NCERT Solutions For Class 8 Science Chapter 13 Sound

Q1. Choose the correct answer.

Sound can travel through

(a) gases only  (b) solids only     (c) liquids only     (d) solids, liquids and gases.

Answer:

Sound can travel through solids, liquids and gases. But the sound does not travel in a vacuum

(d) is correct.

Choose the correct answer.

Q2. 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:

Voice of a man would have the lowest frequency since it is of a lower pitch than the voice of a woman, a baby boy and a baby girl. Pitch is proportional to frequency.

(c) is correct

Q3. In the following statements, tick ‘T’ against those which are true, and ‘F’ against those which are false.

(a) Sound cannot travel in vacuum. (T/F)
(b) The number of oscillations per second of a vibrating object is called its time period. (T/F)
(c) If the amplitude of vibration is large, sound is feeble. (T/F)
(d) For human ears, the audible range is 20 Hz to 20,000 Hz. (T/F)
(e) The lower the frequency of vibration, the higher is the pitch. (T/F)
(f) Unwanted or unpleasant sound is termed as music. (T/F)
(g) Noise pollution may cause partial hearing impairment. (T/F)

Answer:

(a) T .

Sound cannot travel in a vacuum as it requires a medium to travel.

(b) F .

The number of oscillations per second of a vibrating object is called its frequency. Time period is the time taken to complete one oscillation.

(c) F .

The sound of a vibration is directly proportional to the square of the amplitude of vibration , therefore, the sound will be feeble if the amplitude of vibration is low.

(d) T .

For humans, the audible range of frequency is 20 Hz to 20,000 Hz.

(e) F .

The pitch of a sound is directly proportional to its frequency.

(f) F .

Unwanted or unpleasant sound is termed as noise.

(g) T .

Exposure to loud noise for a long period of time may cause partial hearing impairment.

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

Answer:

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

Q5. A pendulum oscillates 40 times in 4 seconds. Find its time period and frequency.

Answer:

Time Period is defined as the time taken to complete one oscillation.

Frequency is defined as the number of oscillations per second.

Q6 . 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:

The time period of vibration is defined as the time taken to complete one oscillation.

Time Period = 0.002 s.

Q7 . Identify the part which vibrates to produce sound in the following instruments.
(a) Dholak (b) Sitar (c) Flute

Answer:

(a) The stretched membrane over the body of the dholak vibrates to produce sound.

(b) The strings stretched over the body of the sitar vibrate to produce sound.

(c) The air inside the flute vibrates when air is blown into it to produce sound.

Q8 . What is the difference between noise and music? Can music become noise sometimes?

Answer:

Noise is sound which is irritating and unpleasant whereas music is sound which is pleasant and soothing. Music can turn into noise at very high volume.

Q9 . List sources of noise pollution in your surroundings.

Answer:

Sources of noise pollution in our surroundings are:

(a) Horns of vehicles on the road.

(b) Loudspeakers and radios.

(c) Crackers.

(d) Machinery inside factories and appliances at homes.

(e) The noise produced by a large number of people.

Q10 . Explain in what way noise pollution is harmful to human.

Answer:

Noise pollution can cause several health-related problems such as:

(i) Insomnia.

(ii) Anxiety and stress.

(iii) Hearing impairment

(iv) Headaches.

Q11 . 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:

I would suggest them to buy the one three lanes away from the roadside as there will be less noise pollution caused by vehicles moving on the road.

Q12 . Sketch larynx and explain its function in your own words.

Answer:

Inside the larynx, there are two vocal cords with a gap between them and when air passes through this gap the vocal cords vibrate and as we know the sound is produced when something vibrates, therefore, this vibration of vocal cords inside the larynx produce sound.

Q13 . 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:

The speed of light is much faster than the speed of sound, due to this the light emitted by lightning reaches our eyes before the sound of thunder reaches our ears and we see lightning before we hear thunder.