Class 9 General Science Chapter 12 Sound Solutions
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Class 9 General Science Chapter 12 Sound Solutions
Answers to Questions Within the Lesson(1)
Q1. How does the sound produced by a vibrating object reach your ear?
Ans: When a vibrating object produces sound in a medium, the small particles of the medium oscillate about their mean positions. As the sound propagates, the particles of the medium undergo compression, and after the sound passes, they undergo rarefaction. In this way, sound travels through the medium and reaches our ear.
Answers to Questions Within the Lesson(2)
Q1. Describe how your school bell produces sound.
Ans: When the school bell is struck with a rod, it starts vibrating. These vibrations produce sound.
Q2. Why are sound waves called mechanical waves?
Ans: Sound waves involve the particles of a material medium and propagate through it. Therefore, sound waves are called mechanical waves.
Q3. Suppose you and your friend are on the Moon. Will you be able to hear any sound produced by your friend?
Ans: We will not be able to hear the sound produced by the friend. This is because sound requires a medium to propagate, and on the Moon there is no air or any other medium for sound transmission.
Answers to Questions Within the Lesson(3)
Q1. Which property of a wave determines (a) loudness and (b) pitch?
Ans: (a) The loudness of a wave depends on its amplitude.
(b) The pitch of a wave is determined by the frequency of the vibrations produced by the source.
Q2. Between the sound of a car horn and a guitar, which has a higher pitch?
Ans: The sound of a car horn generally has a higher pitch.
Answers to Questions Within the Lesson(4)
Q1. Define wavelength, frequency, time period, and amplitude of a sound wave.
Ans: Wavelength: The distance between two successive compressions or rarefactions is called the wavelength.
Frequency: The number of complete oscillations per unit time is called frequency.
Time period: The time taken for two successive compressions or rarefactions to pass a fixed point is called the time period of the wave.
Amplitude: The maximum displacement of the particles of the medium from their mean position is called the amplitude of the wave.
Q2. How are the frequency and wavelength of a sound wave related to its speed?
Ans: The speed of sound is the distance travelled by a compression or rarefaction per unit time. The wavelength is the distance travelled by the wave in one time period. Therefore, the relation between frequency (n), wavelength(lambda), and speed (v) is:
v = n lambda
Q3. Find the wavelength of a sound wave in a given medium where the speed of sound is 440 m s-¹ and the frequency is 220 Hz.
Ans: Given, speed of sound v = 440 m s-¹ and frequency f = 220 Hz.
The wavelength is given by the relation v = lambda.
So, lambda = v / f = 440 / 220 = 2 m.
Therefore, the wavelength is 2 metres.
Q4. A person sitting at a distance of 450 m from the source hears a sound of frequency 500 Hz. What is the time interval between successive compressions emitted from the source?
Ans: The time interval between successive compressions is equal to the time period of the sound wave.
Given frequency f = 500 Hz.
Time period T = 1 / f = 1 / 500 = 0.002 s.
Therefore, the time interval between successive compressions is 0.002 seconds.
Answers to Questions Within the Lesson(5)
Q1. What is the difference between loudness and intensity of sound?
Ans: Intensity of sound is related to the amount of energy carried by the sound wave per unit area, which depends on the amplitude of vibration. On the other hand, loudness is the perception of sound intensity by the human ear, i.e., how strong or weak a sound appears to us.
Answers to Questions Within the Lesson(6)
Q1. In which medium among air, water, and iron is the speed of sound highest at a given temperature?
Ans: In iron medium.
Answers to Questions Within the Lesson(7)
Q1. An echo of a sound is heard after 3 seconds. What is the distance between the reflecting surface and the source? Given that the speed of sound is 342 m s-¹.
Ans: The time of 3 seconds is for the sound to travel to the reflectingsurface and come back.
So, total distance covered by sound = speed × time = 342 × 3 = 1026 m.
The distance between the source and the reflecting surface = 1026 / 2 = 513 m.
Therefore, the distance is 513 metres.
Answers to Questions Within the Lesson(8)
Q1. Why is the roof of a concert hall curved?
Ans: The roof of a concert hall is made curved so that the sound produced during the performance can be reflected and spread to all corners of the hall, reaching every listener clearly.
Answers to Questions Within the Lesson(9)
Q1. What is the normal range of human hearing?
Ans: The normal range of human hearing is from 20 Hz to 20 kHz.
Q2. What are the frequency ranges of (a) infrasound and (b) ultrasound?
Ans: (a) Infrasound has a frequency range below 20 Hz.
(b) Ultrasound has a frequency range above 20 kHz.
Answers to Questions Within the Lesson(10)
Q1. A sonar pulse emitted from a ship hits a hill on the sea floor and returns in 1.02 s. If the speed of sound in salt water is 1531 m s-¹, what is the distance of the hill from the ship?
Ans: The given time 1.02 s is the total time for the sound to go to the hill and come back.
So, total distance travelled = speed × time = 1531 × 1.02 = 1561.62 m.
Distance to the hill = 1561.62 / 2 = 780.81 m ≈781 m.
Therefore, the distance of the hill from the ship is about 781 metres.
Exercise: Questions and Answers
Q1. What is sound and how is it produced?
Ans: Sound is a mechanical wave that produces the sensation of hearing in our ears. It is produced by the vibration of different objects.
Q2. Explain with the help of a diagram how compressions and rarefactions are formed near a sound source.
Ans: When sound is produced from a source, the particles of the medium near it are pushed closer together, forming compressions. After the sound passes, the particles return to their original positions, forming rarefactions. In this way, compressions and rarefactions are produced near the source and the sound wave travels to our ears.
Q3. Mention an experiment to show that sound requires a medium for propagation.
Ans: Take an electric bell inside an airtight glass bell jar. The jar is connected to a vacuum pump. The mouth of the jar is sealed so that air cannot enter or leave. When the switch is turned on, the bell rings and sound is heard. Now air is slowly removed using the vacuum pump. The sound of the bell becomes weaker and finally cannot be heard when air is completely removed. This proves that sound needs a medium to travel.
Q4. Why is sound wave called a longitudinal wave?
Ans: Sound waves are called longitudinal waves because the particles of the medium vibrate back and forth in the same direction as the propagation of the wave, similar to longitudinal waves.
Q5. In a dark room with many people, by which property of sound can you recognize your friend’s voice?
Ans: By the timbre (quality) of sound.
Q6. Lightning and thunder are produced together, but thunder is heard after lightning. Why?
Ans: This is because the speed of light in air is much greater than the speed of sound. Therefore, although both are produced at the same time, the light from lightning reaches us first, and the sound (thunder) is heard later.
Q7. A person has a hearing range from 20 Hz to 20 kHz. What are the corresponding typical wavelengths of sound waves in air for these two frequencies? Take the speed of sound in air as 344 m s-¹.
Ans: We use the relation v = flambda, so lambda = v / f.
For f = 20 Hz:
lambda = 344 / 20 = 17.2 m
For f = 20 kHz = 20000 Hz:
lambda = 344 / 20000 = 0.0172 m = 1.72 cm
Therefore, the wavelength range is from 17.2 m to 0.0172 m (or 1.72 cm).
Q8. Two students are standing at the two ends of an aluminium rod. One of them strikes the rod with a stone. Find the ratio of time taken bythe sound to reach the other student through air and through aluminium.
Ans: The speed of sound in aluminium is much greater than in air.
Let v_air be the speed of sound in air and v_al be the speed of sound in aluminium.
Time taken is inversely proportional to speed, so t_air / t_al = v_al / v_air
Since sound travels much faster in aluminium than in air, the time taken in air is much greater.
Therefore, the ratio t_air : t_al = v_al : v_air.
Using standard values (approximately):
v_air ≈ 344 m s?¹ and v_al ≈ 6420 m s-¹
So, t_air : t_al = 6420 : 344 ≈ 18.7 : 1 ≈ 19 : 1
Therefore, the sound takes about 19 times more time in air than in aluminium.
Q9. A sound source has a frequency of 100 Hz. How many vibrations will it complete in one minute?
Ans: Frequency means number of vibrations per second.
Given frequency = 100 Hz = 100 vibrations per second.
Time = 1 minute = 60 seconds.
Total vibrations = 100 × 60 = 6000
Therefore, the source completes 6000 vibrations in one minute.
Q10. Do the laws of reflection followed by light also apply to sound? Explain.
Ans: Yes, the laws followed by light are also followed by sound. Just as light reflects from a reflecting surface, sound also reflects from a reflecting surface. Light produces an image after reflection, similarly sound produces an echo after reflection.
Q11. An echo is produced when sound is reflected from a distant object. Suppose the distance between the sound source and the reflecting surface is the same. Will an echo be heard on a hotter day than usual?
Ans: Yes, even on a hotter day, an echo will be heard if the distance between the sound source and the reflecting surface remains the same.
Additional Questions & Answers
Q1. Why do we hear thunder after lightning?
Ans: Because the speed of light is much greater than the speed of sound, light reaches us first and sound comes later.
Q2. What is vibration?
Ans: Vibration is the rapid back and forth motion of a body about its mean position.
Q3. Name one device that works on the reflection of sound.
Ans: SONAR.
Q4. What is an echo?
Ans: An echo is the repetition of sound caused by the reflection of sound waves from a surface.
Q5. Why is sound called a longitudinal wave?
Ans: Because the particles of the medium vibrate parallel to the direction of propagation of the wave.
Q6. What is amplitude?
Ans: Amplitude is the maximum displacement of a particle of the medium from its mean position.
Q7. What is frequency?
Ans: Frequency is the number of vibrations or oscillations per second.
Q8. What is the unit of wavelength?
Ans: Metre.
Q9. Why can sound not travel in vacuum?
Ans: Because there are no particles in vacuum to carry sound vibrations.
Q10. What is rarefaction in a sound wave?
Ans: It is the region where particles are far apart and pressure is low.
Q11. What is compression in a sound wave?
Ans: It is the region where particles are close together and pressure is high.
Q12. What is the relationship between frequency and time period?
Ans: Time period is the reciprocal of frequency, T = 1/f.
Q13. Which sound is more shrill, high frequency or low frequency?
Ans: High frequency sound is more shrill.
Q14. What determines the loudness of a sound?
Ans: The amplitude of the sound wave determines loudness.
Q15. Name the phenomenon responsible for hearing sound after it is reflected from a distant object.
Ans: Echo.
Multiple Choice Questions (MCQ)
1. What type of wave is sound?
(a) Electromagnetic wave
(b) Mechanical wave
(c) Light wave
(d) Radio wave
Ans: (b)
2. Sound cannot travel through:
(a) Air
(b) Water
(c) Vacuum
(d) Steel
Ans: (c)
3. The SI unit of frequency is:
(a) Meter
(b) Second
(c) Hertz
(d) Joule
Ans: (c)
4. Sound requires a:
(a) Vacuum
(b) Medium
(c) Mirror
(d) Lens
Ans: (b)
5. The speed of sound is highest in:
(a) Air
(b) Water
(c) Iron
(d) Vacuum
Ans: (c)
6. The phenomenon of sound reflection is called:
(a) Refraction
(b) Diffraction
(c) Echo
(d) Absorption
Ans: (c)
7. Minimum distance required to hear an echo is about:
(a) 5 m
(b) 10 m
(c) 17 m
(d) 50 m
Ans: (c)
8. Sound is produced due to:
(a) Reflection
(b) Vibration
(c) Refraction
(d) Absorption
Ans: (b)
9. Frequency of sound determines:
(a) Loudness
(b) Pitch
(c) Speed
(d) Reflection
Ans: (b)
10. Loudness of sound depends on:
(a) Frequency
(b) Amplitude
(c) Wavelength
(d) Speed
Ans: (b)
11. Unit of amplitude is:
(a) Hertz
(b) Meter
(c) Watt
(d) Newton
Ans: (b)
12. Sound travels fastest in:
(a) Gases
(b) Liquids
(c) Solids
(d) Vacuum
Ans: (c)
13. The human audible range is:
(a) 2 Hz – 200 Hz
(b) 20 Hz – 20 kHz
(c) 200 Hz – 200 kHz
(d) 0 Hz – 100 Hz
Ans: (b)
14. Sound above 20 kHz is called:
(a) Infrasound
(b) Ultrasound
(c) Audible sound
(d) Noise
Ans: (b)
15. Sound below 20 Hz is called:
(a) Ultrasound
(b) Infrasound
(c) Echo
(d) Noise
Ans: (b)
16. The speed of sound in air is approximately:
(a) 340 m/s
(b) 300 m/s
(c) 1500 m/s
(d) 5000 m/s
Ans: (a)
17. The unit of speed is:
(a) Hz
(b) m/s
(c) Joule
(d) Newton
Ans: (b)
18. Echo is produced due to:
(a) Absorption
(b) Refraction
(c) Reflection
(d) Diffraction
Ans: (c)
19. Sound waves in air are:
(a) Transverse
(b) Longitudinal
(c) Electromagnetic
(d) Stationary
Ans: (b)
20. Compression in a sound wave is region of:
(a) Low pressure
(b) High pressure
(c) No pressure
(d) Vacuum
Ans: (b)
21. Rarefaction is region of:
(a) High density
(b) Low density
(c) No motion
(d) High temperature
Ans: (b)
22. Time period is:
(a) f
(b) 1/f
(c) f²
(d) 2f
Ans: (b)
23. The phenomenon where sound becomes weaker with distance is due to:
(a) Increase in amplitude
(b) Absorption and spreading
(c) Reflection only
(d) Increase in frequency
Ans: (b)
24. Pitch of sound depends on:
(a) Amplitude
(b) Frequency
(c) Speed
(d) Distance
Ans: (b)
25. Loudness depends on:
(a) Frequency
(b) Amplitude
(c) Wavelength
(d) Medium
Ans: (b)
26. Sound cannot travel in vacuum because there is no:
(a) Light
(b) Energy
(c) Medium
(d) Heat
Ans: (c)
27. Sound waves carry:
(a) Matter
(b) Energy
(c) Mass
(d) Light
Ans: (b)
28. Reflection of sound is used in:
(a) Thermometer
(b) SONAR
(c) Barometer
(d) Compass
Ans: (b)
29. SONAR stands for:
(a) Sound Navigation and Ranging
(b) Solar Navigation and Radar
(c) Sound Network and Radar
(d) Signal Navigation and Range
Ans: (a)
