NCERT Solutions for Class 8 Science Chapter 13 Sound are provided here with simple step-by-step explanations. These solutions for Sound are extremely popular among Class 8 students for Science Sound Solutions come handy for quickly completing your homework and preparing for exams. All questions and answers from the NCERT Book of Class 8 Science Chapter 13 are provided here for you for free. You will also love the ad-free experience on Meritnationâ€™s NCERT Solutions. All NCERT Solutions for class Class 8 Science are prepared by experts and are 100% accurate.

#### Question 1:

A list of mediums is given below.
(i) wood         (iii) air
(ii) water        (iv) vacuum
In which of these mediums can sound travel?
(a) i & ii only          (c) iii & iv only
(b) i, ii & iii only     (d) ii, iii & iv only

Sound needs material medium to travel. It can travel through solid (wood), liquid (water) and gas (air). But it can not travel in vacuum.
Hence, the correct answer is option (b).

#### Question 2:

The loudness of sound depends on:
(a) its amplitude.      (c) its time period.
(b) its frequency.       (d) its speed.

Loudness of a sound depends on its amplitude. Higher the amplitude, louder the sound.
Hence, the correct answer is option (a).

#### Question 3:

Which of the following statements are correct?
(i) Sound is produced by vibrations.
(ii) Sound requires a medium for propagation.
(iii) Light and sound both require a medium for propagation.
(iv) Sound travels slower than light.
(a) i & ii only
(b) i, ii & iii only
(c) ii, iii & iv only
(d) i, ii & iv only

Sound can be  produced by vibrating objects. Sound requires material medium for propagation. it can not travel in vacuum. Speed of light is very high () as compared to speed of sound (approximately 320 m/s). Light does not require material medium for propagation.
Hence, the correct answer is option (d).

#### Question 4:

An object is vibrating at 50 hertz. What is its time period?
(a) 0.02 s      (c) 0.2 s
(b) 2 s          (d) 20.0 s

Frequency (f) = 50 Hz

Hence, the correct answer is option (a).

#### Question 5:

In order to reduce the loudness of a sound we have to
(a) decrease its frequency of vibration of the sound.
(b) increase its frequency of vibration of the sound.
(c) decrease its amplitude of vibration of the sound.
(d) increase its amplitude of vibration of the sound.

Loudness of the sound can be reduced by decreasing its amplitude.
Hence, the correct answer is option (c).

#### Question 6:

Loudness of sound is measured in units of
(a) decibel (dB)      (c) metre (m)
(b) hertz (Hz)         (d) metre/second (m/s)

Loudness of sound is measured in units of decibel (dB). Wavelength is measured in units of metre (m). Frequency is measured  in units of hertz (Hz). And the speed is measured  in units of metre/second (m/s).
Hence, the correct answer is option (a).

#### Question 7:

The loudness of sound is determined by the
(a) amplitude of vibration
(b) ratio of amplitude and frequency of vibration
(c) frequency of vibration
(d) product of amplitude and frequency of vibration

The loudness of sound  is determined by the amplitude of vibration. Higher the amplitude, higher its loudness.
Hence, the correct answer is option (a).

#### Question 8:

1 hertz is equal to
(a) 1 vibration per minute
(b) 10 vibrations per minute
(c) 60 vibrations per minute
(d) 600 vibrations per minute

Frequency is defined as number of vibrations per second ( Hz).

Hence, the correct answer is option (c).

#### Question 9:

Pitch of sound is determined by its
(a) frequency     (c) speed
(b) amplitude     (d) loudness

Pitch is determined by the frequency of the sound. High pitch means high frequency, and low pitch means low frequency.
Hence, the correct answer is option (a).

#### Question 10:

Ultrasound has frequency of vibration
(a) between 20 and 20,000 Hz
(b) below 20 Hz
(c) above 20,000 Hz
(d) between 500 and 10,000 Hz

Ultrasound has frequency of vibration above 20,000 Hz. It is above the maximum hearing range of normal person.
Hence, the correct answer is option (c).

#### Question 11:

Lightning can be seen the moment it occurs. Paheli observes lightning in her area. She hears the sound 5 s after she observed lightning. How far is she from the place where lightning occurs? (speed of sound = 330 m/s).

Speed of the sound, v = 330 m/s
Time, t = 5 s
Distance travelled,

Paheli is 1650 m far from the place of lightning.

#### Question 12:

Does any part of our body vibrate when we speak? Name the part.

Yes, in humans, the sound is produced by vibrating the vocal cords or the larynx. Muscles attached to the vocal cords can make the cords tight or loose. These function are governed by air pressure produced by lungs.

#### Question 13:

Boojho saw a cracker burst at night at a distance from his house. He heard the sound of the cracker a little later after seeing the cracker burst. Give reason for the delay in hearing the sound.

Speed of light is very high as compared to sound. In other words, sound takes more time to cover the same distance as compared to light So, Boojho heard the sound of the cracker a little later after seeing the cracker burst.

#### Question 14:

When we hear a sound, does any part of our body vibrate? Name the part.

Yes, while hearing, sound vibrations make the eardrum vibrate. The eardrum sends vibrations to the inner ear. From there, the signal goes to the brain. That is how we hear sound.

#### Question 15:

Name two musical instruments which produce sound by vibrating strings?

Guitar and Sitar are musical instruments which produce sound by vibrating strings.

#### Question 16:

A simple pendulum makes 10 oscillations in 20 seconds. What is the time period and frequency of its oscillation?

Number of oscillations, n = 10
Time, t = 20 s
Time period,

#### Question 17:

We have learnt that vibration is necessary for producing sound. Explain why the sound produced by every vibrating body cannot be heard by us?

Our ear is capable to hear the vibrations of the frequency range between 20 Hz to 20,000 Hz. If a vibrating body produces the sound of frequency range above 20,000 Hz or below 20 Hz, then the sound is not audible to us. So, we can not hear vibrations below 20 Hz or above 20,000 Hz.

#### Question 18:

Suppose a stick is struck against a frying pan in vacuum. Will the frying pan vibrate? Will we be able to hear the sound? Explain.

Yes, the frying pan will vibrate by striking the stick on it. As there is vacuum (i.e. no material medium) and sound will propagate in material medium only, so we can not hear the sound of vibrating pan.

#### Question 19:

Two astronauts are floating close to each other in space. Can they talk to each other without using any special device? Give reasons.

In space, there is absence of air i.e. there is vacuum and sound can not propagate in vacuum. So, the astronauts cannot hear sounds of each-other without using any special device.

#### Question 20:

List three sources of noise pollution in your locality.

Loudspeakers during marriage functions and parties, horn of heavy vehicles, sound of metal cutting machines.

#### Question 21:

We have a stringed musical instrument. The string is plucked in the middle first with a force of greater magnitude and then with a force of smaller magnitude. In which case would the instrument produce a louder sound?

Vibration produced will be of higher amplitude if the string is plucked with greater force. And we know that the higher the amplitude of vibrations, the louder the sound. So, the instruments in which the string is plucked with higher force causing higher amplitude, results the louder sound than the other.

#### Question 22:

How is sound produced and how is it transmitted and heard by us?

Sound can be produced by vibrating objects. These vibrations transfers the vibrations to its neighboring material medium particles (air particles). These air particles starts vibrating to and fro and transfer the motion to the next neighboring air particles ...and so on. In this way the sound propagate and the air particle near our ear also starts vibrating and make the eardrum vibrate.
The eardrum is like a stretched rubber sheet. Sound vibrations make the eardrum vibrate. The eardrum sends vibrations to the inner ear. From there, the signal goes to the brain. In this way we can hear the sound.
( Actually, in the whole process, the transfer of energy occurs)

#### Question 23:

An alarm bell is kept inside a vessel as shown in the given figure. A person standing close to it can distinctly hear the sound of alarm. Now if the air inside the vessel is removed completely how will the loudness of alarm get affected for the same person?

Sound needs medium to propagate. The loudness of the alarm clock sound will decrease as the air is removed slowly from the bottle. If the air in the bottle is removed completely, there is vacuum in the bottle. The sound cannot travel through vacuum, the loudness of the sound reduces to zero and hence the person cannot hear the sound of the alarm clock at all.

#### Question 24:

The townhall building is situated close to Boojho’s house. There is a clock on the top of the townhall building which rings the bell every hour. Boojho has noticed that the sound of the clock appears to be much clearer at night. Explain.

In the day time, there are several other sounds of different frequency and loudness ( like...vehicle horn, speaker's sound, machinery sound etc.) apart from clock bell sound, create noise. These sounds affect the loudness and clarity of the clock bell sound.
The noise level is quite low at night. Therefore the sound of the clock appears much clearer at night than in the day.

#### Question 25:

Suggest three measures to limit noise pollution in your locality.