CBSE Class 9 Science Chapter 10: Sound Waves: Characteristics and Applications — Assertion & Reason
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The following Assertion and Reason questions are based on Chapter 10: Sound Waves: Characteristics and Applications from the NCERT Class 9 Science textbook. Each question has four options - read both the Assertion (A) and Reason (R) carefully before selecting your answer.
How to attempt Assertion & Reason questions:
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is NOT the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
(a) Both A and R are true, and R is the correct explanation of A.
(b) Both A and R are true, but R is NOT the correct explanation of A.
(c) A is true, but R is false.
(d) A is false, but R is true.
Assertion & Reason Questions
1
Assertion (A): A stretched rubber band produces sound only while it is vibrating.
Reason (R): Sound is produced by vibrating objects.
Reason (R): Sound is produced by vibrating objects.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. When the vibration stops, the source no longer creates the disturbances responsible for sound.
2
Assertion (A): Human vocal cords vibrate to produce sound during speaking and singing.
Reason (R): The tongue, lips, mouth, and nasal cavity help convert the sound into speech or music.
Reason (R): The tongue, lips, mouth, and nasal cavity help convert the sound into speech or music.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. The reason describes modification of the sound after production; it does not explain why vibrating vocal cords produce it.
3
Assertion (A): Sound in air is a longitudinal wave.
Reason (R): Air particles travel continuously from the source to the listener along with the sound wave.
Reason (R): Air particles travel continuously from the source to the listener along with the sound wave.
Answer: (c)
A is true, but R is false. The particles oscillate about their mean positions; the disturbance, rather than the particles themselves, travels forward.
4
Assertion (A): Sound can travel through the vacuum of outer space.
Reason (R): Electromagnetic waves can travel through a vacuum without a material medium.
Reason (R): Electromagnetic waves can travel through a vacuum without a material medium.
Answer: (d)
A is false, but R is true. Sound is a mechanical wave and requires a medium, unlike electromagnetic radiation.
5
Assertion (A): Touching a vibrating tuning fork to water produces visible disturbances on the water surface.
Reason (R): The vibrating prongs transfer energy to the water and set it into motion.
Reason (R): The vibrating prongs transfer energy to the water and set it into motion.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. The resulting surface disturbance provides evidence that the prongs are vibrating.
6
Assertion (A): Sound can propagate through solids, liquids, and gases.
Reason (R): The speed of sound is generally greatest in solids and least in gases.
Reason (R): The speed of sound is generally greatest in solids and least in gases.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Both statements are correct, but the relative speeds do not explain the basic ability of all three media to transmit sound.
7
Assertion (A): A compression in a sound wave is a region of relatively high pressure and density.
Reason (R): Particles in a compression are farther apart than their average separation.
Reason (R): Particles in a compression are farther apart than their average separation.
Answer: (c)
A is true, but R is false. Particles are closer together in a compression; greater separation characterises a rarefaction.
8
Assertion (A): A rarefaction is a region of maximum density in a longitudinal sound wave.
Reason (R): Particles are more widely separated in a rarefaction than in a compression.
Reason (R): Particles are more widely separated in a rarefaction than in a compression.
Answer: (d)
A is false, but R is true. A rarefaction has relatively low pressure and density.
9
Assertion (A): Frequency is the number of density oscillations passing a fixed point per unit time.
Reason (R): Twenty oscillations in four seconds correspond to a frequency of 5 Hz.
Reason (R): Twenty oscillations in four seconds correspond to a frequency of 5 Hz.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. Frequency equals number of oscillations divided by time, so 20/4 = 5 s-1 or 5 Hz.
10
Assertion (A): The time period of a sound wave is inversely related to its frequency.
Reason (R): Wavelength is the distance between two consecutive compressions or two consecutive rarefactions.
Reason (R): Wavelength is the distance between two consecutive compressions or two consecutive rarefactions.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Both definitions are correct, but the wavelength definition does not explain T = 1/f.
11
Assertion (A): The speed of sound depends on the medium and can change with temperature.
Reason (R): When sound enters a new medium, its source frequency must change to match the new speed.
Reason (R): When sound enters a new medium, its source frequency must change to match the new speed.
Answer: (c)
A is true, but R is false. The frequency is fixed by the source; a speed change is accompanied by a wavelength change.
12
Assertion (A): A louder sound necessarily has a higher frequency than a softer sound.
Reason (R): A larger amplitude is generally perceived as a louder sound.
Reason (R): A larger amplitude is generally perceived as a louder sound.
Answer: (d)
A is false, but R is true. Frequency mainly affects pitch, whereas amplitude is associated with loudness.
13
Assertion (A): For a wave of fixed speed, increasing frequency decreases wavelength.
Reason (R): Wave speed is related to frequency and wavelength by v = f lambda.
Reason (R): Wave speed is related to frequency and wavelength by v = f lambda.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. Rearranging the relation gives lambda = v/f, establishing the inverse relation at fixed speed.
14
Assertion (A): Sound intensity is the sound energy passing per unit area per unit time perpendicular to the direction of propagation.
Reason (R): Amplitude is the maximum change in density from the average density of the medium.
Reason (R): Amplitude is the maximum change in density from the average density of the medium.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Both are correct definitions, but the amplitude definition alone does not explain the definition of intensity.
15
Assertion (A): The pitch perceived by a human listener depends mainly on the frequency of sound.
Reason (R): The loudness of a sound is determined only by its frequency.
Reason (R): The loudness of a sound is determined only by its frequency.
Answer: (c)
A is true, but R is false. Loudness is primarily related to amplitude and intensity, not solely to frequency.
16
Assertion (A): Humans normally hear sound waves with frequencies below 20 Hz.
Reason (R): Sound waves below 20 Hz are called infrasonic waves.
Reason (R): Sound waves below 20 Hz are called infrasonic waves.
Answer: (d)
A is false, but R is true. Frequencies below 20 Hz normally lie below the human audible range.
17
Assertion (A): A sufficiently delayed reflected sound can be heard as a distinct echo.
Reason (R): The reflected sound must reach the ear after enough delay to be distinguished from the original sound.
Reason (R): The reflected sound must reach the ear after enough delay to be distinguished from the original sound.
Answer: (a)
Both A and R are true, and R is the correct explanation of A. A short delay blends the sounds, while a sufficient delay allows separate perception.
18
Assertion (A): Reverberation is the persistence of sound caused by repeated reflections in an enclosure.
Reason (R): Curtains and other soft materials absorb sound and help reduce reverberation.
Reason (R): Curtains and other soft materials absorb sound and help reduce reverberation.
Answer: (b)
Both A and R are true, but R is NOT the correct explanation of A. Absorbing materials reduce the effect; they are not the cause of repeated reflection.
19
Assertion (A): Ultrasonic waves have frequencies above 20 kHz.
Reason (R): Infrasonic waves are used by sonar to detect underwater objects.
Reason (R): Infrasonic waves are used by sonar to detect underwater objects.
Answer: (c)
A is true, but R is false. Sonar uses ultrasonic waves, not infrasonic waves.
20
Assertion (A): Astronauts outside a spacecraft can hear each other's voices directly through empty space without communication equipment.
Reason (R): Sound needs a material medium for propagation.
Reason (R): Sound needs a material medium for propagation.
Answer: (d)
A is false, but R is true. Because space is nearly a vacuum, astronauts require radio communication systems.
❓ Frequently Asked Questions
What is covered in CBSE Class 9 Science Chapter 10 Sound Waves: Characteristics and Applications?
This chapter covers all key topics from Sound Waves: Characteristics and Applications as per CBSE 2026-27 syllabus.
Is this Assertion & Reason useful for CBSE board exams?
Yes, designed for CBSE Class 9 board exam preparation covering the complete syllabus.
Are these CBSE Class 9 Science Assertion & Reason updated for 2026-27?
Yes, all content at eBookPublisher is updated as per the latest 2026-27 CBSE syllabus.
How many chapters are in CBSE Class 9 Science?
All chapters of CBSE Class 9 Science are covered at eBookPublisher with free Assertion & Reason for each chapter.
Can I study Sound Waves: Characteristics and Applications online for free?
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