NCERT Solutions for Class 8 Science Chapter 13 Sound

Choose the correct answer.

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. Sound travels through medium, thus, it can travel through solids liquids and gases.

2. Voice of which of the following is likely to have minimum frequency? – (a) Baby girl (b) Baby boy (c) A man (d) A woman

Answer: (c) A man. Voice of adult man has very low pitch in comparison to a baby girl or a baby boy or a woman.

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

(a) Sound cannot travel in vacuum.

Answer: True (T)

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

Answer: False (F). Number of oscillations per second of a vibrating object is called as frequency.

(c) If the amplitude of vibration is large, sound is feeble.

Answer: False (F). For large amplitude of vibration sound is large, for small amplitude of vibration sound is feeble.

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

Answer: True (T)

(e) The lower the frequency of vibration, the higher is the pitch.

Answer: False (F). With increase in frequency pitch of vibration also increases.

(f) Unwanted or unpleasant sound is termed as music.

Answer: False (F). It is called noise.

(g) Noise pollution may cause partial hearing impairment.

Answer: True (T).

4. Fill in the blanks with suitable words.

(a) Time taken by an object to complete one oscillation is called __________.

Answer: time period

(b) Loudness is determined by the __________ of vibration.

Answer: amplitude

(c) The unit of frequency is __________

Answer: hertz

(d) Unwanted sound is called __________ .

Answer: noise

(e) Shrillness of a sound is determined by the __________ of vibration.

Answer: frequency

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

Answer: Given: number of oscillation = 40, time = 4 seconds,

∴ Frequency = number of oscillation / time = 40 / 4 = 10 Hz

Also, Time period = 1 / frequency = 1 / 10 = 0.1 s

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: Given: Frequency of vibration = 500 Hz

Time period = 1 / Frequency of vibration = 1 / 500 = 0.002 sec

7. Identify the part which vibrates to produce sound in the following instruments.– (a) Dholak (b) Sitar (c) Flute

Answer: a) Dholak produces sound through stretched membrane called as head. Stretched membrane is set into vibration that produces sound in dholak.

b) Sitar produces sound when strings of sitar are set into vibration.

c) Flute produces sound when air inside the flute is set into vibration.

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

Answer: Pleasing sound to ear is called as music. For example sound of flute, guitar, etc. Unwanted sound which is unpleasant to ear is called noise. For example sound produces by vehicle horns, machines, etc. When music is played in large volume it can be considered as noise.

9. List sources of noise pollution in your surroundings.

Answer: Some sources of noise pollution are-

• Sound of television and loud speakers
• Roadside traffic horns
• Burning of firecrackers
• Noise of home appliances like grinders, water pumps, etc

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

Answer: Noise pollution causes various health issues like headache, stress, hypertension, loss of hearing ability, insomnia.

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.

Answer: House three lanes away from road side will be the better option. This is because, it will prevent from roadside noise of vehicles and their horns. Also the sound intensity of vehicles will be lower at house which is three lanes away from the road.

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

Answer: Larynx is responsible for the production of sound. Larynx is consist of two vocal cords separated by small gap, which allows passing of air. While speaking air enter into the gap through lungs, and causes vibration in vocal cords. As sound is produced due to vibration, therefore, vocal cords on vibration produces sound.

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 faster than sound. Thus, when lightning and thunder takes place in the sky at same time and at same distance from us, lightning is seen before the sound is heard.

Check other lessons NCERT Solutions for Class 8

Download free NCERT textbook class 8 Science – Chapter 8 Cell Structure and Functions