NCERT Class 9 Science Solutions: Chapter 12 – Sound Part 8

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Question 20:

Q.A sonar device on a submarine sends out a signal and receives an echo 5 s later. Calculate the speed of sound in water if the distance of the object from the submarine is 3625 m.


Time taken to hear the echo,

Distance of the object from the submarine,

Total distance travelled by the sonar waves during the transmission and reception in water

Velocity of sound in water,

Question 21:

Q. Explain how defects in a metal block can be detected using ultrasound.


Defects in metal blocks do not allow ultrasound to pass through them and they are reflected back. This fact is used to detect defects in metal blocks. Ultrasound is passed through one end of a metal block and detectors are placed on the other end. The defective part of the metal block does not allow ultrasound to pass through it. As a result, it will not be detected by the detector. Hence, defects in metal blocks can be detected using ultrasound.

Q 21 Image Showing Metal Block.

Q 21 Image Showing Metal Block

Question 22:

Q. Explain how the human ear works.


Different sounds produced in our surroundings are collected by pinna that sends these sounds to the ear drum via the ear canal. The ear drum starts vibrating back and forth rapidly when the sound waves fall on it. The vibrating eardrum sets the small bone hammer into vibration. The vibrations are passed from the hammer to the second bone anvil, and finally to the third bone stirrup. The vibrating stirrup strikes on the membrane of the oval window and passes its vibration to the liquid in the cochlea. This produces electrical impulses in nerve cells. The auditory nerve carries these electrical impulses to the brain. These electrical impulses are interpreted by the brain as sound and we get a sensation of hearing.

Q 22 Image Showingh Diagram Of The Ear.

Q 22 Image Showingh Diagram of the Ear

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