Class 8th Chapter Sound

Class 8 Science - Sound

1. Choose the correct answer.

Sound can travel through:

(d) solids, liquids and gases ✅
Sound needs a medium to travel. It can travel through solids, liquids, and gases, but not through vacuum.

2. Choose the correct answer.

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

(c) A man ✅
A man’s voice has a low pitch and low frequency compared to others, so it has the minimum frequency.

3. Tick True or False

(a) Sound cannot travel in vacuum. – ✅ True
(b) The number of oscillations per second is called its time period. – ❌ False
(c) If the amplitude of vibration is large, sound is feeble. – ❌ False
(d) Human audible range is 20 Hz to 20,000 Hz. – ✅ True
(e) Lower frequency means higher pitch. – ❌ False
(f) Unpleasant sound is music. – ❌ False
(g) Noise pollution may cause hearing loss. – ✅ True

4. Fill in the blanks

(a) Time taken to complete one vibration is called – Time period
(b) Loudness is determined by the – Amplitude of vibration
(c) Unit of frequency is – Hertz (Hz)
(d) Unwanted sound is called – Noise
(e) Shrillness of sound is determined by – Frequency of vibration

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

Frequency = 40 ÷ 4 = 10 Hz
Time period = 1 ÷ 10 = 0.1 second
So, the pendulum has a frequency of 10 Hz and a time period of 0.1 second.

6. Mosquito sound is produced at 500 vibrations per second. Find its time period.

Time period = 1 ÷ 500 = 0.002 seconds
The mosquito sound has a very small time period because it vibrates very fast.

7. Identify the vibrating part of each instrument.

Dholak: Stretched membrane
Sitar: Stretched string
Flute: Air column inside the flute vibrates

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

Music is soft and pleasant. Noise is loud and unpleasant. Music can become noise if played very loudly or continuously.

9. List sources of noise pollution in your surroundings.

• Traffic and horns
• Loudspeakers during events
• Construction work
• Loud music
• Firecrackers

10. How is noise pollution harmful to humans?

It causes stress, hearing loss, sleep problems, and affects brain and heart health if continued for long.

11. Which house should your parents buy – near or away from the road? Why?

They should buy the house away from the road. It will be quieter, with less pollution and safer for living.

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

The larynx is also called the voice box. It has vocal cords that vibrate and help us to produce sound.

13. Why do we see lightning first and hear thunder later?

Light travels faster than sound. So, we see the lightning first and hear the thunder a few seconds later.