12. Sound Class 9 NCERT Exemplar Solutions Chapter 12 (CBSE Science)
CBSE Class 9 Science Chapter 12: Sound – NCERT Exemplar Questions with Answers
Chapter 12: Sound
Multiple Choice Questions
1. Note is a sound
(a) of mixture of several frequencies (b) of mixture of two frequencies only (c) of a single frequency (d) always unpleasant to listen
Answer : (c) of a single frequency
[ A note is a musical sound having a single definite frequency (pure tone), while noise is a mixture of several frequencies. ]
2. A key of a mechanical piano struck gently and then struck again but much harder this time. In the second case
(a) sound will be louder but pitch will not be different (b) sound will be louder and pitch will also be higher
(c) sound will be louder but pitch will be lower (d) both loudness and pitch will remain unaffected
Answer: (a) sound will be louder but pitch will not be different
[ Striking harder increases the amplitude of vibration and the sound becomes louder. Pitch depends on frequency and it does not change so the pitch remains the same.]
3. In SONAR, we use
(a) ultrasonic waves (b) infrasonic waves (c) radio waves (d) audible sound waves
Answer: (a) ultrasonic waves
[ SONAR uses ultrasonic waves (frequency above 20,000 Hz). These waves travel in water, reflect back from objects and help to detect distance and position underwater.]
4. Sound travels in air if
(a) particles of medium travel from one place to another
(b) there is no moisture in the atmosphere
(c) disturbance moves
(d) both particles as well as disturbance travel from one place to another.
Answer: (c) disturbance moves
[ In sound waves, air particles only vibrate about their mean position. They do not travel long distance. Only the disturbance (energy) moves through the medium.]
5. When we change feeble sound to loud sound we increase its
(a) frequency (b) amplitude (c) velocity (d) wavelength
Answer: (b) amplitude
[ Loudness depends on amplitude. When sound becomes louder, its amplitude increases. Frequency, velocity and wavelength remain unchanged. ]
6. In the curve (Fig.12.1) half the wavelength is
(a) A B (b) B D (c) D E (d) A E
Answer: (b) BD
[Thus, the distance between the chest (B) and though (D) represents exactly half of the wavelength .]
7. Earthquake produces which kind of sound before the main shock wave begins
(a) ultrasound (b) infrasound (c) audible sound (d) none of the above
Answer: (b) infrasound
[ Before the main shock waves, earthquakes produce infrasonic waves (frequency less than 20 Hz). These are not audible to humans but can be detected by instruments.]
8. Infrasound can be heard by
(a) dog (b) bat (c) rhinoceros (d) human beings
Answer: (c) rhinoceros
[ Infrasound (below 20 Hz) cannot be heard by humans. Animals like rhinoceros can detect infrasound, but dogs and bats mainly hear ultrasonic sounds.]
9. Before playing the orchestra in a musical concert, a sitarist tries to adjust the tension and pluck the string suitably. By doing so, he is adjusting
(a) intensity of sound only
(b) amplitude of sound only
(c) frequency of the sitar string with the frequency of other musical instruments
(d) loudness of sound
Answer: (c) frequency of the sitar string with the frequency of other musical instruments
[ By adjusting the tension, the sitarist changes the frequency (pitch) of the string so it matches other instruments. This process is called tuning. ]
Short Answer Question
10. The given graph (Fig.12.2) shows the displacement versus time relation for a disturbance travelling with velocity of 1500 m/s . Calculate the wavelength of the disturbance.
Fig.12.2
Solution: Here, Velocity (v) = 1500 m/s , 
W have, 
Again, 
Therefore, the wavelength of the disturbance is 
.
11. Which of the above two graphs (a) and (b) (Fig.12.3) representing the human voice is likely to be the male voice? Give reason for your answer.
Fig. 12.3
Answer: In graph (a), the voice belongs to a male because it shows fewer vibrations per second, i.e., a lower frequency and hence a deeper sound. On the other hand, graph (b) shows a female voice, which has higher frequency and produces a higher-pitched sound.
12. A girl is sitting in the middle of a park of dimension 12 m × 12 m. On the left side of it there is a building adjoining the park and on right side of the park, there is a road adjoining the park. A sound is produced on the road by a cracker. Is it possible for the girl to hear the echo of this sound? Explain your answer.
Answer: No, the girl will not hear a clear echo. An echo is heard only if the reflecting surface is at least 17.2 m away for a 0.1 s delay. Here the park is only 12 m wide so the building is at most 12 m from the girl. The sound will reflect but return too quickly less than 0.1 s and mix with the original sound. Thus no separate echo is heard.
13. Why do we hear the sound produced by the humming bees while the sound of vibrations of pendulum is not heard?
Answer: Humming bees produce sound in the audible frequency range about 20–20,000 Hz due to rapid wing vibrations. A pendulum vibrates at a very low frequency less than 20 Hz which is infrasonic and cannot be heard by the human ear.
14. If any explosion takes place at the bottom of a lake, what type of shock waves in water will take place?
Answer: The explosion will produce longitudinal waves also called pressure waves or shock waves in water. In these waves the particles of water vibrate back and forth along the direction of wave propagation creating compressions and rarefactions.
15. Sound produced by a thunderstorm is heard 10 s after the lightning is seen. Calculate the approximate distance of the thunder cloud. (Given speed of sound = 340 m/s .)
Solution: Given: Speed of sound = 340 m/s , Time = 10 s
We have, Distance = Speed × Time
Distance = 340 × 10
Distance = 3400 m
So, the thunder cloud is 3400 m away.
16. For hearing the loudest ticking sound heard by the ear, find the angle x in the Fig.12.4.
Fig.12.4
Answer: We know that that, the loudest sound is heard when the angle of incidence equals the angle of reflection .
So, the angle of incidence = 90° – 50° = 40° .
According to the laws of reflection, the angle of incidence = the angle of reflection
.
17. Why is the ceiling and wall behind the stage of good conference halls or concert halls made curved?
Answer: The ceiling and wall behind the stage are made curved to reflect sound evenly in all directions. Curved surfaces help sound waves spread uniformly throughout the hall. This ensures that every listener hears the sound clearly and loudly, without echo or disturbance.
Long Answer Questions
18. Represent graphically by two separate diagrams in each case
(i) Two sound waves having the same amplitude but different frequencies?
(ii) Two sound waves having the same frequency but different amplitudes.
(iii) Two sound waves having different amplitudes and also different wavelengths.
Answer: (i) The graph shows two sound waves having the same amplitude but different frequencies :
(ii) The graph shows two sound waves having the same frequency but different amplitudes:
(iii) The graph shows two sound waves with different amplitudes and different wavelengths :
19. Establish the relationship between speed of sound, its wavelength and frequency. If velocity of sound in air is 340 m/s , calculate
(i) wavelength when frequency is 256 Hz.
(ii) frequency when wavelength is 0.85 m.
Answer: When a sound wave travels through a medium, it moves with a certain speed (v), and it covers one complete wavelength (λ) in one time period (T).
We know, 
Therefore, the relationship between speed, wavelength and frequency of sound is
Given, velocity of sound in air, v = 340 m/s
(i) Here,
We have, 
(ii) Here,
We have, 
20. Draw a curve showing density or pressure variations with respect to distance for a disturbance produced by sound. Mark the position of compression and rarefaction on this curve. Also define wavelengths and time period using this curve.
Answer: A sound wave graph shows density/pressure vs distance as a sinusoidal curve.
High peaks are compressions (C) and low valleys are rarefactions (R). Wavelength (
) is the distance between two successive compressions (C) or rarefactions (R). Time period (T) is the time taken for one complete oscillation of a particle of the medium.
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