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Sound

ICSE Class 7 Physics • Chapter 6

Chapter Overview

Sound plays a vital role in our lives, helping us communicate. In this chapter, we will learn how sound is produced by vibrations, how it requires a material medium to propagate, the characteristics of a sound wave, and how different types of musical instruments function.

6.1 Production of Sound

Sound is a form of energy that produces the sensation of hearing in our ears.

Vibration: The rapid back-and-forth movement of an object about a central mean position is called a vibration. Sound is strictly produced by vibrating bodies.

When you speak, your vocal cords inside your voice box (larynx) vibrate rapidly to produce your voice. When a school bell is struck, the metal plate vibrates wildly to produce an energetic ringing sound.

6.2 Propagation of Sound

Unlike light energy, which can travel through the vacuum of space, sound always strictly requires a physical material medium (such as solid, liquid, or gas) to travel from the source to our ears.

The Bell Jar Experiment

This famous experiment proves that sound cannot travel in a vacuum.

An electric bell is securely suspended inside an airtight glass bell jar.
The jar is connected to a powerful vacuum pump located outside.
When the bell circuit is switched on, you can clearly hear the loud ringing sound.
As the vacuum pump gradually sucks the air out of the jar, the ringing sound physically becomes fainter and fainter.
Finally, when all the air is removed (a vacuum is created), you can still see the metallic hammer hitting the gong, but you explicitly hear absolutely no sound whatsoever.

Conclusion: Sound waves fundamentally require air or any other material medium for their propagation.

AI Image Prompt: A clean 3D illustration of the Bell Jar Experiment. A glass dome sits on a shiny metal plate. Inside is an electric bell with wires. The vacuum pump at the bottom is physically sucking red air particles out of the jar. Text "No Air = No Sound" is written boldly.

6.3 Characteristics of a Sound Wave

Sound travels through a medium in the form of longitudinal waves (forming compressions and rarefactions).

Amplitude: The maximum physical displacement of the vibrating particle from its stable mean position. Loudness directly depends on amplitude. Higher amplitude produces louder sounds.
Time Period ($T$): The specific time taken by a vibrating body to complete precisely one full vibration.
Frequency ($f$): The exact number of vibrations completed by a body in one single second. Its unit is the Hertz (Hz). Pitch directly depends on frequency. Higher frequency produces a shriller, higher-pitched sound.

Practice Zone

Q1. Astronauts on the Moon use special radio communication to talk. Why can they not simply yell at each other?

Ans: The Moon is completely devoid of any atmospheric air (it is a literal vacuum). Since sound requires a material medium to propagate, their voices cannot travel through the empty space between them. Radio waves, however, are electromagnetic and do not require a physical medium.

Q2. What controls the loudness of a sound?

Ans: The physical amplitude of the vibration completely controls the loudness.