Science Exploration Notes Class 9 English Medium 2026-27

Science Exploration Chapter 10. Sound Waves: Characteristics and Applications

CBSE Notes New Syllabus: Class 9

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Chapter 10. Sound Waves: Characteristics and Applications

This section explains how sound is produced, the sources of sound and how it propagates through different media. It also demonstrates why sound requires a material medium for propagation and why it cannot travel through vacuum. 

Production of Sound

Sound is produced whenever an object vibrates. These vibrations create disturbances in the surrounding medium, which travel as sound waves and reach our ears.

Definition

Sound is a form of energy produced due to the vibration of objects.

Sources of Sound

• Vibrating Strings – Guitar, violin and sitar produce sound when their strings vibrate.
• Vibrating Membranes – Drums and tablas produce sound due to vibrating stretched membranes.
• Vibrating Air Columns – Flutes, trumpets and whistles produce sound through vibrating air columns.
• Vocal Cords – Humans produce sound through the vibration of vocal cords present in the larynx.

Characteristics of Sound Production

• Vibration is Essential – No vibration means no sound.
• Energy Transfer – Vibrating objects transfer energy to the surrounding medium.
• Frequency of Vibration – Faster vibrations produce higher-pitched sounds.
• Amplitude of Vibration – Greater amplitude produces louder sounds.

Propagation of Sound

After being produced, sound travels from one place to another through a material medium. During propagation, particles of the medium vibrate and transfer energy without moving from their original positions.

Mediums Through Which Sound Travels

• Solids – Sound travels fastest in solids because particles are closely packed.
• Liquids – Sound travels slower than in solids but faster than in gases.
• Gases – Sound travels slowest in gases due to larger distances between particles.

Sound Needs a Material Medium

Sound is a mechanical wave and therefore requires a material medium such as air, water or solids to propagate.

• Particles of the medium transfer sound energy.
• Without particles, sound cannot propagate.
• The medium may be solid, liquid or gas.

Why Sound Cannot Travel Through Vacuum

A vacuum contains no particles to transfer vibrations. Therefore, sound cannot travel through vacuum.

Bell Jar Experiment

• A ringing electric bell is placed inside a glass bell jar.
• Initially, the sound is heard clearly because air is present.
• As air is gradually removed using a vacuum pump, the sound becomes weaker.
• When almost all the air is removed, the bell continues to vibrate but no sound is heard.
• This experiment proves that sound requires a material medium for propagation. 

Applications of Sound Propagation

• Communication through speech and telephones.
• Musical instruments.
• Public address systems.
• Medical diagnostic instruments.
• Industrial sound detection devices.

ATP Education Concept Builder

Always remember that vibration produces sound, but a material medium carries sound. These are two different concepts. A vibrating object produces sound, while particles of the medium help it travel from one place to another.

ATP Education Exam Booster

• Remember that sound is produced by vibrations.
• Learn the different sources of sound with examples.
• Understand why sound requires a material medium.
• Explain the Bell Jar Experiment clearly.
• Remember that sound cannot travel through vacuum.
• CBSE competency-based questions often ask students to explain why astronauts cannot hear each other directly in space.