Propagation of Sound

Students’ Learning Outcomes

·         Demonstrate that sound can travel through solids, liquids and gases but can’t travel through vacuum.

Information for Teachers

·         A sounding body produces vibrations in the air around it.

·         The sound we hear is carried by the vibrating air.
·         If there is no air in surroundings of a vibrating object, the sound can’t be heard, e.g. in outer space or in vacuum.
·         Sound vibrations can also travel through solids and liquids, e.g. metal and water. 

Material / Resources

Table, alarm clock, paper bag, plastic container, metal can, wooden block, plastic bag, water tube, two steel spoons, round bottom flask, bunch of keys, thread, cork, burner, textbook

Worm up activity                 

·         Ask the students: Do you know the three states of matter? Name them.
(Expected response: Solid, liquid and gases)
·         Ask them what the states of table, water and air are.
·         Then introduce today’s topic, how sound travels through solids, liquids and gases.
·         Ask the students how they hear different sounds around them, from a loudspeaker or a music player.
·         Ask how these sounds travel and spread all around and reach their ears.
(Expected response: Through air)
·         Ask one student to put his/her ear on the table.
·         Ask another student to tap the table with his fingers.
·         Ask the student, put ear on the table, does he/she hear some kind of sound.
·         Ask the students: Does sound travel through solids?

(Expected response: Sound can travel through solids)


Activity 1

·         Put a ringing alarm clock in a bag. Close the bag.
·         Ask students: Do you hear the clock ringing?
·         Now put the clock in a plastic container and close its mouth tightly.
·         Ask students: Do you hear clock ringing now?
·         Put the clock in a metal can and close it tightly and ask do they hear any sound.
·         Ask them: Can sound travel through paper, plastic and metal?
·         Conclude by mentioning that sound can easily travel through solids e.g. paper, plastic and metal.

Activity 2

·         Show the students a tub full of water.
·         Ask a student to bring his/her ear close to the water tub.
·         Ask another student to hit two steel spoons together under water.
·         Then ask do you hear any sound? Does sound travel through water?

(Students’ response: Sound can travel through liquids i.e. water)

Activity 3

·         Take a glass flask, with some water in it. As a student to tie a bunch of keys with a thread. Ask him/her to pass the thread through a crock and suspend the keys inside the flask by fitting the cork on the mouth of flask. Shake the flask. Ask the students weather they hear the sound of keys.
(Expected response: Yes)
·         Now ask a student to heat the flask after removing the crock. When water starts boiling and flask is filled up with steam, ask him/her to turn off the burner and again fit the cork tightly on the mouth of the flask. Leave the flask to cool down. When all the steam has condensed into water, ask a student to shake the flask again. Ask the student do they hear although the keys are shaking. Ask the students why it is so?

·         Now explain that when water was boiled, the air inside the flask was replaced but the steam. On cooling, the steam changed into water and vacuum was produced in the flask. Sound can’t travel through vacuum. Sound requires a medium for its propagation.

Sum up / Conclusion
·         Ask the students what they have learnt about sound in this lesson. Conclude their answers by writing on the board.
v  Sound can travel through solids.
v  Sound can travel through liquids.
v  Sound can travel through gases.
v  Sound can’t travel through vacuum.
v  Material medium is required to carry sound from one place to another.
·         Ask the following question verbally at the end of lesson.
v  How does the sound get to your ears from someone talking?
v  Which materials can sound travel through?
v  Why sound can’t travel through vacuum/
v  How does sound travel when you have a conversion with your friends
Follow up
·         Ask the students to think about what it is like to hear things under water.

·         Debate whether travels sound better in liquids, gases (air), or a solids.

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