwater. The US submarines make use of a system that enables them to accurately reverse engineer the surface ship’s position by tuning in to their active sonar. Furthermore, they are able to resolve even the smallest of variations in machinery noise that is collected by passive sonar thus are able to track enemy contacts without even making a hint of a sound. That is not all; submarines can pick up and receive voice transmissions as well. All of the surface ships and submarines have an ‘underwater telephone.’ You can simply speak into it, and the voice is transmitted by using a second array that can be heard and responded by submarines.