Sonar or Sound Navigation Ranging is a technology which involves the use of sound to detect vessels and other objects underwater or on the surface to aid navigation and avoid collisions. Sonar are of two types: active sonar and passive sonar. Active sonar emits pulses of sounds and listen to echoes from other vessels or objects. Passive sonar is used for listening for sounds made from other vessels. Sonar may be used for locating vessels or other objects by emitting sounds which hit the object being located and bounces back as an echo and is displayed on the sonar screen of the locating vessel. Sonars were first developed in World War 1 to counter the threat from submarines with an operational passive sonar being used in 1918. Modern active sonars being used today employ an acoustic transponder to generate a sound wave which is reflected back when it strikes a target object.
Active sonar uses a sound transmitter and a receiver. When the two are at the same place it is called monostatic operation. When the two are separated it is called bistatic operation. When more transmitters or receivers are used but spatially separated it is called multi static operation.
Active sonars create a pulse of sound or “ping” and then listens for reflections known as echo of the pulse. This pulse of sound is usually created electronically using a sonar projector consisting of a signal generator,power amplifier and electro- acoustic transducer. A beam former is used to concentrate the acoustic power into a beam which may be swept to cover the required search angles. To measure the distance to an object, the time from transmission of a pulse to reception is measured and converted into a range by knowing the speed of sound. When active sonar is used to measure the distance from the transducer to the bottom, it is known as echo sounding. Active sonar is also used to measure the distance through water between two sonar transducers or a combination of a hydrophone ( underwater acoustic microphone) and projector (underwater acoustic speaker). A transducer is a device that can transmit or receive acoustic signals known as pings.
In combat situations an active pulse can be detected by an enemy and will reveal a submarine’s position at twice the maximum distance that the submarine can itself detect the contact and give clues as to the submarine’s identity based on the characteristics of the outgoing ping. For these reasons active sonars are not frequently used by military submarines.
It is an active sonar device that receives a stimulus or signal and immediately or with a slight delay retransmit the received signal or a predetermined one.
This sonar type listens without transmitting. They are usually employed for military applications such as submarines and underwater warfare but find civil applications in science such as detecting the presence or absence of fish and other marine life in a particular marine geographical location. Passive sonar have a wide variety of techniques for identifying the source of a detected sound. An example is vibrations coming from a submarine that uses 50Hz or 60 Hz alternating current power systems. If the generators are mounted without proper vibration insulation from the hull of the submarine, then the vibration noises can easily be detected by the passive sonar and the nationality of the vessel will be known. Noises such as dropping of wrenches produce noise that can also be easily detected by passive sonar. All these can easily be detected by specialized computers installed on board these submarines specifically for this purpose.