What is sonar and how does it detect objects underwater

What do animals such as dolphins, whales or bats have in common with a ship or submarine? Apparently nothing. But seas and submarines, as well as some animals, use the sonar as a navigation aid. This instrument is based on the propagation of sound underwater and is used to communicate, measure distances below sea level and to detect the presence of artificial objects, but also to aid fishing and for scientific and military purposes.

What is SONAR

The sonaracronym for Sound NAvigation And Ranging, is an instrument present on board submarines, boats but also helicopters (used for military purposes in the search phases for submarines). Its operating principle is based on two steps:

  1. L’emission of a signal acoustic which propagates through the marine environment;
  2. the reception ofecho produced when the signal encounters a obstacle, natural or artificial. Basically, the sound comes reflex and comes back, a bit like the echo of our voice in an empty room.

The fact of receiving an echo indicates the presence of an object, of which one can measure the distance starting from the time elapsed between the emission of the sound signal and the reception of the echo.

In principle it’s a fairly simple principle that our technology has borrowed from nature: is called echolocation. Marine mammals such as dolphins and whales, but also bats, employ sonar to be able to individuate And to hunt the preys that for orient yourself in the surrounding environment. In fact, think about bats: they can fly in the dark without “bumping” into obstacles.

Image

How sonar works and what it is used for

The underwater discovery of obstacles and objects through echolocation occurs essentially thanks to three fundamental factors:

  1. the presence of a obstacle (natural or artificial) that generates a discontinuity of the marine environment;
  2. the different nature of the material of which the obstacle is made compared to the surrounding water translates into a variation of density and this generates a reflection of the sound wave that hits him;
  3. every moving object emits noise. Ships, small boats, submarines, etc.: each generates noise, with the movement of the propellers, with the roar of the engines or with the friction of the hull itself as it cuts through the water. These factors are very important in the military field.

Let’s imagine getting on board a submarine and being able to actually “see” the acoustic wave generated by the sonar: we will see it propagate through the sea water as long as it does not encounter an obstacle. At this point, the sound wave bounces and returns towards our boat, where it is “collected” and analyzed by our sonar. We will therefore have the data to be able to say at least 3 things:

  1. there presence of an obstacle;
  2. there direction (the detection) of the obstacle with respect to our boat;
  3. there distance of the obstacle.

We will practically be able to navigate. Obviously it’s not all that simple: there are many factors to take into account in order to evaluate those three aspects we listed above. Temperature of water and presence of others noises can influence the speed of sound propagation in water, the distance of propagation and the ability to discriminate the echo received.

Types of sonar: active and passive

Sonars can be grouped into two broad categories: those active and those passive.

Sonars active they are instruments capable of emitting a sound signal: the classic “ping” that is heard in many films set underwater. Inside they have a transmission system called transducer, which generates the sound signal. In addition to this active part, there are sensors capable of receiving the echo of the signal sent and analyzing it.

Sonars passive, sayings hydrophones, instead they work like large “ears” continuously listening below the surface of the sea: they do not emit any type of sound but they also receive the slightest noise. They are often organized into long chains of sensors called hydrophonic curtains to maximize their efficiency. For this reason, they represent an important factor especially in military contexts.

A submarine, in fact, was born as a means of transport discretion its strong point. Avoid doing it intercept is critical. In particular contexts, in which it is necessary to be hidden, meticulous checks are carried out, called noise hygienewith the aim of identifying and eliminating any possible accidental source of noise that could unmask the boat.

Image

The difference between sonar and radar

Sonar and radar use the same principles and operate in a conceptually similar way. The main difference is that radars emit (and receive) electromagnetic waves instead of sound waves. While sonars are mainly used underwater (but not only), radars are mostly used on the ground or in the air.

Image

A medical application: ultrasound

An area in which sound waves are used for purposes completely different from those described is the medicine. The ultrasoundsin fact, they use sound waves to visualize the various organs inside the body, or during the pregnancy. Based on the same principle as sonar, taking advantage of the different density of tissues within the body, making it possible to visualize them.

Image