NCERT Class 9 Science Solutions: Chapter 12 – Sound Part 4

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Question 1:

Q. What is sound and how is it produced?

Answer:

Sound is produced by vibration. When a body vibrates, it forces the neighbouring particles of the medium to vibrate. This creates a disturbance in the medium, which travels in the form of waves. This disturbance, when reaches the ear, produces sound.

Question 2:

Q. Describe with the help of a diagram, how compressions and rarefactions are produced in air near a source of sound.

Answer:

When a vibrating body moves forward, it creates a region of high pressure in its vicinity. This region of high pressure is known as compressions. When it moves backward, it creates a region of low pressure in its vicinity. This region is known as a rarefaction. As the body continues to move forward and backwards, it produces a series of compressions and rarefactions (as shown in the following figure).

Q 2 Image Showing Tuning Fork Producing Sound Waves In Air.

Q 2 Image Showing Tuning Fork

Question 3:

Q. Cite an experiment to show that sound needs a material medium for its propagation.

Answer:

Take an electric bell and hang this bell inside an empty bell-jar fitted with a vacuum pump (as shown in the following figure).

Q 3 Image Showing Bell Jar Experiment.

Q 3 Image Showing Bell Jar Experiment

Initially, one can hear the sound of the ringing bell. Now, pump out some air from the bell-far using the vacuum pump. It will be observed that the sound of the ringing bell decreases. If one keeps on pumping the air out of the bell-jar, then at one point, the glass-jar will be devoid of any air. At this moment, no sound can be heard from the ringing bell although one can see that the prong of the bell is still vibrating. When there is no air present inside, we can say that a vacuum is produced. Sound cannot travel through vacuum. This shows that sound needs a material medium for its propagation.

Question 4:

Q. Why is sound wave called a longitudinal wave?

Answer:

The vibration of the medium that travels along or parallel to the direction of the wave is called a longitudinal wave. In a sound wave, the particles of the medium vibrate in the direction parallel to the direction of the propagation of disturbance. Hence, a sound wave is called a longitudinal wave.

Question 5:

Q. Which characteristics of the sound helps you to identify your friend by his voice while sitting with other in a dark room?

Answer:

Quality of sound is that characteristic which helps us identify a particular person. Sound produced by two persons may have the same pitch and loudness, but the quality of the two sounds will be different.

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