Production and Medium Of Propagation of Sound Lesson 3 For Class 9th

Production of sound

  •   if we touch a silent bicycle bell ,we do not feel any thing but when it is ringing we may feel the vibration when we touch it
  • Sound is produced by vibrating objects
  • The energy required to make vibration is produced by some outside source like (our hand,wind etc.)

Sound is produced

  • By vibrating strings
  • By vibrating air
  • By vibrating membrane
  • By vibrating plates

How sound waves travel in air ?

  • Sound waves in air consists of compression and rare fraction of air

  • Suppose set the original position of the air layer
  • Pass a sound on a turning fork by tuning it against a rubber fork
  • It moves back and forth continuously and produce sound

Remember Sound waves are longitudinal waves

Propagation of Sound

  • when an object vibrates (and makes sound)then the air layer around it also starts vibrating in exactly the same way and carry sounds producing objects to our ears.
  • Sound needs a medium to travel
  • The substance through which sound can travel is called medium
  • Sound can travel through Solid, Liquid as well as gases. although the speed may be different in all the three mediums

Take an example here

  • A train is coming from far away. you may not hear its sound through air
  • when you put your ear on the track you may hear the sound even if it is far away.
  • Above statements shows that sound travels faster in solid
  • In fact sound can travel 15 times faster in solid compared to air

in your childhood you must have tried  a toy telephone. in which two tin cans are joined by a metal thread. when one speak something on one side your friend can here it on the other side.

so the sound is travelling through  the metal thread which is solid.

Sound can also travel through water.

this is used to detect submarines inside the sea. the sound of submarine is transmitted through water and is detected by hearing aids present outside the sea water.

sound cannot travel through vacuum.

You  can test it by putting a ringing electric bell inside a bottle but after removing all the air inside it by using a vacuum pump.

How astronauts talk when they are in moon or outer space?

  • We learnt that sound needs a medium to travel.
  • Sound can not be heard directly in space/moon because there is no air in the moon to carry sound waves.
  • So we cannot talk directly in the moon.
  • So astronauts use devices which use radio waves to communicate
  • The radio waves can travel even in vacuum

On what factors the speed of sound depends

  • On the nature of materials
    – At room temperature the Sound travels faster in Solid>>Liquid>>Gases
    – At room temperature The speed of sound in Iron(5130 m/s m/s)>>Water(1500)>>Air (344 m/s)
  • On Temperature
    – More Temperature More sound speed
    Example at 00C the speed of sound in air is 332 m/s and at 200c The Speed of sound in air is 342 m/s
    – So speed of sound will be more on a hot day than on a cold day
  • On Humidity
    -More Humidity More Speed


Supersonic Speed-The Speed of an object which is greater than the speed of the speed of the sound in air .

Example – speed of an Aircraft

Which is more faster  Miss Sound or Miss Light ?

The speed of sound in air is just 344 m/s but the speed of light in air is 300000000m/s which is way higher than speed of sound.


  • In Rainy Days we first see the lightning then the sound of thunder
  • In cricket we can see the ball hitting the bat fast then the noise of hitting
  • If a gun his fired from a distance we first see the flash of the gun and then hear its noise

a small problem for you 

If a thunder is heard by a man 4 seconds after the lightning is seen, how far is the lightning from the man( take speed of sound in air is 330 m/s)

Hint: Use the formula speed = Distance/Time. and answer is 1320 m)

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