Physics (NTSE/Olympiad)  

5. Sound


The word 'SONAR' stands for
Sound Navigation and Ranging.
(A) Principle of Sonar : Sonar is an apparatus which is used to find the depth of a sea or to locate the under water things like shoals of fish, enemy submarines etc. Sonar works by sending short bursts of ultrasonic sound from a ship down into sea water and then gets echo produced by the reflection of ultrasonic sound from under-water objects like bottom of sea, shoal of fish, a submarine.
(B) Working of Sonar :

(i) A transmitter (for emitting ultrasonic waves) and (ii) a receiver (for detecting ultrasonic waves). Now suppose a sonar device is attached to the under-side of a ship and we want to measure the depth of sea (below the ship). To do this the transmitter of sonar is made to emit a pulse of ultrasonic sound with a very high frequency of about 50,000 hertz. This pulse of ultrasonic sound travels down in the sea-water towards the bottom of the sea. When the ultrasonic sound pulse strikes the bottom of the sea, it is reflected back to the ship in the form of an echo. This echo produces and electrical signal in the receiver part of the sonar device. The sonar device measures the time taken by the ultrasonic sound pulse to travel from the ship to the bottom of the sea and back to the ship. Half of this time gives the time taken by the ultrasonic to travel from the ship to the bottom of the sea.
d = Depth of sea
v = Velocity of sound in sea water
t = time recorded by the recorder

(i) Ultrasonic waves have a very high frequency due to which they can penetrate deep in sea water without being absorbed.
(ii) Ultra sonic waves cannot be confused with the noise, such as the voice of engines of ship. It is because the ultrasonic waves are not perceived by human ear.


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