Propagation of Sound

Science class 7 lesson 11 (Maharashtra Board)

1. Propagation of Sound 1 © Jnana Prabodhini Educational Resource Centre

2. Vibration of source is necessary for the production of sound . When object begins to vibrate , the layer of air in contact with it also begins to vibrate with same frequency as that of the object. As a result the layer of air next to that layer also begins to vibrate. Thus, sound travels away from source. The substance through which sound travels is called the medium. Air is one medium through which sound travels. 2 © Jnana Prabodhini Educational Resource Centre

3. Connect two paper cups with a hole at the base , pass a thread through the holes at the base. This works as a telephone. Tell your friend to hold one cup to his ear and hold the other cup to your mouth and speak into it softly. 3 © Jnana Prabodhini Educational Resource Centre

4. The paper cup vibrates when you speak. These vibrations travel through string to the other cup and that cup , too vibrates. This is how your friend can hear you. 4 © Jnana Prabodhini Educational Resource Centre

5. If the string is wet you hear better sound. If the string breaks and you knot it together , the telephone does not work any more because the vibrations are not propagated. 5 © Jnana Prabodhini Educational Resource Centre

6. A medium is necessary for the propagation of sound. Sound can be propagated through solid , liquid , as well as gaseous medium. Sound travels faster through a liquid than through a gaseous medium and fastest through a solid medium. 6 © Jnana Prabodhini Educational Resource Centre

7. While travelling at mountains or hills , you are almost certain to find echo point there. If you shout standing on top of a mountain, the sound is reflected back from the mountain opposite you and you hear your own shout again after some time . This is called as echo. When sound travels through air and reflected back from a flat surface and travels back in the direction from which it came . The reflected sound is called as echo. 7 © Jnana Prabodhini Educational Resource Centre

8. At temperature of 20 °C the speed of sound in air is 340 meters/second . It increases as temperature increases. The speed of light is much faster than speed of sound. In rainy days , when there is thunder and lightning, we see the flash of lightning instantaneously but we hear the sound much later. The speed of light is 10 6 times faster than the speed of sound. It means the speed of light is 3 Χ 10 8 m/s. 8 © Jnana Prabodhini Educational Resource Centre

9. In a music concert a singer is accompanied by other instrument played by experts. The main instruments used are of strings instruments or, percussion instruments or wind instruments . The string instruments are Tambora , Sitar, Sarod , Violin etc. The percussion instruments like drums , tabla , etc. The wind instruments are flute, Sanai , saxophone, etc. 9 © Jnana Prabodhini Educational Resource Centre

10. String instruments have several strings. The strings are plucked to produce sounds. Each string has a different frequency. The desired frequency can be obtained by adjusting the tension in the string. In percussion instruments like drum , a stretched membrane is struck to produce sound. The tension in the membrane can be varied. Hence the desired frequency can be obtained. 10 © Jnana Prabodhini Educational Resource Centre

11. Wind instruments are column like structure with holes at different length. Sound is produced by blowing into the column to cause vibrations in a column of air. By placing the finger on different holes , the length of vibrating column can be varied. Thus sound of desired frequency can be obtained. 11 © Jnana Prabodhini Educational Resource Centre

12. In diwali we hear noise of crackers , music played in loud sound in social events , the sound of traffic jam all these sounds do not have any rhythm , they have different frequency , and are not in synchronization with each other. The sound is not pleasant, you don’t want to listen this sound. 12 © Jnana Prabodhini Educational Resource Centre

13. But in music the singer and the instruments are in synchronization with each other, their frequencies are in harmony , the sound has specific regularity. Hence the music has a melodious tune. 13 © Jnana Prabodhini Educational Resource Centre

14. Now a days in cities noise is increasing due to the population in cities, the crowed, and increasing number of vehicles, causing high frequency sound. If the intensity of sound crosses a certain limit it can cause deafness, psychological disorders and some physical disorders too. 14 © Jnana Prabodhini Educational Resource Centre

15. Special care need to be taken for patient in hospital, old age people, small children , school going children to avoid noise. Unsynchronized sound is called as noise . Increasing noise is called as noise pollution. Now a days we see boards saying ‘ Silence’ or ‘Silence Zone’ or ‘ No Horn Zone’ near hospitals, schools, residential buildings etc. If we follow the rule and regulations , it will help in reducing noise pollution. 15 © Jnana Prabodhini Educational Resource Centre