Science STD 8 Chapter 10: Sound - Exercises

Science STD 8 Chapter 10: Sound - Exercises

1. Choose the correct answer.

Sound can travel through

  1. (a) gases only
  2. (b) solids only
  3. (c) liquids only
  4. (d) solids, liquids and gases.

2. Voice of which of the following is likely to have minimum frequency?

  1. (a) Baby girl
  2. (b) Baby boy
  3. (c) A man
  4. (d) A woman

3. In the following statements, tick ‘T’ against those which are true, and ‘F’ against those which are false.

  1. (a) Sound cannot travel in vacuum. (T/F)

    T

  2. (b) The number of oscillations per second of a vibrating object is called its time period. (T/F)

    F

  3. (c) If the amplitude of vibration is large, sound is feeble. (T/F)

    F

  4. (d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T/F)

    T

  5. (e) The lower the frequency of vibration, the higher is the pitch. (T/F)

    F

  6. (f) Unwanted or unpleasant sound is termed as music. (T/F)

    F

  7. (g) Noise pollution may cause partial hearing impairment. (T/F)

    T

4. Fill in the blanks with suitable words.

  1. (a) Time taken by an object to complete one oscillation is called time period.
  2. (b) Loudness is determined by the amplitude of vibration.
  3. (c) The unit of frequency is hertz (Hz).
  4. (d) Unwanted sound is called noise.
  5. (e) Shrillness of a sound is determined by the frequency of vibration.

5. A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.

Time Period: Time taken to complete one oscillation = Total time / Number of oscillations = 4 s / 40 = 0.1 s

Frequency: Number of oscillations per second = Number of oscillations / Total time = 40 / 4 s = 10 Hz

6. The sound from a mosquito is produced when it vibrates its wings at an average rate of 500 vibrations per second. What is the time period of the vibration?

Frequency = 500 Hz

Time Period = 1 / Frequency = 1 / 500 = 0.002 s

7. Identify the part which vibrates to produce sound in the following instruments.

  1. (a) Dholak

    Stretched membrane

  2. (b) Sitar

    Stretched string

  3. (c) Flute

    Air column (air blown into the flute's hole)

8. What is the difference between noise and music? Can music become noise sometimes?

Noise Music
It is an unpleasant or unwanted sound. It is a sound that is pleasing to the ear.
It is often irregular and non-periodic. It is regular and has a pleasant, rhythmic pattern.

Yes, music can become noise sometimes. If music is played at a very high volume, it can cause discomfort and be a source of noise pollution. For example, loud music from a party can be considered noise by neighbours.

9. List sources of noise pollution in your surroundings.

Sources of noise pollution in my surroundings include:

  • Sounds from vehicles like buses, trucks, and horns.
  • Loudspeakers and music systems played at high volume.
  • Construction work and industrial machines.
  • Home appliances such as mixers, desert coolers, and air conditioners.
  • Explosions, including the bursting of crackers.

10. Explain in what way noise pollution is harmful to human.

Noise pollution can cause several health-related problems in humans. It can lead to a lack of sleep, which is essential for a healthy body. It can also cause hypertension (high blood pressure) and anxiety. Continuous exposure to loud noise can even result in temporary or permanent hearing impairment. Therefore, noise pollution is a serious health hazard.

11. Your parents are going to buy a house. They have been offered one on the roadside and another three lanes away from the roadside. Which house would you suggest your parents should buy? Explain your answer.

I would suggest that my parents buy the house that is three lanes away from the roadside. The house on the roadside would be exposed to a constant source of noise pollution from vehicle horns and traffic. This could lead to a lack of sleep and other health problems. The house away from the road would be quieter and more peaceful, making it a better living environment.

12. Sketch larynx and explain its function in your own words.

The larynx, also known as the voice box, is the organ responsible for producing sound in humans. It is located at the upper end of the windpipe. The larynx contains two vocal cords that are stretched across it. When the lungs force air through the narrow slit between the vocal cords, they vibrate and produce sound. Muscles attached to the vocal cords can make them tight or loose, which changes the quality of the voice.

13. Lightning and thunder take place in the sky at the same time and at the same distance from us. Lightning is seen earlier and thunder is heard later. Can you explain why?

This happens because the speed of light is much faster than the speed of sound. Both lightning and thunder are produced at the same time. However, since light travels at a much higher speed, we see the lightning first. Sound travels much slower, so the thunder is heard later. The time difference between seeing the flash and hearing the sound can be used to estimate the distance of the storm.

Suggested Activities and Projects

1. Visit the music room of your school. You may also visit musicians in your locality. Make a list of musical instruments. Note down the parts of these instruments that vibrate to produce sound.

Outline: This activity is about observing different musical instruments and understanding how they produce sound. You would find that instruments like the sitar have stretched strings that vibrate, while drums have stretched membranes. The flute produces sound from the vibration of an air column. This activity helps you connect the concept of vibration with the production of sound in different instruments.

2. If you play a musical instrument, bring it to the class and demonstrate how you play it.

Outline: This is a demonstration activity. By bringing a musical instrument to class, you can show your classmates how different parts of the instrument vibrate to produce different sounds. This can be an engaging way to share your hobby and teach others about the physics of music and sound production.

3. Prepare a list of famous Indian musicians and the instruments they play.

Outline: This project is a research activity on famous Indian musicians and their instruments. You would create a list of musicians like Zakir Hussain (Tabla), Ravi Shankar (Sitar), Bismillah Khan (Shehnai), and Hariprasad Chaurasia (Flute). This activity helps you appreciate Indian musical heritage and the diverse range of instruments used in our culture.

4. Take a long thread. Place your hands over your ears and get some one to place this thread round your head and hands. Ask her to make the thread taut and hold its ends in one hand. Now ask her to draw her finger and thumb tightly along the thread (Fig. 10.19). Can you hear a rolling sound like that of a thunder? Now repeat the activity while another friend stands near both of you. Can he hear any sound?

Outline: This experiment demonstrates that sound can travel through solids. When the thread is taut and rubbed, the vibrations travel along the thread and are transmitted to your hands and ears, creating a sound that you can hear. However, your friend standing nearby will not hear the sound because the vibrations are primarily contained within the thread and are not efficiently transferred to the air.

5. Make two toy telephones. Use them as shown in Fig. 10.20. Make sure that the two strings are taut and touch each other. Let one of you speak. Can the remaining three persons hear? See how many more friends you can engage in this way. Explain your observations.

Outline: This project is a fun way to demonstrate that sound can travel through solids (the string). When one person speaks into the cup, the vibrations travel along the taut string to the other cup, allowing the other person to hear. This activity shows how a toy telephone works and provides a hands-on experience of sound propagation through a solid medium.

6. Identify the sources of noise pollution in your locality. Discuss with your parents, friends and neighbours. Suggest how to control noise pollution. Prepare a brief report and present it in the class.

Outline: This project is about identifying local sources of noise pollution and suggesting solutions. You would list sources like traffic, construction, and loud music. The report would then suggest solutions such as planting more trees along roads, using noise-cancelling headphones, and ensuring that noisy activities are kept away from residential areas. The report would highlight the importance of collective action to control noise pollution and its harmful effects on health.