RRB ALP Stage-2 exam: Physics notes- SOUND (PART 2)

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  • It is a reflection of sound that arrives at the listener with a delay after the direct sound. 
  • The sensation of sound persists in our brain for about 0.1 second. 
  • To hear a distinct echo, the time interval between the original sound and the reflected one must be at least 0.1 second.
  • For hearing distinct echoes, the minimum distance of the obstacle from the source of sound must be 17.2 m. This distance will change with the temperature of air. Echoes may be heard more than once due to successive or multiple reflections.

The phenomenon of prolongation of sound due to successive reflections of sound from surrounding objects is called reverberation.

In stethoscopes the sound of the patient’s heartbeat reaches the doctor’s ears by multiple reflection of sound.

Ultrasounds are high frequency waves. They are able to travel along well defined paths even in the presence of obstacles. Ultrasounds are used extensively in industries and for medical purposes.


  • 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. 
  • Ultrasound is generally used to clean parts located in hard-to-reach places,for example, spiral tube, odd shaped parts, electronic components etc.
  • 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. It helps the doctor to detect abnormalities, such as stones in the gall bladder and kidney or tumours in different organs. The technique is called ‘ultrasonography’.
  • Ultrasound may be employed to break small ‘stones’ formed in the kidneys into fine grains. These grains later get flushed out with urine.

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 echo ranging.
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.

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