2015 Dec

Physics of SONAR

December 3, 2015

Physics of Sonar

The deepest place on earth is around 10.97 kilometers below the sea level and is located in Pacific Ocean. It is the deepest point in a trench (an underwater canyon) called as the Mariana trench. Mariana trench was first discovered during Challenger expedition in 1875, where they dropped lead weight into ocean to measure its depth every 150 kilometers. This process would take ages for us map the entire ocean floor and it’s better to talk less about the resolution. How do we measure depth of sea floor at any given point now? Do we still follow the age-old-lead-weight technique? The answer is obviously a no, and we use SONAR technology to measure the depths more accurately, with better resolution and less demand for manpower. Let’s discuss how SONAR works in greater detail.

SONAR is an acronym for Sound Navigation And Ranging. As the name suggests, sound waves are employed for the purpose. Remember those echoes you hear when you shout your lungs out at hills; the mechanism with which SONAR works is almost the same. To understand the working of SONAR easily let us do a thought experiment, assume that there is an imaginary ball that travels from your house to your friend’s and back to yours at a speed of 10 meters per second and the distance between your houses is unknown. Now the total distance traveled by the ball is twice the distance between your houses. We know that the speed at which the ball is travelling is constant and has a value of 10 meters per second. Speed is defined as the distance traveled in unit time (in our case its one second). Let’s say you measure the time taken by the ball to go all the way from your home to your friend’s and come back and let’s assume it is t seconds. Now from the definition of speed, when the speed is equal to 10 meters per second, the ball travels 10 meters in one second. If the ball travels for a time of t seconds then the distance traveled by the ball is 10 x t meters. The distance between your homes and it is half of 10 x t. From the value of t one can calculate the distance.

Now replace the ball with a click sound that travels from that is triggered from your home and is reflected back. So from the time the click sound takes to travel all the way one can measure the distance between your homes. But we seldom observe such phenomenon in daily life as several other factors come into play. For example most of the sound is absorbed by the buildings and other objects, secondly the intensity of the sound wave decreases to immeasurably low levels under the background noise during the process of travelling all the distance and remember sound waves can be deflected and actually are. In fact the entire sea floor is mapped using SONAR. This use of sound waves is also taught in our Physics Tuition classes as it is a knowledge prerequisite for Physics examinations.

And finally if you are not aware of this, bats use the same phenomenon to echolocate their surroundings.

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