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 the nature of sound waves, the formation of compressions and rarefactions, the transfer of energy through a medium, and the graphical representation of sound waves. It also shows why sound is classified as a longitudinal mechanical wave. 

Sound as a Longitudinal Wave

Sound travels through a material medium in the form of longitudinal waves. In these waves, the particles of the medium vibrate back and forth in the same direction as the wave travels.

Definition

A longitudinal wave is a wave in which the particles of the medium vibrate parallel to the direction of wave propagation. 

As sound travels through a medium, it produces alternate regions of high and low pressure.

Compression

• Definition – A region where particles are closely packed together.
• Pressure – High pressure and high density.
• Energy – More energy is concentrated in this region.

Rarefaction

• Definition – A region where particles are spread farther apart.
• Pressure – Low pressure and low density.
• Energy – Less energy is present compared to compression.

How Sound Travels

• The vibrating source pushes nearby particles to form a compression.
• The particles then move back, creating a rarefaction.
• A series of compressions and rarefactions moves through the medium.
• Energy is transferred from one particle to another.
• The particles themselves do not travel with the wave.

Transfer of Energy

Sound waves transfer energy without transferring matter. Each particle of the medium only vibrates about its mean position and passes the disturbance to the next particle. 

Graphical Representation of Sound Waves

Sound waves can be represented graphically to understand the arrangement of compressions, rarefactions and wave properties.

Density–Distance Graph

• Shows the variation of particle density with distance.
• Peaks represent compressions.
• Valleys represent rarefactions.
• Helps determine the wavelength of the sound wave.

Density–Time Graph

• Shows how particle density changes with time.
• Represents the periodic nature of sound waves.
• Helps explain oscillations and time period.

Features of a Longitudinal Wave

Importance of Graphical Representation

• Helps visualise compressions and rarefactions.
• Makes it easier to understand wavelength and time period.
• Useful for analysing sound waves in physics.
• Forms the basis for solving numerical problems related to waves.

ATP Education Concept Builder

Do not confuse the movement of the wave with the movement of the particles. In a longitudinal wave, the wave moves forward, but the particles only vibrate back and forth about their mean positions. This is why sound transfers energy but not matter.

ATP Education Exam Booster

• Remember that sound is a longitudinal mechanical wave.
• Differentiate clearly between compression and rarefaction.
• Understand why sound transfers energy but not matter.
• Learn the significance of density–distance and density–time graphs.
• Practise drawing and labelling compressions and rarefactions.
• CBSE competency-based questions often ask students to explain the propagation of sound using particle motion.