sound - Class 8

 SOUND

1:- What is sound? 
Ans:- Sound is a form of energy that travels in the form of vibrations through the air or any another medium. It is measured in hertz (Hz)
2:- What is vibration?
Ans:- a rapid to and fro movement of an object about its mean position is called vibration.
3:-Why sound is also known as web?
Ans:-    A sound is a form of energy that is produced when air molecules vibrate in a particular pattern called waves. Hence, the sound is also known as a wave.
 4:- Write two example of vibratory motion?
Ans:- pendulum, swing 
5:- Which is the sound producing organ in human body?
Ans:- larynx or voice box is the sound producing organ of human body. It is located at the upper end of the windpipe.
6:- Write about the structure and function of voice box.
Ans:- Larynex or voicebox is a sound producing organ of human body. It is located at the upper end of the windpipe. Two vocal cords are stretched across the voice box. A narrow slit is present between the vocal cords for the passage of air. When the lungs force air through the slit, the vocal cords vibrate to produce sound. Tighter the vocal cords, higher the sound produced. 
7:- How do the women and children produce higher frequency sound then men?
Ans:- Women and children have shorter vocal chords then men, therefor women and children produce higher frequency sound then men.
8:- How does a musical instrument produce sound?
Ans:- A musical instrument produces sound when a part of it vibrates rapidly.
9:- How many types of musical instrument on the basis of their vibrating part?
Ans:- There are three types of musical instrument on the basis of their vibrating part.
1:- String instruments
2:- wind instruments
3:- percussion instruments
10:- What are string instruments? give example.
Ans:- Those musical instrument that produce sound by vibrations of stretched strings are called strins instruments. Ex:- guitar, violin, sitar, piano etc.
11:- What are wind instruments? give example.
Ans:- Those musical instrument that produce sound by vibrations of the air column are called wind instruments. Ex:- flute, shehnai, trumpet, clarinet etc.
12:- What are percussion instruments? give example.
Ans:- Those musical instrument that produce sound by vibrations of a stretched membrane when beaten are called percussion instruments. Ex:- tabla, congo, gong, drum etc.   
13:- How does sound travel from one place to another?
Ans:- Sound travels in mechanical waves. A mechanical wave is a disturbance that moves and transports energy from one place to another through a medium. In the case of sound the source of disturbance is the vibrating object. The vibrating body transfers its energy to the neighbouring molecules in a medium. The neighbouring molecules pass the energy to the other molecules. Thus sound is propagated in the form of a wave that spreads in all directions. We hear the sound when the vibrations reach our ears.
14:- What is mechanical wave?
Ans:- A mechanical wave is a disturbance that moves and transports energy from one place to another through a medium.
15:- How does sound propagates from one place to another?
Ans:-Sound is a mechanical wave. All mechanical wave need a medium to propagate from one place to anothere. So sound also requires a medium to propogate from one place to another. The mediums are either solid, liquid or gas. Sound travels fastest in solids and slowest in gases. The speed of sound in air at 0˚ c is 332 m/s
16:- Prove that sound needs a medium for propagation.
Ans:- Sound needs a medium for propagation. We will prove it by Bell jar activity.


► Aim :- To show that Sound cannot travel through vacuum.

 

                      

► Materials Required :- A bell jar, electric bell, cotton thread or rubber band, a cork.

► Procedure :- Take the bell jar, Suspend an electric bell inside it with the help of a cotton thread or rubber band through an air tight cork.

Connect a vacuum pump through another opening in the jar. Connect the bell to a battery through a key Initially, the bell jar is full of air. Now press the switch. The bell starts ringing and you can hear its sound clearly.

Now, pump out the air from the bell jar gradually by switching on the vacuum pump attached in it.

► Observation :- You will notice as more and more air is removed from the bell jar, the sound of the bell becomes fainter and fainter. and when a vacuum is created , no sounds is heard, in spite of the fact that hammer is striking against the gong of bell.

Stop the vacuum pump and allow the air to enter into the bell jar. You will be able to hear the sound of the ringing bell again.

► Conclusion : - Sound cannot travel through vacuum, it always need a medium to propagate.

This Experiment Shows that :

◆ Sound cannot travel through vacuum, but can travel through air.

◆ Thus, a material medium is necessary for the propagation of sound.

 

 17:- Write an experiment that prove that sound can travel through liquid.

Ans:-             

Take a tub filled with water. Hold a bell in one hand and dip it in water. Keep one of your ears gently on the surface of water without letting water into the ear. Now ring the bell inside water. You will be able to hear the sound clearly. This shows that sound can travel through liquids.

18:- Write an experiment that prove that sound can travel through solid.

                     

Ans:- Take two empty ice-cream cups. Make a small hole at the bottom of each cup and pass a long thread (about 20 m long) through them. Tie a knot or match-stick at each end of the thread so that the thread does not slip out through the holes. This makes a toy – telephone .Now use the toy-telephone to talk to your friend. You will be able to hear the sound of your friend. This shows that sound travels through the thread and reaches your ear. Thus, sound can travel through a solid. 

19:- Write an experiment that prove that sound can travel through solid.

                         

Ans:- Sit at one end of a long wooden or metallic table. Place your ear at this end of the table. ask your friend to gently tap the other end of table. you can hear the tapping sound very clearly. This show that that the sound can travel through solid. 

20:- Why do we see lightning before we hear thunder?

Ans:- The speed of sound in air is 332m/s while the speed of light in air is 3 × 10⁸ m/s. The speed of light is much more than the speed of sound in air. Therefor we always see the lightning much before than the thunder.

21:- What is hearing?

Ans:- Hearing, is the ability to perceive sounds by detecting vibrations. Our ears are incharge of collecting sounds, processing them and sending sound signals to the brain. This process is called hearing.

22:- How many sections in a human ear?

Ans:- There are three sections in a human ear.

              

          1- The outer ear

          2- The middle ear

          3- The inner ear

23:- What is the function of outer ear?

Ans:- The outer ear is called pinna. It is funnel shaped. its main job is to collect sound waves. After collecting the sound waves, outer ear passes  the sound waves to the middle ear by the ear canal.

24:- What is the function of middle ear?

Ans:- The middle ear converts the sound waves into vibrations by the help of thin membrane called ear drum. The thin eardrum stretched tightly at the end of ear canal.The eardrum sends these vibrations to the inner ear.

25:- What is the function of inner ear?

Ans:- The inner ear receives vibrations from the middle ear and convert them into nerve impulses, which are then transmitted to the brain by the auditory nerve. The brain perceives these impulses as sound.              

                          

 26:- What is oscillatory motion?

Ans:- the to and fro motion of an object from its mean position in a fixed time interval is called oscillatory motion. 

Ex:- Oscillation of simple pendulum, the beating of the human heart

               

27:-  What is frequency of oscillation?

Ans:- The number of oscillations made by an object in one second is called the frequency of oscillation.

The SI unit of frequency is hertz that is denoted by 'Hz'. Frequency is represented by 'f '.

28:- What is amplitude?

Ans:- The maximum displacement of the object from its mean position is called amplitude. Amplitude is the measure of loudness of sound. Larger the amplitude, louder is the sound.

29:- What is time period?

Ans:- The time taken by an object to complete one oscillation is called its time period. It is denoted by a symbol 'T'

30:- What is relation between frequency (oscillation ) and time period?

Ans:-   T = 1 / f

31:- If the frequency of a wave is 500 Hz.What is its time period?

Ans:-  f = 500 Hz

          T = 1 / f

             = 1 / 500 second

            = 0.002 second

32:- What is loudness?

Ans:- Loudness of sound depends on the amplitude of vibration or oscillation. When the amplitude is large then the sound produced is loud. When the amplitude is small then the sound produced is feeble. The loudness of sound is expressed in a unit called decibal and it is represented by "dB".

33:- Which sound is painful for our ear?

Ans:- Sound having a value above 90 dB is painful for the ear.

34:- What is the relation between loudness and amplitude?

Ans:- Loudness is proportional to the square of the amplitude of the vibration. 

If amplitude becomes twice then loudness increases four times.

35:- What is pitch?

Ans:- The frequency of the vibration determines the pitch or shrillness of the sound. The more is the frequency of the vibration, higher is the pitch of the sound. 

Ex:-

 👉A drum has low pitched sound because it vibrates with low frequency.

👉 A whistle has high pitched sound because it vibrates with high frequency.

36:- What are audible sounds?

Ans:- The sound ranges whose frequency is between the range of 20Hz-20KHz. These sound ranges can be detected by human ears. Such sounds are called audible sounds.

 

37:- What are inaudible sounds?  

Ans:- The sound ranges whose frequency is less than 20Hz and greater than 20KHz. These sound ranges can not detected by human ears. Such sounds are called inaudible sounds.

38:- What are ultrasonic waves?

Ans:- Sound waves whose frequency lie above 20,000Hz are called ultrasonic waves. animals like dogs and bats can hear ultrasonic waves.

39:- What are infrasonic waves?

Ans:- Sound waves whose frequency lie below 20 Hz are called infrasonic waves. such waves are produced by whales, elephants, earthquakes and vibrating pendulum.

40:- What is noise?

Ans:- A sound that produces an unpleasant or jarring effect on our ear is called a noise. It is produced when vibrations produce waves in an irregular pattern.

Ex:- sound of horns of buses and trucks

41:- What is a music sound?

Ans:- A sound that produces a pleasant effect on our ear is called a music sound. It is produced when vibrations produce waves in a regular pattern.

42:- What is noise pollution? Write six major sources of noise pollution.

Ans:- Unwanted or unpleasant sounds in the environment is called noise pollution. High pitched noises are more annoying than low-pitched noise.

The six major sources of noise pollution are

1-aircrafts

2-motor vehicles

3-explosions including bursting of crackers

4-machines including domestic appliances

5-loudspeakers

6-loud musics

43:- Write five harmful effects of noise pollution.

Ans:- Five harmful effects of noise pollution are

1- Noise pollution that is above to 85 dB can cause tiredness, nausea ( उल्टी  ), headache, and gradual loss of hearing.

2- Sudden exposure to loud noise that is above to 120 dB can cause rupture of ear drum and thus can cause permanent hearing loss.

3- Noise pollution can be a contributing factor to high blood pressure, anxiety ( व्याकुलता  ), and nervousness.

4- Lack of sleep may be caused by noise pollution.

5- Noise of under water drilling and blasting can damage hearing of marine animals like seals, whales etc.

44:- Write five measures that help us to control noise pollution.

Ans:- The five measures that help us to control noise pollution are

1- silencing devices must be installed in aircraft engines and transport vehicles.

2- quieter engines should be designed to reduce vibrations

3- Use of loudspeakers should be controlled.

4- use of automobiles horns should be minimised.

5- TV and music system should be run at low volume.  

Ex:- sound produced by musical instrument.

 

45:- Write some important point about sound.

Ans:-

👉 Sound is a form of energy that is produced by vibrating bodies. 

 👉 Sound helps us to communicate with each other.

👉 Some sounds are pleasant. like sounds of cuckoos, sound of musical instrument 

👉 Some sounds are unpleasant. like sounds produced by horns of vehicles.

👉singing, shouting, banging, talking, whispering are some specific types of sound  

👉It produces the sensation of hearing in our ears.