Sound Waves - Medium For Their Propagation

Sound is something without which we cannot imagine our life. Everything around us is associated with sound waves. Chirping of birds, drilling of machine, alarm clock beeping, playing a guitar, bouncing of ball, etc., everything produces sound. But have you ever thought how do we hear these sounds? What really is sound?

Sound is a form of energy produced by the vibration of particles in a medium. Let us understand how sound waves are produced with the help of an example: In a drum, when drum sticks hit the drum, the outer membrane of the drum vibrates up and down. Whenever the drum membrane moves up it compresses the air just above it. While the air molecules expand when the drum membrane vibrates down. This compression and expansion of air produces a difference in air pressure. This pressure difference in air is carried like ripples in ponds and result in the production of sound by drum.

Why does sound waves need a medium for their propagation?

Now think how the sound produced by drums is heard by us?  How is this transferred to our ears? Let us understand this with the help of an experiment.

  1. Take a bell jar. Bell jar is a laboratory equipment used for creating vacuum. Its shape is similar to that of a bell.
  2. Place an electric bell in the bell jar.
  3. Start pumping out the air of the sealed bell jar. As we start pumping out the air, the sound of electric bell coming out of bell jar starts fading.
  4. As soon as vacuum is achieved in the bell jar, no more sound will be audible from the bell jar. However in the bell jar, hammer continues to hit the gong.

This means that still the sound is being produced but now we cannot hear it. This is because sound waves always need a medium to propagate. In the bell jar, it was , hence sound waves cannot travel.

Sound Waves

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Practise This Question

The speed of sound in a vacuum is?