NCERT Solutions for Class 9 Science Chapter 11 Sound

NCERT Solutions for Class 9 Science Chapter 11 Sound help students understand the concepts of sound waves, their properties, and applications. This chapter explains how sound is produced, how it travels through different media, and important phenomena like reflection of sound, echo, and ultrasound.

Prepared according to the latest CBSE Class 9 Science syllabus, these solutions provide clear and structured answers to help students build strong concepts and perform well in exams.

NCERT Solutions for Class 9 Science Chapter 11 Sound

Sound
Easy
Q.
What is sound and how is it produced?
Sound
Medium
Q.
A stone is dropped from the top of a tower 500 m high into a pond of water at the base of the tower. When is the splash heard at the top? Given, g = 10 m s–2 and speed of sound = 340 m s–1.
Sound
Medium
Q.
Explain how defects in a metal block can be detected using ultrasound.
Sound
Medium
Q.
A sonar device on a submarine sends out a signal and receives an echo 5 s later. Calculate the speed of sound in water if the distance of the object from the submarine is 3625 m.
Sound
Medium
Q.
Explain the working and application of a sonar.
Sound
Difficult
Q.
How is ultrasound used for cleaning?
Sound
Medium
Q.
Explain how bats use ultrasound to catch a prey.
Sound
Easy
Q.
What is loudness of sound? What factors does it depend on?
Sound
Easy
Q.
What is reverberation? How can it be reduced?
Sound
Medium
Q.
A sound wave travels at a speed of 339 m s–1. If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible?
Sound
Medium
Q.
Give two practical applications of reflection of sound waves.
Sound
Medium
Q.
Describe with the help of a diagram, how compressions and rarefactions are produced in air near a source of sound.
Sound
Medium
Q.
When a sound is reflected from a distant object, an echo is produced. Let the distance between the reflecting surface and the source of sound production remains the same. Do you hear echo sound on a hotter day?
Sound
Easy
Q.
Does sound follow the same law of reflection as light does? Explain
Sound
Medium
Q.
The frequency of a source of sound is 100 Hz.  How many times does it vibrate in a minute?
Sound
Medium
Q.
Two children are at opposite ends of an aluminium rod. One strikes the end of the rod with a stone. Find the ratio of times taken by the sound wave in air and in aluminium to reach the second child.
Sound
Difficult
Q.
A person has a hearing range from 20 Hz to 20 kHz. What are the typical wavelengths of sound waves in air corresponding to these two frequencies? Take the speed of sound in air as 344 m s–1.
Sound
Easy
Q.
Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why?
Sound
Medium
Q.
Which characteristic of the sound helps you to identify your friend by his voice while sitting with others in a dark room?
Sound
Easy
Q.
Why is sound wave called a longitudinal wave?
Sound
Medium
Q.
Cite an experiment to show that sound needs a material medium for its propagation.
Sound
Medium
Q.
Explain how the human ear works.

NCERT Solutions for Class 9 Science Chapter 11 Sound

Q1. What is sound and how is it produced?

Ans: Sound is produced due to vibrations of objects, which create waves in a medium.

Q2. How are compressions and rarefactions produced?

Ans: Vibrating objects create high-pressure (compression) and low-pressure (rarefaction) regions in air.

Q3. Show that sound needs a medium

Ans: In a vacuum (bell jar experiment), sound is not heard, proving that sound needs a material medium.

Q4. Why is sound a longitudinal wave?

Ans: Because particles vibrate parallel to the direction of wave propagation.

Q5. Which property helps identify a person’s voice?

Ans: Quality (timbre) of sound.

Q6. Why is thunder heard after lightning?

Ans: Because light travels faster than sound.

Q7. Does sound follow reflection laws?

Ans: Yes, sound follows laws of reflection similar to light.

Q8. Why echo is not heard on hot days?

Ans: Speed of sound increases, reducing time gap, so echo may not be heard.

Q9. Applications of reflection of sound

Ans: Soundboards and stethoscopes.

Q10. What is reverberation?

Ans: Persistence of sound due to repeated reflections; reduced using sound-absorbing materials.

Q11. What is loudness?

Ans: Loudness depends on amplitude of sound waves.

Q12. How do bats use ultrasound?

Ans: Bats use reflected ultrasonic waves to locate prey.

Q13. How is ultrasound used for cleaning?

Ans: High-frequency waves remove dirt from objects.

Q14. What is SONAR?

Ans: A device using ultrasound to detect underwater objects and measure depth.

Q15. How are defects detected using ultrasound?

Ans: Ultrasound reflects from defects, helping detect flaws in materials.

Q16. How does the human ear work?

Ans: Ear converts sound vibrations into electrical signals sent to brain.

FAQs – Class 9 Science Chapter 11 Sound

Q1. What is sound?
Sound is a form of energy produced by vibrations.

Q2. Can sound travel in vacuum?
No, sound needs a material medium.

Q3. What are sound waves?
Sound waves are longitudinal waves.

Q4. What is echo?
Echo is the reflection of sound waves.

Q5. Why is this chapter important for exams?
This chapter includes numericals, theory, and application-based questions, frequently asked in exams.