# NCERT Solution For Class 8 Science, Chapter 13 – Sound

## Class 8 Science, Chapter 13 – Sound

NCERT Solutions is known as an extremely helpful resource for preparing for the exam. Takshila Learning provides its learners with access to a wealth of NCERT problems and their solutions. CBSE Class 8 Science NCERT Solutions are built by subject matter experts, so be sure to train learners for a good grade. The questions set out in the NCERT Books are prepared in compliance with the requirements of the CBSE.

NCERT Solution For Class 8 Science, Chapter 13 – Sound provides us with all-inclusive information on all concepts. As students would have to learn the basics about the subject in class 8, this curriculum for class 8 is comprehensive study material, which explains the concepts in a great way.

Important Formulae

Q1. Choose the correct answer.

Sound can travel through

(a)        gases only        (b)        solids only

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

Solution:

(d)        Sound can travel through solids, liquids, and gases.

Sound requires a medium to travel through. As all solid, liquid and gas provide the medium for sound, sound can travel through solids, liquids and gases.

Q2. Which of the following voices is likely to have minimum frequency?

(a)        Baby girl          (b)        Baby boy

(c)        A man              (d)        A woman

Solution:

(c) A man

The voice of an adult man is of lower pitch in comparison to the voices of a baby boy, a baby girl or woman. As frequency of a sound is directly proportional to its pitch, man’s voice is of minimum frequency in comparison to a boy, a girl, or a woman’s voice.

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)

Solution:

(a)        True

Sound requires a medium to travel through. Since vacuum doesn’t have any medium, sound cannot travel in vacuum.

(b)        False

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

(c)        False

Loudness of a sound is proportional to the square of the amplitude of its vibration. When the amplitude of vibration of a sound is large, the sound is very loud. The sound is feeble for small amplitude.

(d)        True

Humans cannot hear sounds of all frequencies. Humans can hear a sound whose frequency falls in the range of 20 Hz−20,000 Hz. The sound having frequency out of this range is inaudible to humans.

(e)        False

The pitch of a sound is proportional to its frequency. As the frequency of vibration increases, the pitch of the sound also increases and vice-versa. A sound is said to be high pitched if its frequency of vibration is high, and is low pitched for a small frequency of vibration.

(f)        False

Unwanted or unpleasant sounds are known as noise. Sounds that are melodious and pleasing to ear are known as music.

(g)        True

Unwanted or unpleasant sounds are known as noise. If one is subjected to loud unpleasant sound continuously for a long time, then it may cause temporary hearing impairment.

Q4. 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.

Solution:

(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 (Hz) .

(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.

Solution:

Frequency of oscillations is defined as the number of oscillations of a vibrating body per second.

It is given by

Frequency = (Number of oscillations)/ Total Time = 40 / 4 = 10 Hz

The time required to complete one oscillation is known as time period. It is given by the inverse of the frequency.

Time period   = 1/ Frequency = 1/10 = 0.1 s

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?

Solution:

The time required to complete one oscillation is known as time period. It is given by the inverse of the frequency.

Time period  = 1/ Frequency

Frequency of oscillations = 500 Hz

Time period  = 1/500 = 0.002 s

Q7. Identify the part which vibrates to produce sound in the following instruments.

(a)        Dholak            (b)        Sitar    (c)        Flute

Solution:

(a)       Dholak is a instrument which consists of a stretched membrane called its head. When the top is beaten gently, the stretched membrane sets into vibration. Since sound is produced when an object vibrates, the dholak produces a sound.

(b)        Sitar is  a instrument which consists of stretched strings. When a string is plucked, it sets into vibration. Since sound is produced when an object vibrates, the sitar produces a sound.

(c)        Flute is a hollow pipe. When air is blown over its mouth, the air inside the pipe is about into vibration. As a result, a pleasing sound is produced.

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

Solution:

The sound that’s pleasing to the ear is termed music. Like, the sound produced by violins, pianos, flutes,  etc.

The sound that’s unpleasing to the ear is termed noise.

Some examples of noise are as follows:

(i)         Sound produced by horns of buses and trucks

(ii)        Sound of electrical generators

(iii)       Sound of a gun shot

(iv)       Sound produced by jackhammers

Q9. List sources of noise pollution in your surroundings.

Solution:

Some sources of pollution are as follows:

(i)         Televisions and transistors running at high volumes

(ii)        Loudspeakers and crackers

(iii)       Horns of buses, cars and trucks

(iv)       Home appliances like mixer, desert cooler, etc.

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

Solution:

Noise pollution can cause variety of health-related problems like:

(i) hearing disorder

(ii) Insomnia; inability to sleep

(iii) Hypertension

(v) Stress

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.

Solution:

There will be more noise within the house which is along the roadside. The noise produced by transportation vehicles may cause trouble to the residents. The intensity of noise decreases with the gap between the source and therefore the listener. So, it’s better to require the house that’s three lanes far from the roadside.

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

Solution:

Larynx is a part of the throat which is responsible for production of sound.

A sketch of a human larynx is shown in the following figure.

Larynx moves when we swallow something. Inside the larynx, there are two vocal cords. There is a small gap between them. This small gap allows air to pass through. When we speak, air is forced into this small gap by the lungs. This prompts vocal cords to vibrate. Since vibrating objects produce sound, sound is produced due to the vibration of vocal cords.

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?

Solution:

The speed of sound is less than the speed of light. So, light reaches us before the sound during a lightning, which is accompanied by thundering.

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