10.1. Sound is Produced by a Vibrating Body

Touch the school bell when not in use. What do you feel? Again touch it when producing sound. Can you feel it vibrating?

When we touch a school bell that is not ringing, it feels still and does not vibrate. However, when the bell is struck to produce sound, it starts to vibrate. This vibration is a back-and-forth motion that creates sound waves, which we perceive as sound. This is because the vibrating body disturbs the surrounding air particles, resulting in sound propagation.

Think of a trampoline. When someone jumps on it, the surface vibrates up and down. Just like the trampoline creates waves when someone jumps, the bell’s vibration creates sound waves in the air.

Activity 10.1: Take a metal plate (or a pan). Hang it at a convenient place in such a way that it does not touch any wall. Now strike it with a stick. Do you hear a sound? Touch the plate or pan gently with your finger. Do you feel the vibrations?

In this experiment, when you strike the metal plate with a stick, it vibrates and produces a sound. The sound occurs because the vibrating plate creates waves in the air. Touching the plate after hitting it allows you to feel the vibrations. These vibrations are physical movements in the material that produce sound waves.

Consider a water wave. When you throw a stone into a pond, it creates ripples. Similarly, the vibrations of the plate create sound waves that travel through the air and reach your ears.

As you learnt in Class VII, the to and fro or back and forth motion of an object is called vibration. When a tightly stretched band is plucked, it vibrates and produces sound. When it stops vibrating, it does not produce any sound.

Vibration as a concept refers to the rapid oscillation of an object around a central position. When an object vibrates, it moves up and down or back and forth. An example of this is a rubber band; when plucked, it vibrates and creates sound waves. However, once it stops vibrating, the sound ceases because no more waves are being produced.

Imagine plucking a guitar string. The string vibrates when plucked, producing music. Once you stop plucking, the sound disappears, just like silence fills the room after the music ends.

Activity 10.3: Take a metal dish. Pour water in it. Strike it at its edge with a spoon. Do you hear a sound? Again strike the dish and then touch it. Can you feel the dish vibrating? Strike the dish again. Look at the surface of water. Do you see any waves there? Now hold the dish. What change do you observe on the surface of water?

This activity helps visualize sound through the medium of water. When the metal dish is struck, it vibrates, causing sound waves to travel through the air and water. As it vibrates, the energy is transferred to the water, creating visible ripples on the surface. These ripples are the result of the vibrations that cause the water particles to move.

Think of dropping a pebble into a calm lake. Just like the ripples spread out across the surface of the water, sound waves spread through the air when an object vibrates.

We see that a vibrating object produces sound. In some cases, the vibrations are easily visible to us. But in most cases, their amplitude is so small that we cannot see them. However, we can feel them. For example, a violin string when bowed vibrates to produce sound.

Different musical instruments produce sound through their unique vibrating parts. In a violin, the string vibrates when played with a bow, creating sound. While some instruments show visible vibrations (like a drum membrane when struck), others have vibrations too fine to see but can still be felt. The different characteristics of these vibrations lead to the creation of various musical tones.

Consider the vibrations when you strum a guitar. While playing, you can feel the strings vibrating beneath your fingers, producing melodious sound that fills the room, which highlights how vibrations create music in different forms.

--