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Study notes on “SOUND” (Part-III) for SSC CGL Pre 2017

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Study notes on "SOUND" (Part-III) for SSC CGL Pre 2017_40.1

SSC CGL 2017 Pre exam in scheduled in the month of June/July 2017. At SA, We are providing to you the NOTES ON GENERAL SCIENCE, these notes will help you build your basic understanding of all topics and you’ll be able to score maximum marks in SSC CGL Pre exam. Study, Revise and Learn all these notes by heart. 


Sounds of frequencies below 20 Hz are called infrasonic sound or infrasound.
Rhinoceroses communicate using infrasound of frequency as low as 5 Hz.  Whales and 
elephants produce sound in the infrasound range. It is observed that some animals get disturbed before earthquakes. Earthquakes produce low-frequency infrasound before 
the main shock waves begin which possibly alert the animals.


Frequencies higher than 20 kHz are called ultrasonic sound or ultrasound. 
Ultrasound is produced by dolphins, bats and porpoises.

  • Ultrasounds can be used to detect cracks and flaws in metal blocks.
  • Metallic components are generally used in construction of big structures like buildings, bridges, machines and also scientific equipment. 
  • The cracks or holes inside the metal blocks, which are invisible from outside reduces the strength of the structure. Ultrasonic waves are allowed to pass through the metal block and detectors are used to detect the transmitted waves. If there is even a small defect, the ultrasound gets reflected back indicating the presence of the flaw or defect.
  • Ultrasonic waves are made to reflect from various parts of the heart and form the image of the heart. This technique is called ‘echocardiography’.
  • Ultrasound scanner is an instrument which uses ultrasonic waves for getting images of internal organs of the human body. A doctor may image the patient’s organs such as the liver, gall bladder, uterus, kidney, etc. It helps the doctor to detect abnormalities, such as stones in the gall bladder and kidney or tumours in different organs. In this technique the ultrasonic waves travel through the tissues of the body and get reflected from a region where there is a change of tissue density. These waves are then converted into electrical signals that are used to generate images of the organ. These images are then displayed on a monitor or printed on a film. This technique is called ‘ultrasonography’.

Study notes on "SOUND" (Part-III) for SSC CGL Pre 2017_50.1

The acronym SONAR stands for Sound Navigation And Ranging.

  • Sonar is a device that uses ultrasonic waves to measure the distance, direction and speed of underwater objects.
  • Sonar consists of a transmitter and a detector and is installed in a boat or a ship. The transmitter produces and transmits ultrasonic waves. These waves travel through water and after striking the object on the seabed, get reflected back and are sensed by the detector. The detector converts the ultrasonic waves into electrical signals which are appropriately interpreted. 
  • The distance of the object that reflected the sound wave can be calculated by knowing the speed of sound in water and the time interval between transmission and reception of the ultrasound. 

Let the time interval between transmission and reception of ultrasound signal be t and the speed of sound through seawater be v. The total distance, 2d travelled by the ultrasound is then, 

2d = v × t

The above method is called echoranging. The sonar technique is used to determine the depth of the sea and to locate underwater hills, valleys, submarine, icebergs, sunken ship etc.


Again if the speed of any substance, specially of an air-craft, be more than the speed of sound in air, then the speed of the substance is called supersonic speed. 


The ratio of the speed of a body and that of sound in air is, however, called the Mach number of the body. If the Mach number of a body is more than 1 , it is clear that the body has supersonic speed.

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