CBSE Class 9 Science Exemplar Chapter 11: Sound NCERT Solutions

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NCERT Solutions PDF Class 9 PDF

This chapter delves into the fundamental concepts of sound, a crucial topic in Class 9 Science. The NCERT Solutions provided here offer detailed explanations and step-by-step solutions for various questions, including multiple-choice questions and conceptual problems. Students will explore the nature of sound waves, factors affecting loudness and pitch, the role of amplitude and frequency, and applications like SONAR. The solutions clarify how sound travels, the difference between audible, infrasonic, and ultrasonic waves, and how to interpret wave characteristics from graphs. These resources are designed to help students understand complex ideas, solve problems accurately, and build a strong foundation for their upcoming examinations, aiding in effective revision and concept reinforcement.

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BoardCBSE
ClassClass 9
SubjectScience Exemplar
Session2026
LanguageEnglish
TypeNCERT Solutions
ChapterChapter 11

Chapter summary

Chapter 11 focuses on the physics of sound for Class 9 Science. The NCERT Solutions cover key concepts such as the definition of sound, its propagation through mediums, and the characteristics of sound waves like loudness and pitch. It addresses the distinction between different types of sound waves (audible, infrasonic, ultrasonic) and their applications, for instance, in SONAR. The solutions also explain how to identify wave properties like wavelength from graphical representations and the relationship between sound intensity and amplitude. This chapter's solutions are essential for grasping the basic principles of acoustics.

Learning outcomes

  • Understand the nature of sound as a mixture of frequencies.
  • Differentiate between loudness and pitch and the factors affecting them.
  • Identify the type of sound waves used in applications like SONAR.
  • Explain the mechanism of sound propagation through a medium.
  • Interpret graphical representations of sound waves to determine wavelength.
  • Relate the amplitude of a sound wave to its loudness.

Topics covered

Paper topics

  • Nature of Sound
  • Sound Propagation
  • Loudness and Amplitude
  • Pitch and Frequency
  • Wave Characteristics (Wavelength)
  • Types of Sound Waves (Infrasonic, Audible, Ultrasonic)
  • Applications of Sound Waves (SONAR)
  • Wave Motion
  • Graphical Representation of Waves

Important topics

  • Loudness and Amplitude relationship
  • Pitch and Frequency relationship
  • Sound Propagation mechanism
  • Types of Sound Waves and their uses
  • Interpreting Wave Diagrams

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Questions and Solutions

Multiple Choice Questions

1. What is the nature of sound?
  1. A note is a sound
  2. of mixture of several frequencies
  3. of mixture of two frequencies only
  4. of a single frequency
  5. always unpleasant to listen
Solution: The correct option is (b) of mixture of several frequencies. Sound is typically a complex wave that results from the combination of multiple frequencies, which contributes to its unique quality or timbre. While a pure tone might consist of a single frequency, most sounds we encounter are mixtures.
2. A key of a mechanical piano is struck gently and then struck again, but much harder this time. In the second case:
  1. sound will be louder but pitch will not be different
  2. sound will be louder and pitch will also be higher
  3. sound will be louder but pitch will be lower
  4. both loudness and pitch will remain unaffected
Solution: The correct option is (a) sound will be louder but pitch will not be different. When a piano key is struck harder, the force applied increases the amplitude of the sound wave produced. A larger amplitude corresponds to a louder sound. The pitch of the sound, however, is determined by the frequency of vibration of the string, which is primarily dependent on the string's properties (length, tension, mass) and not the force with which it is struck. Therefore, the pitch remains the same.
3. In SONAR, we use:
  1. ultrasonic waves
  2. infrasonic waves
  3. radio waves
  4. audible sound waves
Solution: The correct option is (a) ultrasonic waves. SONAR (Sound Navigation and Ranging) systems emit high-frequency sound waves, known as ultrasonic waves, into the water. These waves travel through the water, reflect off objects, and return to the SONAR device. By analyzing the time it takes for the waves to return, the system can determine the distance and location of objects. Ultrasonic waves are used because their high frequency allows for better directionality and detection over distances.
4. Sound travels in air if:
  1. particles of medium travel from one place to another
  2. there is no moisture in the atmosphere
  3. disturbance moves
  4. both particles as well as disturbance travel from one place to another.
Solution: The correct option is (c) disturbance moves. Sound is a form of energy that travels as waves. When sound travels through a medium like air, it is the disturbance created by the vibration of the source that propagates from one place to another. The particles of the medium themselves oscillate about their mean positions but do not travel along with the wave. Therefore, sound travels as a moving disturbance.
5. When we change feeble sound to loud sound, we increase its:
  1. frequency
  2. amplitude
  3. velocity
  4. wavelength
Solution: The correct option is (b) amplitude. The loudness of a sound is directly proportional to the square of its amplitude. A feeble sound has a small amplitude, while a loud sound has a large amplitude. When we increase the loudness of a sound, we are essentially increasing the amplitude of the sound wave, which means the particles of the medium vibrate with a greater displacement from their equilibrium positions.
6. In the curve (Fig.12.1), half the wavelength is represented by:
  1. A B
  2. B D
  3. D E
  4. A E

Fig. 12.1

Solution: The correct option is (b) B D. In a wave diagram, the wavelength (\lambda) is the distance between two consecutive points in the same phase, such as two crests or two troughs. In the given figure (assuming it shows a wave pattern), the distance between point B (a crest) and point D (the next trough) represents half of a complete wavelength. A full wavelength would be the distance between B and the next crest, or D and the next trough.
7. Earthquakes produce which kind of sound before the main shock wave begins?
  1. ultrasound
  2. infrasound
  3. audible sound
  4. none of the above
Solution: The correct option is (b) infrasound. Earthquakes often generate infrasound, which are sound waves with frequencies below the range of human hearing (typically below 20 Hz). These low-frequency waves can travel long distances through the Earth's crust and atmosphere and are sometimes detected before the main seismic shock waves arrive.

Common mistakes

  • Confusing the factors that determine pitch and loudness.
  • Incorrectly identifying the type of sound waves used in specific technologies.
  • Misinterpreting graphical representations of waves, especially regarding wavelength.
  • Not understanding that sound travels as a disturbance, not by particle transfer.

Revision tips

  • Focus on understanding the relationship between physical actions (striking a key harder) and sound properties (loudness, pitch).
  • Memorize the types of sound waves (infrasonic, audible, ultrasonic) and their applications.
  • Practice interpreting wave diagrams to find wavelength and relate it to other wave properties.
  • Review the fundamental concept that sound propagates as a disturbance moving through a medium.

Practice MCQs

Q1. What is the nature of sound?

Q2. When a piano key is struck harder, what changes?

Q3. What type of waves are used in SONAR?

Q4. How does sound travel in air?

Q5. When a feeble sound is made louder, what is increased?

Q6. In a wave diagram, what does the distance between two consecutive crests or troughs represent?

Q7. What kind of sound is produced by earthquakes before the main shock wave?

Frequently asked questions

What is the main topic of CBSE Class 9 Science Exemplar Chapter 11?

Chapter 11 of the CBSE Class 9 Science Exemplar focuses on the topic of Sound, covering its nature, propagation, characteristics like loudness and pitch, and related applications.

How do these NCERT Solutions help students?

These solutions provide clear, step-by-step explanations for all questions in the chapter, helping students understand complex concepts, solve problems accurately, and prepare effectively for their exams.

What is the difference between loudness and pitch?

Loudness is related to the amplitude of the sound wave, while pitch is related to the frequency of the sound wave. A larger amplitude means a louder sound, and a higher frequency means a higher pitch.

What are ultrasonic waves and where are they used?

Ultrasonic waves are sound waves with frequencies higher than the upper audible limit of human hearing. They are used in applications like SONAR for navigation and detection.

How does sound travel through a medium?

Sound travels through a medium as a disturbance. The particles of the medium vibrate about their mean positions, transferring energy and causing the sound wave to propagate.

What does the wavelength represent in a sound wave?

The wavelength represents the distance between two consecutive identical points on a wave, such as two crests or two troughs. It is a measure of the spatial extent of one complete wave cycle.

Important links

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