Sound Free Lesson For Class 9th NCERT

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Sound- The sensation felt be our ears is called sound

  • Sound is a form of energy which makes us hear.

Examples of sound- The sound of television, The sound of  music, The sound of our teachers, The sound of Birds Etc.

Wave– A wave is a vibratory disturbance which carries energy from one point to another without being direct contact with one another

  • For example the sound wave produced by a ringing bell comes to our ear through the air.
  • Similarly when a water wave passes over the surface of water in a pond,it does not drive the water to one side of the pond,only the water molecules vibrate up and down about their fixed position.

Types of waves

  • Longitudinal waves
  • Transverse waves


Longitudinal waves– A wave in which the particles of the medium vibrate back and forth in the same direction in which the wave is moving is called longitudinal waves.

Example- When we pull a spring when it is pushed and pulled at one end,are longitudinal waves.

Compression- When the spring coils are pushed it is called compression

Rare-fraction- When the spring goes  beyond its normal position after removing compression it is called rare fraction.″ title=”Longitudinal waves and transverse waves animation

The Sound waves in air are longitudinal waves

  • When a sound wave passes through air the particles vibrate back and forth parallel to the direction of sound waves
  • Thus when a sound waves passes the particles of the medium also vibrates.
  • When compression is there the particles are closer to each other have a high pressure
  • The region of rare fraction is the part of low pressure area.


Transverse waves- A wave in which the particles of the medium vibrate up and down at right angles to the direction in which the wave is moving is called transverse waves.

Examples- The waves produced by moving one end of a long (Slinky)up and down rapidly , whose other end is fixed, are transverse waves.

In transverse waves there is elevation or crest is that part which is above the line of zero disturbance

The depression or hollow or trough is that part of the transverse wave which is below the line of the zero disturbance


Graphical representation of the longitudinal waves

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  • In one side we have shown density and on other time.
  • In Compression more density and in rare fraction less density
  • The part above the normal are compression and shows high density
  • The part below the normal shows rare fraction and less denser


Graphical representation of the Transverse waves


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  • Here one axis shows displacement ( in longitudinal that was Density) from the distubance
  • Another axis shows the distance from the origin of the sound
  • So transverse is represented in displacement distance graph.