soundWHERE cd.courseId=12 AND cd.subId=16 AND chapterSlug='sound' and status=1SELECT ex_no,page_number,question,question_no,id,chapter,solution FROM question_mgmt as q WHERE courseId='12' AND subId='16' AND chapterId='376' AND ex_no!=0 AND status=1 ORDER BY ex_no,CAST(question_no AS UNSIGNED) CBSE Class 8 Free NCERT Book Solution for Science

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Chapter 13 : Sound


  • Vibrating objects produce sound.
  • In humans sound is produced by vibration of the vocal cords.
  • Sound cannot travel in vacuum.
  • Sound can travel through a solid, liquid or a gas medium as well.
  • The vibration of sound are senses by the eardrum.
  • Eardrum is a stretched membrane.
  • Eardrum send signals to brain. This process is known as hearing.
  • Frequency of oscillation may be defined as the number of oscillation or vibrations in one second.
  • Hertz (Hz) is the unit of frequency.
  • The unpleasant and unwanted sound is called as noise. It may lead to noise pollution.
  • Many health hazards are produced by noise pollution.
  • Loudness of sound is called as amplitude or we can say the displacement of vibrating object from its main position is called amplitude.
  • Eardrum is a stretched membrane which separates the middle ear from the outer ear which vibrated to produce sound.
  • Larynx is known as the voice box which produces sound.
  • Loudness is the characteristics of sound which depends upon the amplitude.
  • Oscillation is the complete to and fro motion of an object about at its mean position.
  • Pitch is known as the flatness or shrillness of a sound.
  • Time period may be defined as the time taken by vibrating objects to complete one oscillation.
  • Vibration is the to and fro movement of a body.
  • Windpipe connects the larynx to the lungs, it is tube like structure of respiratory tract.

Exercise 1 ( Page No. : 171 )
Q:
A:

(d) Solid, liquids and gases

Sound can travel through solid, liquid as well as through gas medium. Sound cannot travel in vacuum.


Exercise 1 ( Page No. : 171 )
Q:
A:

(a) Baby girl

A baby girl voice is likely to have less or minimum frequency in comparison to baby boy, a man and a woman. A baby girl produced a low pitch sound.


Exercise 1 ( Page No. : 171 )
Q:
A:

(a) True

This statement is true because sound cannot travel in vacuum. Sound is produced by vibrations. If there, air is not present then sound cannot be produced.

(b) False

This statement is false because the number of oscillations per second of a vibrating object is called its frequency.

(c) False

This statement is false if the amplitude of vibration is large, then the sound will be sharp and having a high pitch.

(d) True

This statement is true because the range of hearing a sound is 20 Hz to 20000 Hz. Above 20,000 Hz the sound is not audible and harmful for ears.

(e) False

This statement is false because as higher is the frequency, the pitch of the sound is high.

(f) False

This statement is false because the unwanted and unpleasant sound is termed as noise pollution whereas the pleasant sound is known as music.

(g) True

This statement is true because noise pollution is harmful to the ears and it may cause partial hearing impairment. The unwanted and unpleasant sound is known as noise pollution.


Exercise 1 ( Page No. : 171 )
Q:
A:

(a) Time period

Time period may be defined as the time taken by an object or a body to complete the one oscillation or a cycle.

(b) Amplitude

Amplitude shows the loudness of the sound. When the amplitude of the vibration is larger, then the sound produced will be very loud. When the amplitude is small then the produced sound will be feeble.

(c) Hz

Hertz is the unit of frequency. Frequency may be defined as the number of oscillation in one second.

(d) Noise

The sound which is unpleasant and unwanted to ears known as noise whereas the sound which us pleasant to the ears is known as music.

(e) Pitch

If the frequency of the vibration is higher, then the produced sound will be shrill and it has high pitch whereas if the frequency of vibration is lower, then the sound will contain lower pitch.


Exercise 1 ( Page No. : 171 )
Q:
A:

Number of oscillations of pendulum = 40

Time taken = 4 second

Frequency = Number of oscillation                                                                                                                                                                                                                                   Time taken

Frequency = 40/4 = 10 Hz


Exercise 1 ( Page No. : 171 )
Q:
A:

Number of vibrations of mosquito wings in one second = 500

Frequency = 500 Hz

Time period =          1                                                                                                                                                                                                                                                   Frequency

= 1/500

= 0.02 second

 


Exercise 1 ( Page No. : 171 )
Q:
A:
  1. Dholak: In Dholak, when stretched membrane vibrates, then it produces sound. The stretched membrane is beaten up for vibrations, then it produces sound.
  2. Sitar: In Sitar, vibrations are produced by the stretched string. When the strings are pulled or we can say pushed, then production of vibrations occurs and then it produces pleasant sound.
  3. Flute: In Flute, vibrations are produced by the air columns. In this, a number of holes are present, through them air produces sound. It produced a pleasant sound.

Exercise 1 ( Page No. : 171 )
Q:
A:

The sound which is unpleasant or unwanted to ears are known as noise whereas the sound which is pleasant to our ears known as music. Music gives us relaxation whereas noise produces so many health problems like loss of hearing, headache and many other problems.

Yes, music can become a noise sometime when the instrument like Dholak produces a very high volume of sound.


Exercise 1 ( Page No. : 171 )
Q:
A:

There are many sources of noise pollution around us. Some are as follows:

  1. Sound produced by vehicles.
  2. Sound of a loud speaker
  3. Sound of bog machines in factories.
  4. Sound produced at construction sites.
  5. Sound produced by television at high volume.
  6. Bursting of crackers produces noise pollution and air pollution too.

Exercise 1 ( Page No. : 171 )
Q:
A:

The unpleasant or unwanted sound is called as noise. Noise pollution is harmful for the human beings. Some of the health hazards produced by the noise pollution are as follows:

  1. Lack of sleep
  2. Hypertension
  3. Anxiety
  4. Headache
  5. Loss of hearing etc.

Exercise 1 ( Page No. : 171 )
Q:
A:

I would suggest my parents to buy the house which is three lanes away from the roadsides. It will protect us from the noise pollution. The house should bought away from roads in a peaceful place which gives positive energy and keeps you calm.


Exercise 1 ( Page No. : 171 )
Q:
A:

larynx

Larynx is also known as the voice box. The sound is produced by the larynx. It feels like a hard bump on touch that seems to move when you swallow. This is also known as the voice box.


Exercise 1 ( Page No. : 171 )
Q:
A:

In the sky, lightning and thunder takes place at the same time and at the same distance from us. Lightning is seen earlier and then thunder is heard later because the speed of the light is very large in comparison to the speed of sound in air.

Speed of light = 3ooooo km/s

Speed of sound in air = 330 m/s air


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