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SOUND NEEDS A MEDIUM FOR PROPAGATION
SOUND NEEDS A MEDIUM FOR PROPAGATION
When you call up your friend who is standing at a distance, your friend is able to hear your voice. How does the sound propagate or travel to her?
Activity 13.7: (Sound needs medium for propagation)
* Take a metal or glass tumbler. Make sure that it is dry.
* Place a cell phone in it.
(Remember that the cell phone must not be kept in water.)
* Ask your friend to give a ring on this cell phone from another cell phone.
* Listen to the ring carefully.
* Now, surround the rim of the tumbler with your hands (Fig. 13.10).
* Put your mouth on the opening between your hands.
* Indicate to your friend to give a ring again.
* Listen to the ring while sucking air from the tumbler.
* Does the sound become fainter as you suck air?
* Remove the tumbler from your mouth.
* Does the sound become loud again?
Figure 13.10: Sound needs a medium to travel
Can you think of an explanation? Is it possible that the decreasing amount of air in the tumbler had something to do with the decreasing loudness of the ring?
Indeed, if you had been able to suck all the air in the tumbler, you will not listen to any sound. Actually, sound needs a medium to travel. When the air has been removed completely from a vessel, it is said that there is a vacuum in the vessel. The sound cannot travel through vacuum .
Does sound travel in liquids? Let us find out.
Activity 13.8:
* Take a bucket or a bathtub.
* Fill it with clean water.
* Take a small bell in one hand.
* Shake this bell inside the water to produce sound.
* Make sure that the bell does not touch the body of the bucket or the tub.
Figure 13.11: Sound travelling through water
* Place your ear gently on the water surface (Fig. 13.11).
(Be careful: the water should not enter in your ear.)
* Can you hear the sound of the bell?
* Does it indicate that sound can travel through liquids?
Let us find out if sound can travel through solids also.
Activity 13.9:
* Take a metre scale or a long metal rod and hold its one end to your ear.
* Ask your friend to gently scratch or tap at the other end of the scale (Fig. 13.12).
Figure 13.12: Sound travelling through a metre scale
* Can you hear the sound of the scratching?
* Ask your friends around you if they were able to hear the same sound?
You can also perform the above activity by placing your ear at one end of a long wooden or metallic table and asking your friend to gently scratch the other end of the table (Fig. 13.13).
Figure 13.13: Sound can travel through solids
We find that sound can travel through wood or metal. In fact, sound can travel through any solid. You can perform interesting activities to show that sound can also travel through strings. Have you ever made a toy telephone (Fig. 13.14). Can you say that sound can travel through strings?
Figure 13.14: A toy telephone
We have learnt so far that vibrating objects produce sound and it is carried in all directions in a medium. The medium could be a gas, a liquid or a solid. How do we hear it?
Source: This topic is taken from NCERT TEXTBOOK