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01 Sound

  • Sound is a form of energy like electricity, heat or light, which produces the sensation of hearing in our ears.
  • It is also defined as oscillations or auditory sensation evoked by oscillations in particle displacement or velocity, propagated in a medium with internal force.
  • Sound travels in the form of longitudinal waves.


  • If we pull and release a rubber band, it moves to and fro about the central axis and while doing so it also produces a sound.
  • Vibration is the to and fro motion of the body.
  • Vibrating objects produce sound.
  • Some vibrations are visible while others are not.
  • A bell makes a loud ringing noise when we strike it. If we put our finger on the bell after striking it we can feel it vibrating.
  • The sound moves through a medium by alternatively contracting and expanding parts of the medium it is travelling through.
  • Examples:
    • Guitars and Drums: The vibration is even more prominent in guitars and drums. The wires vibrate every time we pluck it. When the bell or the guitar stops vibrating, the sound also stops.
    • Human Voice: Our voice too is produced by the vibration of strings known as the vocal chords which is inside the Adam’s apple.
    • Tuning Forks: In the laboratory experiments, sound is produced by vibrating tuning forks.


  • Medium: Medium is the matter or substance through which sound is transmitted.
  • Sound requires a medium to travel:
    • On hearing a very loud sound, we cover our ears to shut off the air inside our ears from the rest of the atmosphere. The sound waves travelling around us are now unable to get through to your ear or the intensity of the sound you hear is greatly reduced. So, blocking our ears creates a discontinuity in the medium due to which the flow of sound energy is disturbed.
    • Sound travels through medium like solid, liquid or gas to travel.
    • Sound vibrations travel through air and when it reaches our brain through our ears, it is interpreted as sound.
    • The speed of sound when it is travelling through a medium depends on the type of medium. The speed of sound when travelling through air is 343 m/s or 1235 km/h.
  • Sound cannot travel through vacuum:
  • Consider an electric bell to be enclosed within a bell jar which is placed over a disc. Through a hole in the disc, air can be removed by using a vacuum pump. On gradually removing the air, the sound of the bell becomes feebler and eventually becomes inaudible when all the air is removed from the tumbler. This implies that sound waves need a medium for propagation and it cannot travel through vacuum.
  • Sound cannot travel through vacuum because there are no molecules that can be compressed and expanded in space or vacuum.

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