Sound Waves - Medium For Their Propagation

Introduction

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 the guitar, bouncing of the ball, etc., everything produces sound. But have you ever thought how do we hear these sounds? What really is sound?

A 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 produce a difference in air pressure. This pressure difference in the air is carried like ripples in ponds and result in the production of sound by a drum.

Why do sound waves need a medium for their propagation?

Sound needs a material medium for their propagation like solid, liquid or gas to travel because the molecules of solid, liquid and gases carry sound waves from one point to another. Sound cannot progress through the vacuum because the vacuum has no molecules which can vibrate and carry the sound waves.

Experiment:

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 a 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 an electric bell coming out of the bell jar starts fading.
4. As soon as the vacuum is achieved in the bell jar, no more sound will be audible from the bell jar. However, in the bell jar, the 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.