GSEB Class 8 Science Solutions Chapter 13 Sound

Choose the Correct Answer

Question 1. Sound can travel through:
(a) gases only
(b) solids only
(c) liquids only
(d) solids, liquids, and gases.
Answer: (d) solids, liquids, and gases.
In simple words: Sound needs something to travel through. It can move through solid things, liquid things, and even air.

Exam Tip: Remember that sound always requires a material medium to propagate, unlike light which can also travel through empty space.

Question 2. Voice of which of the following is likely to have a minimum frequency?
(a) Baby girl
(b) Baby boy
(c) A man
Answer: (c) A man
In simple words: A man's voice usually has the lowest frequency. This means it has a deep sound, unlike the higher-pitched voices of children.

Exam Tip: Recall that frequency determines the pitch of a sound: lower frequency means lower pitch (deep sound), and higher frequency means higher pitch (shrill sound).

Question 3. In the following statements, tick 'T' against those which are true, and 'F' against those which are false:
(a) Sound cannot travel in a vacuum.
(b) The number of oscillations per second of a vibrating object is called its time period.
(c) If the amplitude of vibration is large, the sound is feeble.
(d) For human ears, the audible range is 20 Hz to 20,000 Hz.
(e) The lower the frequency of vibration, the higher is the pitch.
(f) Unwanted or unpleasant sound is termed as music.
(g) Noise pollution may cause partial hearing impairment.
Answer:
(a) True: Sound requires a medium to move, so it cannot go through empty space.
(b) False: The number of oscillations per second is called frequency, not time period. Time period is the time for one oscillation.
(c) False: A large amplitude means a loud sound, not a feeble (weak) one.
(d) True: Human ears can typically hear sounds within the 20 Hz to 20,000 Hz range.
(e) False: Lower frequency means lower pitch.
(f) False: Unwanted sound is noise, while music is generally pleasant.
(g) True: Excessive noise can damage hearing, leading to partial impairment.
In simple words: Sound needs something to travel through, so it can't move in a vacuum (True). Frequency is oscillations per second, not time period (False). Big vibrations make loud sounds, not weak ones (False). Humans hear from 20 Hz to 20,000 Hz (True). Low frequency means low pitch (False). Unpleasant sound is noise, not music (False). Too much noise can harm your hearing (True).

Exam Tip: Thoroughly understand the definitions of key terms like frequency, amplitude, pitch, loudness, and time period to correctly answer true/false questions.

Question 4. Fill in the blanks with suitable words:
(a) Time taken by an object to complete one oscillation is called ..........
(b) Loudness is determined by the .......... of vibration.
(c) The unit of frequency is ..........
(d) Unwanted sound is called ..........
(e) The shrillness of a sound is determined by the ....... of vibration.
Answer:
(a) The duration an object needs to complete one full oscillation is known as its time period.
(b) The magnitude of a sound, or its loudness, depends on the amplitude of vibration.
(c) Hertz is the standard unit for measuring frequency.
(d) Undesired or irritating sound is referred to as noise.
(e) The pitch or shrillness of a sound is determined by its frequency of vibration.
In simple words: (a) How long it takes for one full back-and-forth swing is the time period. (b) How loud a sound is depends on how big the vibration is, which is amplitude. (c) We measure frequency in hertz. (d) Sound you don't want to hear is called noise. (e) How high or low a sound is, its shrillness, is set by its frequency.

Exam Tip: Memorize the definitions and units of all fundamental sound properties, as fill-in-the-blanks often test this basic knowledge.

Question 5. A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.
Answer:
Number of oscillations = 40
Total time = 4 seconds
Time taken to complete one oscillation \( = \frac {4}{40} = \frac {1}{10} = 0.1 \) second
So time period = 0.1 second.
Frequency \( = \frac {1}{Time period} = \frac {1}{0.1} = 10 \) Hz.
In simple words: To get the time period, divide the total time by how many times it swung. Then, for frequency, just flip that time period number upside down.

Exam Tip: Remember the inverse relationship between time period and frequency: if you know one, you can easily calculate the other. Always include units in your final answer.

Question 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?
Answer:
Total vibrations = 500
Time taken = 1 second
Time taken to complete one vibration \( = \frac {1}{500} = 0.002 \) seconds.
So time period = 0.002 seconds.
In simple words: If a mosquito flaps its wings 500 times in one second, its time period is how long it takes for just one flap. You find it by dividing 1 by 500.

Exam Tip: Pay close attention to the units provided in the question. 'Vibrations per second' directly gives you the frequency, from which you can find the time period.

Question 7. Identify the part which vibrates to produce sound in the following instruments.
(a) Dholak
(b) Sitar
(c) Flute
Answer:
(a) In a dholak, a stretched membrane vibrates to generate sound.
(b) The strings of a sitar vibrate to produce musical notes.
(c) An air column vibrates within a flute to create its distinct sound.
In simple words: (a) A dholak makes sound from its tight skin vibrating. (b) A sitar uses its strings to make sounds. (c) A flute makes sound from the air inside it vibrating.

Exam Tip: For musical instruments, think about what part physically moves back and forth or vibrates to initiate the sound production.

Question 8. Difference between noise and music? Can music become noise sometimes?
Answer:
Sound that feels unpleasant to our ears is known as noise, whereas music is generally considered enjoyable. Sometimes, music can transform into noise if its loudness exceeds the comfortable listening range for our ears.
In simple words: Noise is any sound we don't like, and music is sound we enjoy. But even music can turn into noise if it gets too loud for our ears.

Exam Tip: Define noise and music based on their pleasantness to the listener, and then explain the condition under which music's perception can shift.

Question 9. List sources of noise pollution in your surroundings.
Answer:
Common sources of noise pollution in our daily environment include vehicle horns, loudspeakers, noisy factory machinery, high-volume televisions, radios, and various household appliances.
In simple words: Things that make a lot of unwanted noise around us are car horns, loud speakers, machines in factories, loud TVs, radios, and home gadgets.

Exam Tip: When listing sources of pollution, aim for a diverse range from different categories (e.g., transport, industry, domestic) to demonstrate comprehensive understanding.

Question 10. Explain in what way noise pollution is harmful to humans.
Answer:
Harmful effects of noise pollution:

• It causes deafness
• It causes mental illness
• It causes headache and high blood pressure

Exam Tip: When explaining harmful effects, focus on distinct physiological and psychological impacts, as these are critical points for full marks.

Question 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 to your parents to buy? Explain your answer.
Answer:
I would advise my parents to choose the house located three lanes away from the main road. This option would better protect their health and ensure a calmer living environment.
In simple words: I would tell my parents to get the house far from the main road. This will help them stay healthy and have quiet minds.

Exam Tip: When answering scenario-based questions, clearly state your choice first, then provide a concise and logical justification based on relevant environmental factors like noise pollution.

Question 12. Sketch larynx and explain its function in your own words.
Answer:
The larynx, commonly known as the voice box, houses the vocal cords. These cords vibrate when air passes through them, generating sound in humans.

Exam Tip: When describing the larynx, mention its alternate name (voice box), the presence of vocal cords, and their role in producing sound through vibration.

Question 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?
Answer:
Light travels much faster than sound. Because of this difference in speed, the light from lightning reaches our eyes first, causing us to see it earlier. The sound of thunder, traveling more slowly, arrives afterwards.
In simple words: Light is super fast, much faster than sound. So, when lightning and thunder happen at the same time, we see the light right away, but the sound takes longer to reach us.

Exam Tip: This phenomenon illustrates the significant difference in the speeds of light and sound. Clearly state that light is faster and connect that speed difference to the observation.