8 science sound

What is sound?
Ans: Sound is a form of energy produced due to the vibration of an object.

The sound absorbing materials are used in the walls of the auditoriums. Why?
Ans: The sound absorbing materials are used in the walls of the auditoriums to minimize reverberation.

What is amplitude?
Ans: The maximum displacement of a vibrating body from its mean position is called amplitude.

What is echo?
Ans: The repetition of sound caused due to its reflection is called echo.

What is frequency?
Ans: The number of complete vibrations made by a particle of a body in one second is called frequency.

What is wavelength?
Ans: The distance between two successive compression or rarefaction is called wavelength.

What is infrasonic sound?
Ans: The sound with frequency less than 20 Hz is called infrasonic sound.

What is ultrasound?
Ans: The sound with frequency greater than 20 kHz is called ultrasound.

What is time period?
Ans: The time taken by a vibrating body to complete one vibration is called time period.

What is an audible sound?
Ans: The sound with frequency between 20 Hz to 20 kHz is called audible sound.

What is transmission of sound?
Ans: The phenomenon due to which the sound travels from one place to another is called transmission of sound.

The sound of thunder is heard later than the flash of lightning. Why?
Ans: The sound of thunder is heard later than the flash of lightning because the speed of sound is very less than the speed of light.

What is frequency? What do you mean by frequency of sound is 50 Hz? 
Ans:  Frequency is defined as the number of complete vibrations made by the vibrating particles in one medium.
Frequency of sound is 50 Hz; it means that 50 complete sound waves are being produced in 1 second.
 
Write any two differences between echo and reverberation. 
Ans: The differences between reverberation and echo are as followings:
  Reverberation            EchoReverberation is defined as prolongation of sound produced by a series of reflection.The repetition of sound caused due to its reflection is called an echo.The minimum distance between the source of sound and the reflecting body should be 17 m.The distance between the source of sound and the reflecting body should be less than 17 m.

What is sound? Write unit of frequency and wavelength. 
Ans: Sound is one type of energy which is possessed due to vibration of bodies and can be experienced by our ears.
The unitsof frequency and wavelength are Hz and metre respectively. 
 
What is wavelength? Calculate the velocity of sound having frequency 100Hz and wavelength 3.3m.
Ans: The distance between two consecutive compressions or refractions of a sound wave is called wavelength of that wave.

Here,
Given,
Frequency (f) = 100 Hz
Wavelength (ʎ) = 3.3 m
Velocity (V) =?
We have,
V = f х ʎ = 100 х 3.3 = 330 m/s
Hence, the velocity of sound wave is 330 m/s.
 
 What is transverse wave? Write one effect of reverberation.
Ans:  If the particles medium vibrate perpendicular to the direction of the motion of the wave, such a wave is called transverse wave.

Due to effect of reverberation, original sound gets reflected  and reflected sound mixes with original sound and prolongation of sound occurs. 
 
Astronauts wear special device on their ear, why? Define wavelength. 
Ans: Sound wave is mechanical wave i.e. it need a material medium to for their propagation. In space there is vacuum i.e. no air is present for the propagation of sound, so astronauts cannot communicate with each other. Hence, astronauts wear special device on their ear for communication.

The distance between two consecutive compressions or rarefactions of a sound wave is called wavelength of that wave. It is denoted by 'ʎ'. 

What do you mean by sources of sound? Give some examples.
Ans: The objects which vibrate continuously or the objects that we make vibrate are called the sources of sound.
For example: Guitar, Madal, Radio, Whistle etc.
 
 Sound is not heard on the surface of the moon. Why?
Ans: Sounds are mechanical waves i.e. they need material medium to travel. The earth has atmosphere consisting air which acts as the medium of sound. But it is vacuum on the surface of moon i.e. there is no material medium. Hence, sound cannot be heard on the surface of the moon.
 
Why is sound called a longitudinal wave?
Ans: Sound wave is called a longitudinal wave because the particles of the medium vibrate in the direction of the propagation of the wave.    
 
What is a wave?
Ans:  A wave is a disturbance that travels in a medium due to repeated periodic motion of particle about their mean position, - such that the disturbance is handed over from one particle to the other without the actual motion of the medium.
 
What is oscillation, time period, frequency and vibration?
Ans: Each complete movement of an oscillating body is called an oscillation.
    
Time taken to complete one oscillation is called time period.
  
The number of complete oscillation made in one second is called frequency. 
  
The up and down or to and fro movement of a body is called vibration.
 
What is compression and rarefraction?
Ans: A compression is a region in a longitudinal wave where the particles are closest together.
    
A rarefaction is a region in a longitudinal wave where the particles are furthest apart.
 
What is the audible range of human ear? Write the range of Infrasonic and Ultrasonic sound.
Ans: The audible range of human ear is 20 Hz to 20,000 Hz.
    i. Infrasonic Sound: 1 Hz to 20 Hz
    ii. Ultrasonic Sound: Above 20,000 Hz  
 
Explain sound.
Ans:  Sound is a form of energy which produces sensation of hearing to our ears. It is a mechanical wave because it needs material medium to travel. Human ears cannot hear every sound that is produced because the range of human ear is 20 Hz to 20,000 Hz. The sound lower than this frequency is known as infrasonic sound and the sound above this range of frequency is known as ultrasound.
 
Describe an experiment to show that sound can travel in solids.
Ans: Go to near a table with your friend. Tell your friend to stand on the other side of the table. Keep your one ear pressed gently on the surface of the table and close the other ear with your finger. Tell your friend to gently knock at the other edge of the table. You will hear the sound clearly. This experiment describes the ability of sound to travel in solid medium.
 
Describe hearing process in brief.
Ans: The hearing process mainly includes these three steps:
i.    Source of sound: A vibrating body produces sound. Thus, the source of sound is a vibrating body.
ii.   Medium for propagation of sound: As sound is a mechanical wave it needs material medium to travel. It can travel through all the states of matter namely: solid, liquid and gas.
iii.  Sound receiver: Our ear acts as a sound receiver. When the sound hits the diaphragm of ear, it starts to vibrate. The vibration is further processed by brain to give a clear sense             of sound.
 
Write short notes on propagation of sound in different medium.
Ans: Sound is a mechanical wave and it needs material medium to propagate. The sound wave can travel through all the states of matter i.e. solid, liquid and gas. But the velocity of sound is not equal in the entire medium. The velocity of the sound is maximum in the solid medium and least in the gaseous medium. In solids sound propagate in the form of transverse wave and in liquid and gas it travels in the form of longitudinal wave.
 
Describe three examples which show that sound is produced due to vibration.
Ans: We have seen people playing guitar. In guitar when they pluck the strings of guitar, it vibrates and produces sound. Sound produced by it travels through air which acts as the medium.

Another example is a tuning fork. Fork consists of two tines and a handle. When the fork is struck on hard surface the two tines starts vibrating. The back and forth motion of tines produces disturbances in the surrounding air molecules resulting sound to be heard.
    
The sound produced by us is also the result of vibration. When we speak the vocal chords vibrate which comes out from our mouth and travel through air.
 
The frequency of an oscillating body is 10 Hz. Calculate the total oscillations made in 50 second.
Ans:  Given,
    Frequency (f) = 10 Hz
    Time period (T) =?
    Total oscillation in 50 second =?
    We know time and frequency are related by the relation:
    Frequency =  1/(Time Period) 
    or,     10    = 1/(Time Period)
    ∴ Time Period = 1/10 = 0.1 s 
    Now,
    In 0.1 s the body oscillates 1 time.
    In 1 s the oscillation made by the body =    1/0.1 = 10
In 50 s the total oscillations made by the body = 10 ⨉ 50 = 500 oscillations.
Therefore,
Time period = 0.1 s and the total oscillations made in 50 seconds = 500 

A stone takes 2 seconds to complete 10 oscillations. Calculate the time period of oscillation and frequency.
Solution: For 10 oscillations, time = 2s
For 1 oscillation, time = 2/10s
∴     Time period of oscillation 0 0.2s
    Also,
    Frequency is calculated as:
    In 2 second, the stone completes 10 oscillations 
    In 1 second, the stone completes 10/2 = 5 oscillations.
∴ Frequency = 5 oscillations per second.
Alternatively,
    Frequency = 1/(Time Period)
                      = 1/0.2   
                      = 5 oscillation per second or 5 Hz.
 

Differentiate between music and noise.
Ans: The differences between noise and music are as follows:

Music

Noise

1. It is produced by periodic vibrations.

1. It is produced by irregular vibrations.

2. It has regular wave pattern.

2. It has irregular wave pattern.

3. They are pleasing to our ears.

3. They are unpleasant to our ears.

4. As they are periodic waves they can be reproduced.

4. They are non-periodic wave so they cannot be reproduced.

Example: Sound produced by musical instruments like guitar, drum etc.

Example: Sound produced by moving bus, train, mixer grinder etc.

 

Differentiate between transverse and longitudinal wave.
Ans: The differences between transverse and longitudinal waves are as follows:

Transverse Wave

Longitudinal wave

1. In transverse waves, the particles of the medium vibrate perpendicular to the direction of wave.

1. In longitudinal waves, the particles of the medium vibrate along the direction of wave motion.

2. These waves travel in the form of alternate crest and troughs.

2. These waves travel in the form of alternate compressions and refractions.

3. These waves can be transmitted through solids or over liquids.

3. These waves can be transmitted through all the three type of media i.e. solid, liquid and gas.

Examples:

i) Ripples formed over water surface.

ii) Waves produced in stretched strings.

Examples:

i. Sound waves in air.

ii) Waves formed along a compressed spring.

 What is wave motion? Define different types of wave.
Ans: Wave motion is a form of disturbance which travels through the medium due to particles of the medium about their mean position.

There are two types of wave:
•    Transverse wave: The wave where the particles of the medium vibrate perpendicular to the direction of wave is known as transverse wave. The maximum positive displacement is known as crest and the maximum negative displacement is known as trough.

 
•    Longitudinal wave: The wave where the particles of the medium vibrate along the direction of wave motion is known as longitudinal waves. In this type of wave, particles move to and fro to the direction of the wave giving rise to compressions and refractions.

 
What is crest and trough?
Ans: The maximum displacement in the positive direction produced by transverse wave is known as crest.

Trough is a depression i.e. maximum displacement in the negative direction.

 

Name the medium A, B and C according to the speed of sound.MediumVelocity of soundA5130 m/sB1500 m/sC344 m/s
Ans: In the above table the mediums of sound are:
A - Iron
B - Water
C - Air

Describe three examples which show that sound is produced due to vibration.
Ans: We have seen people playing guitar. In guitar when they pluck the strings of guitar, it vibrates and produces sound. Sound produced by it travels through air which acts as the medium.

Another example is a tuning fork. Fork consists of two tines and a handle. When the fork is struck on hard surface the two tines starts vibrating. The back and forth motion of tines produces disturbances in the surrounding air molecules resulting sound to be heard.
    
The sound produced by us is also the result of vibration. When we speak the vocal chords vibrate which comes out from our mouth and travel through air.
 
Write short notes on propagation of sound in different medium.
Ans: Sound is a mechanical wave and it needs material medium to propagate. The sound wave can travel through all the states of matter i.e. solid, liquid and gas. But the velocity of sound is not equal in the entire medium. The velocity of the sound is maximum in the solid medium and least in the gaseous medium. In solids sound propagate in the form of transverse wave and in liquid and gas it travels in the form of longitudinal wave.
 

9Science Sound

1. Why is sound wave called a longitudinal wave?

Ans: A sound wave is called a longitudinal wave because in a sound wave the particles of the medium move in to and fro motion in the direction of the wave.

2. What is transverse wave?

Ans: The wave in which the particles of the medium move up and down perpendicular to the direction of propagation of the wave is called transverse wave.

3. What is the wavelength of sound wave?

Ans: The distance between two consecutive compressions or rarefactions in a longitudinal wave is the wavelength of the sound wave.

4. What is frequency of the wave?
Ans: The number of complete vibration made by a particle of a medium in one second is called the frequency of the wave.

5. What is the wavelength? A radio station's broadcasting is at 1000 MHz. Calculate the wavelength if the speed of the radio wave is 3 х 10⁸m/s.

Ans: The distance between two consecutive troughs or crests in a transverse wave or the distance between two consecutive compressions or rarefactions is called wavelength. It is denoted by ʎ.

Fig: Wavelength
Here,
Given,
Frequency (f) = 1000 MHz = 1000 х 106 Hz = 1 х 10⁹ Hz
Wave length (ʎ) =?
Wave velocity (v) = 3 х 10⁸ m/s
We have,
v = f х ʎ
Or, 3 х 10⁸ = 1 х 10⁹х ʎ
ʎ = 0.3 m
Hence the wavelength is 0.3 m.
6. What is rarefaction?

Ans: The region where the air molecules are loosely arranged is called rarefaction.

7. What is an echo?

Ans: The repetition of a sound caused by the reflection of a sound wave is called an echo.

8. What is reverberation?
Ans: The prolongation of original sound due to the mixing of the reflected sound to the original sound is called reverberation.

9. What is the refraction of a sound wave?

Ans: The phenomenon of bending of sound waves when it passes from one medium to another medium is called refraction.

10. What is noise?
Ans: The unwanted and unpleasant sound produced by the irregular vibrations is called noise.

11. What is pitch?

Ans: Pitch of a sound is a physical cause which enables us to distinguish a shrill sound from a dull sound of the same intensity.

12. What are the factors which affect pitch of a sound?

Ans: Pitch of a sound is affected by the frequency, distance between source and observer and thickness of the wire.

13. How are speed, wavelength and frequency related?

Ans: Speed, wavelength and frequency are related by the relation,

speed = frequency × wavelength

14. What is compression?

Ans: The region where the air molecules are tightly packed is called compression.

15. The speed of sound is in solid different and liquid. Why?

Ans: The speed of sound is different in solid and liquid because the molecules are tightly arranged in solids whereas the molecules are loosely arranged in liquids.

16. Why is the voice of a child shriller than that of an old man?

Ans: It is because the voice of a child has higher pitch due to higher frequency but the voice of an old man has low pitch due to low frequency.

17. What is call ultrasound?

Answer: The sound having the frequency greater than 20 kHz is called ultrasound.

18. What is an infrasonic wave?

Ans: The sound wave with frequencies below 20 Hz is called infrasonic sound.

19.What is the frequency of sound that can be heard by human ear?
Ans: 20 Hz to 20 kHz is the frequency of sound that can be heard by human ear.

20. Why are mineral wools or porous blocks used in the ceiling, floor or walls of a room?

Ans: Mineral wools or porous blocks are used in the ceiling, floor or walls of a room because to prevent the reflection of sound.

21. What is intensity of sound?

Ans: The rate at which sound waves carry energy from the source is called intensity of a sound.
22. How is intensity related to loudness?Ans: The increase in the intensity of the sound increases the loudness of the sound.

23. What is sound?

Ans: Sound is a form of energy that is produced by the vibration of objects.
24. What is spectrum of sound wave?

Ans: The group of sound waves having different frequencies is called the spectrum of sound wave.

25. What happens to the wavelength of a sound, when its pitch increases?

Ans: When pitch of a sound is increased, the wave length of the sound is decreased.

27. What is a wave?

Ans: Wave is a disturbance travelling through a medium in which energy is carried from one place to another.

28. Write two differences between sound wave and light wave.
Ans:Differences between the sound wave and light wave are as followings:

Sound wave: 1. Sound waves are longitudinal waves i.e. sound waves need a material medium for propagation.
2. Sound waves have velocity 330m/s in the air.

Light wave: 1. Light waves are transverse wave i.e. light waves do not need a material medium to propagate.
2. Lightwave has speed 3 х 10⁸ m/s in the air.

29. What is the difference between the sounds produced by a big-bell in a temple and small one of same shape? Write down in terms of two physical quantities related with sound.

Ans: The sound produced by a big bell is louder and have greater intensity than the sound produced by the small bell of the same shape in a temple because larger the size of sound producing the body, greater is the intensity and louder the sound produced.

30. List out any two factors that affect the speed of sound in air. Why is ultrasound used in the field of medical science?
Ans: The factors affecting the speed of sound in the air are as followings:
Density: The speed of sound in a gas is inversely proportional to the square root of the density of the gas. It means sound travels faster in gases of lower density and vice versa.

Temperature: The velocity of sound varies directly as the square root of the absolute temperature of the gas.

Humidity: Velocity of sound is more in humid air.

Direction of air blowing: Velocity of sound along the direction of air blowing is more.

31. In which medium among solid, liquid and gas does the sound travel fast? Write with reason.

Ans: A sound is a form of energy. It is produced by a vibrating body. Sound travels in the form of a wave. As sound is a longitudinal wave it needs material for its propagation. Propagation of sound is facilitated by the collisions of molecules of the medium, like one molecule hitting the next molecule, and then that molecule hitting the next, and so forth.
The distances between molecules in solids are very small, i.e., solids are denser compared to liquids and gases. Because they are so close, they can collide very quickly, i.e. it takes less time for a molecule of the solid to 'bump' into its neighbouring molecule. Solids are packed together tighter than liquids and gases, hence sound travels fastest in solids. The distances between the molecules in liquids are shorter than in gases but longer than in solids. Liquids are denser than gases but less dense than solids, so sound travels faster in liquids as compared to gases and slower as compared to solid. Gases are the slowest medium for transfer because they are the least dense: the molecules in gases are very far apart, compared with solids and liquids.
32. Why cannot we hear the sound of a bat? Is the earthquake wave subsonic or ultrasonic wave and why?

Ans: A man cannot hear sounds of all frequencies. The range of frequencies which a listener can distinguish is known as the range of audibility. The range of audibility for human ears is 20 Hz to 20,000 Hz. The sound which has frequency is below 20 Hz and above 20,000 Hz is not heard by a person.

Bat produces an ultrasonic sound which has frequency over 20,000 Hz. As sound having the frequency above 20,000 Hz is not audible to a human ear, we cannot hear the sound of the bat.
We cannot hear the infrasonic and ultrasonic sound because they have a frequency below 20 Hz and above 20,000 Hz respectively.

33. Suggest any four methods of reducing noise.
Ans: Methods for reducing noise are as followings:
Cinema halls, saw mills and other noise producing factories should be constructed outside residential area.
Public transport should be used and use of private vehicles should be reduced as far as possible.
Plantation of vegetation should be done in living and working areas.
Sound absorbing materials such as glass, wool, carpet, etc. reduce noise. Hence, they should be used properly.
34. What will be the intensity of sound in the following works?

Talking
Moving leaves
Running motorcycle
Flying jet-plane
Ans: The intensity of sound in the given works is as followings:
Talking - 50 dB
Moving leaves - 20 dB

Running motorcycle - 90 dB
Flying jet-plane - 140 dB

35. When the amplitude of sound is wave is increased two times, what is effect in the intensity of sound? Why?

Ans: The intensity of sound is directly proportional to the square of the amplitude.
i.e.
I ∝ a²
When the amplitude is doubled, the amplitude becomes 2a. So,
I ∝ (2a) ²
I ∝ 4a²
Hence, when the amplitude of the sound wave is increased, the intensity of sound also increases by four times.
36. Observe the given table answer the questions asked below:

Sound ******** Frequency

A ************** 200 Hz

B ************** 10 Hz

C ************** 50 KHz
Which sound can be heard by human ear?
What is the type of sound "B" called? Why?
Write an application of sound "C"
From which source the sound "B" is produced?

Ans: Sound "A" can be heard by human ear.
Sound "B" is infrasonic sound because its frequency is below 20 Hz and it is inaudible to human ear.
Sound "C" is ultrasonic sound. They are used for sterilization purposes, as strong beams of ultrasonic sound kill bacteria.
Infrasonic sound sometimes results naturally from severe weather, surf, lee waves, avalanches, earthquakes, volcanoes, and bolide.

38. By what property of sound a sound can be heard distinctly at night than at day and how that happens?

Ans: Sound also follows the laws of refraction as light does, this property of sound causes a sound can be heard distinctly at night than at day. Speed of sound changes in magnitude and direction as it passes from one medium to another medium. Sound is clearly heard at night than during day time. It is because at day-time, the temperature of air is maximum near the ground and it diminishes upwards. Therefore, layers of air gradually behaves as a denser medium as we go up. Hence, the ray of sound diverges upwards from sources and less sound wave reaches to listener.
At night, the situation is just opposite. The layer of air near the ground behaves as denser medium and it gradually becomes rarer and rarer as we go up. Hence, a ray diverging upwards from a source on or near the surface of the earth is refracted continuously away from the normal. Finally, the ray undergoes total internal reflection and then it begins to travel downwards to reach the listener. It makes the sound more distinct at night.

39. Why echo (sound) is experienced more in newly constructed room than in the living room?

Ans: The repetition of sound, which is reflected from walls of a large room or distant surface, is called an echo. Sound also follows the laws of reflection just like light. When sound is produced, it's reflected wave returns after striking a wall or any object to the listener. We say an echo is heard. For echo to be heard reflection of sound is an essential condition and there should not be more sound absorbing materials on the way of the sound wave. As newly constructed walls has less or literally no sound absorbing material as compared to living rooms, echo is experienced more in newly constructed room than in the living room.