CBSE Class 9 Science Chapter 12: Sound NCERT Solutions

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NCERT Solutions PDF Class 9 PDF

This chapter delves into the fundamental concepts of sound, exploring how it is produced, how it travels through different mediums, and its characteristics. The NCERT Solutions for Class 9 Science, Chapter 12: Sound, provide detailed explanations for each question, clarifying topics such as the production of sound by vibrating objects, the nature of sound waves as mechanical waves, and the necessity of a medium for sound propagation. It also addresses why sound cannot travel in a vacuum, using the example of the moon. These solutions are designed to help students grasp the core principles of acoustics and prepare effectively for their examinations by offering clear, step-by-step explanations.

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Quick info

BoardCBSE
ClassClass 9
SubjectScience
Session2026
LanguageEnglish
TypeNCERT Solutions
ChapterChapter 12: Sound

Chapter summary

Chapter 12 of the NCERT Class 9 Science textbook focuses on Sound. The provided solutions explain the mechanism of sound production through vibrations, the definition and necessity of a medium for mechanical waves like sound, and the absence of sound in a vacuum. Key concepts covered include compression, rarefaction, and the transmission of sound energy.

Learning outcomes

  • Understand how sound is produced by vibrating objects.
  • Explain why sound waves are classified as mechanical waves.
  • Describe the process of sound propagation through a medium.
  • Recognize that sound requires a medium and cannot travel in a vacuum.

Topics covered

Paper topics

  • Sound Production
  • Vibrating Objects
  • Sound Waves
  • Mechanical Waves
  • Medium for Sound
  • Compression
  • Rarefaction
  • Sound Propagation
  • Sound in Vacuum
  • Moon Environment

Important topics

  • Sound Production by Vibrations
  • Mechanical Nature of Sound Waves
  • Requirement of a Medium for Sound Travel
  • Sound Propagation (Compression & Rarefaction)
  • Absence of Sound in Vacuum

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Questions and Solutions

Question 1

How does the sound produced by a vibrating object in a medium reach your ear?
Solution: When an object vibrates, it causes the particles of the surrounding medium (like air) to vibrate as well. These vibrations create disturbances in the medium, resulting in alternating regions of compression (where particles are crowded together and pressure is high) and rarefaction (where particles are spread apart and pressure is low). These compressions and rarefactions travel outwards from the source as a wave. As the wave reaches your ear, it causes your eardrum to vibrate, which is then interpreted by your brain as sound. The energy of the vibration is transferred from one particle to the next through the medium.

Question 1

Explain how sound is produced by your school bell?
Solution: When your school bell is struck, its surface vibrates. These vibrations are the source of sound. The vibrating bell causes the surrounding air particles to vibrate. These vibrations are then transmitted through the air as sound waves, traveling to our ears. When these waves reach our ears, they cause our eardrum to vibrate, and our brain interprets these vibrations as the sound of the bell.

Question 2

Why are sound waves called mechanical waves?
Solution: Sound waves are called mechanical waves because they require a material medium to travel from one place to another. This medium can be a solid, a liquid, or a gas. The particles of the medium vibrate and transfer the sound energy. Sound waves cannot travel through a vacuum because there are no particles to vibrate and carry the wave.

Question 3

Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?
Solution: No, you will not be able to hear any sound produced by your friend on the moon. This is because the moon has virtually no atmosphere, meaning there is no medium like air present. Sound waves are mechanical waves and require a medium to travel. Since there is a vacuum on the moon, sound cannot propagate, and therefore, no sound can be heard.

Common mistakes

  • Confusing mechanical waves with other types of waves.
  • Not understanding the role of compression and rarefaction in sound travel.
  • Assuming sound can travel through a vacuum.

Revision tips

  • Review the definitions of compression and rarefaction.
  • Focus on the 'why' behind sound being a mechanical wave.
  • Use the moon example to solidify the concept of sound needing a medium.

Practice MCQs

Q1. What causes sound to be produced by a vibrating object?

Q2. Why are sound waves called mechanical waves?

Q3. On the moon, can you hear your friend speaking?

Q4. What is a region of low pressure in a sound wave called?

Frequently asked questions

How is sound produced according to NCERT Class 9 Science?

Sound is produced by vibrating objects. When an object vibrates, it disturbs the particles in the surrounding medium, causing them to vibrate and transfer energy.

What are mechanical waves and why does sound fall into this category?

Mechanical waves are waves that require a material medium (like air, water, or solids) to propagate. Sound waves are mechanical because they need particles of a medium to vibrate and carry the sound energy.

Can sound travel through space or on the moon?

No, sound cannot travel through space or on the moon because these are essentially vacuums, lacking the necessary medium (like air) for sound waves to propagate.

What happens to the air particles when sound travels through them?

When sound travels through air, the particles are pushed together to form compressions (regions of higher pressure) and then spread apart to form rarefactions (regions of lower pressure), creating a wave motion.

How do these NCERT Solutions help students?

These solutions provide clear, step-by-step explanations for each question in Chapter 12, helping students understand the concepts of sound production, propagation, and its properties for better exam preparation.

Important links

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