Sound Class 9 Science Chapter 11 Important Questions, Download PDF

Class 9 Science Chapter 11 Important Questions:  Chapter 11 of Class 9 Science is all about sound. It talks about what sound is, how it is made and how it travels. Some important questions to study include the main features of sound, like pitch (how high or low a sound is), loudness (how loud or soft a sound is) and timbre (the quality of the sound).

Important Questions Class 9 Science Chapter 11 Sound

1. Sound waves are classified as:
   (A) Transverse waves
   (B) Longitudinal waves
   (C) Electromagnetic waves
   (D) None of the above
   Ans: (B) Longitudinal waves

2. Which of the following is not a method of sound production?
   (A) Vibrations of vocal cords
   (B) Striking a tuning fork
   (C) Plucking a guitar string
   (D) Refracting light
   Ans: (D) Refracting light

3. The transmission of sound through a medium is called:
   (A) Production of sound
   (B) Propagation of sound
   (C) Reflection of sound
   (D) Range of hearing
   Ans: (B) Propagation of sound

4. The speed of sound is highest in which of the following mediums?
   (A) Air
   (B) Water
   (C) Solid
   (D) Vacuum
   Ans: (C) Solid

5. Which of the following best describes a sound wave?
   (A) Transverse motion of particles
   (B) Oscillation of electromagnetic fields
   (C) Longitudinal motion of particles
   (D) Movement of light particles
   Ans: (C) Longitudinal motion of particles

6. When a sound wave strikes a hard surface and bounces back, it is called:
   (A) Refraction
   (B) Absorption
   (C) Reflection
   (D) Diffraction
   Ans: (C) Reflection

7. What happens when you shout in a canyon and hear your own voice come back to you?
   (A) Reflection of sound
   (B) Echo
   (C) Reverberation
   (D) Range of hearing
   Ans: (B) Echo

8. The persistence of sound after the original sound has stopped is called:
   (A) Echo
   (B) Reverberation
   (C) Reflection
   (D) Propagation
   Ans: (B) Reverberation

9. Multiple reflections of sound are used in:
   (A) Radar systems
   (B) Sonar systems
   (C) Ultrasound imaging
   (D) All of the above
   Ans: (D) All of the above

10. The range of frequencies that can be heard by a human ear is approximately:
    (A) 1 Hz to 100 Hz
    (B) 100 Hz to 1000 Hz
    (C) 20 Hz to 20,000 Hz
    (D) 20 Hz to 200 Hz
    Ans: (C) 20 Hz to 20,000 Hz

11. Guess which sound has a higher pitch: guitar or car horn?
    Sol: Our ear reacts to the wavelength of the sound known as pitch. The higher the frequency, the higher the pitch will be. Comparatively, the guitar works on a higher frequency, so the pitch of the guitar will be higher.

12. In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature?
    Sol: The utmost speed of sound is in solids which decrease in liquids. However, sound travels much more slowly in the air. Amongst the given options, the fastest medium for sound is iron.

13. What is the audible range of the average human ear?
    Sol: For the normal human ear, the range of frequencies varies from 20 Hz to 20,000 Hz and this is considered an audible range. Lower than 20 Hz and higher than 20 kHz is not audible for humans.

14. Which characteristic of the sound helps you to identify your friend by his voice while sitting with others in a dark room?
    Sol: Timbre and pitch are the characteristics of sound which help to identify the sound of different voices. Therefore, because of timbre and pitch, a person can identify the voice of others while sitting in a dark room.

15. 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.
    Sol:
16. Two children are at opposite ends of an aluminum rod. One strikes the end of the rod with a stone. Find the ratio of times taken by the sound wave in the air and in aluminum to reach the second child.
    Sol:

17. The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute?
    Sol: Frequency 100 Hz means 100 vibrations per second. In one minute (60 seconds), vibrations = $100\times 60=6000$.

18. Does sound follow the same laws of reflection as light does? Explain.
    Sol: Yes, sound follows the same laws of reflection as light because the angles of incidence and reflection are equal and all lie in the same plane.

19. 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 remain the same. Do you hear the echo sound on a hotter day?
    Sol: The speed of sound increases with temperature, so you hear the echo sooner on a hotter day.

20. Give two practical applications of reflection of sound waves.
    (i) Megaphones or loudhailers: Designed to send sound in a specific direction without spreading it all around.
    (ii) Stethoscope: Uses multiple reflections to transmit sounds within the body like heartbeat to the doctor's ears.

21. 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?

    Sol:

22. A sound wave travels at a speed of 339 m/s. If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible?
    Sol:

23. What is reverberation? How can it be reduced?
    Sol: Reverberation is the persistence of sound due to repeated reflection making sound last longer. It can be reduced by using sound-absorbent materials on walls and ceilings, like fiber boards or curtains.

24. What is the loudness of sound? What factors does it depend on?
    Sol: Loudness is how loud or soft a sound is perceived. It depends on the intensity of sound waves, which relates to amplitude and distance from the source.

25. Explain how sound is produced by your school bell.
    Sol: When the bell vibrates, it causes air molecules around it to vibrate, producing compressions and rarefactions that create sound waves.

26. Why are sound waves called mechanical waves?
    Sol: They are called mechanical waves because they need a medium (like air, water, or solids) to travel and cannot travel through a vacuum.

27. Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?
    Sol: No, because the moon has no atmosphere or medium for sound to travel through.

28. Which wave property determines:
    (a) Loudness
    Sol: Amplitude of the wave; higher amplitude means louder sound.
    (b) Pitch
    Sol: Frequency of the wave; higher frequency means higher pitch.

29. How are the wavelength and frequency of a sound wave related to its speed?
    Sol: Speed $v=f\times \lambda$ (frequency $\times$ wavelength). The speed depends on the medium.

30. Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m/s in a given medium.
    Sol:

31. A person is listening to a 500 Hz tone from a source 450 m away. What is the time interval between successive compressions?
    Sol:

32. Distinguish between loudness and intensity of sound:

    Loudness	Intensity
    Reaction of the ear to sound	Physical quantity
    Unit: Decibel (dB)	Unit: Watt per m²
    Depends on ear's power to hear	Independent of hearing ability
    Determined by amplitude	Determined by energy

     

     

33. Why are the ceilings of concert halls curved?
    Sol: Curved ceilings help sound waves reflect and reach all parts of the hall evenly.

34. Why do we see light first and hear sound later during a thunderstorm?
    Sol: Speed of light (~3×1083×108 m/s) is much faster than speed of sound (~330 m/s), so light reaches first.

35. What are the wavelength, frequency, time period, and amplitude of a sound wave?
    Sol:

    • Wavelength: Distance between identical parts of the wave

    • Frequency: Number of vibrations per second

    • Time period: Time for one vibration, inverse of frequency

    • Amplitude: Maximum displacement of particles from rest

36. When a wave travels from one medium to another, wavelength changes but frequency stays the same.
    Wavelength in helium and water given:

37. How can ultrasound be used to detect defects in a metal block?
    Sol: Ultrasound waves reflect differently if there's a defect in the metal, helping detect faults by their reflections.

38. What is SONAR? How does it work?
    Sol: SONAR (Sound Navigation and Ranging) uses ultrasonic echoes to detect underwater objects by measuring reflected sound waves.

Assertion and Reason Questions

39. Assertion: The flash of lightning is seen before the sound of thunder is heard.
    Reason: Speed of sound is greater than speed of light.

40. Assertion: Two persons on the surface of moon cannot talk to each other.
    Reason: There is no atmosphere on moon.

41. Assertion: Sound would travel faster on a hot summer day than on a cold winter day.
    Reason: Velocity of sound is directly proportional to the square of its absolute temperature.

42. Assertion: Echo is produced when sound hits a hard, polished surface.
    Reason: Sound energy can be totally reflected by objects with soft and loose texture.

43. Assertion: The speed of sound in solids is maximum though their density is large.
    Reason: The coefficient of elasticity of solid is large.

44. Assertion: Compression and rarefaction involve changes in density and pressure.
    Reason: When particles are compressed, density increases; when rarefied, density decreases.

45. Assertion: Velocity of sound in hydrogen gas is less than in oxygen gas.
    Reason: Density of oxygen is higher than hydrogen.

46. Assertion: The velocity of sound increases with increase in humidity.
    Reason: Velocity of sound does not depend on the medium.

47. Assertion: Waves with particle motion perpendicular to wave direction are transverse waves.
    Reason: Loudness is determined by amplitude.

48. Assertion: Sound is a form of energy producing hearing sensation.
    Reason: When you clap, a sound is produced.

Benefits of Practicing Important Questions Class 9 Science Chapter 11 

Effective Exam Preparation: Practicing important questions is a important part of exam preparation. It familiarizes students with the exam format and the types of questions they may encounter, allowing them to approach the exam with confidence.

Identification of Weak Areas: Working through important questions helps students identify topics they find challenging. Recognizing these weak areas allows them to concentrate their study efforts where needed, leading to improved performance.

Development of Analytical Skills: Analyzing important questions promotes the development of analytical skills. Students learn to interpret information, evaluate scenarios and draw conclusions based on sound principles, which is valuable in both academic and real-world situations.

Time Management Skills: Practicing important questions can improve students time management skills during exams.

Increased Confidence: Successfully answering important questions boosts students confidence in their knowledge and abilities. This confidence reduces anxiety during assessments and encourages a more positive attitude toward learning.

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