Sound
Ans: Sound is the form of energy which gives the sensation of hearing.
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Speed of
Sound in Different Medium |
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Medium |
Substances |
Speed of sound in m/s |
|
|
Solid |
Iron |
5900 |
|
|
Glass |
3962 |
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|
Wood |
4000 - 5000 |
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Steel |
5000 - 7000 |
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|
Liquid |
Water |
1400 |
|
|
Alcohol |
1210 |
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Gas |
Air |
332 |
|
|
Hydrogen |
260 |
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Oxygen |
316 |
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2. What is reverberation? What is meant by the frequency of a wave is 50Hz? (2073 Banke)
Ans: The phenomenon of multiple reflection of sound when reflector is at a distance of less than 17m is called reverberation.
The frequency of wave is 50Hz means that the sound wave completes 50 vibrations in one second.
3. What is time period of a wave?
Ans:
The time required to complete one vibration is called time period of a wave.
i.e. Time period = 1/frequency
4. Define echo. Soubnd absorbing materials are used in ceiling and walls of a conference hall, why? (2073 Bhaktapur)
Ans: The phenomenon of returning the incident sound wave from a distance more than 17m is called an echo.
We know that conference hall is large and its distance is more than 17m. So, there may be echo and sound may not be clear. To avoid it sound absorber like wood, cotton etc are kept on the walls to reduce reflection of sound.
5.Sound does not propage through a vacuum, why? (2073 Dhanusha/Jhapa)
Ans: Sound needs material medium to propagate but in vacuum nothing is present. It means no vibration occurs in vacuum. Hence, sound does not propagate through a vacuum.
6. What is the main causes of producing sound? Write any two necessary conditions for producing echo. (2073 Kaski)
Ans: Vibrations of molecules is the main causes of producing sound.
The following are the two necessary conditions for producing echo:
- The sound should be so loud that after reflection also listener must listen.
- For an echo distance of reflecting object must be more than 17m.
7. Write
any two factors affecting sound. Differentiate bewteen echo and reverberation
in any two points.
Ans:
|
Difference between echo and reverberation |
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S.No |
Echo |
Reverberation |
|
1 |
Its is reflection of sound from an object. |
It is the prolongation of sound reflected from an object. |
|
2 |
Distance of reflector must be more than 17m. |
Distnace of reflector must be less than 17m. |
8. Sound of thundering is heard some while later of lighting, why? (2017 Lalitpur)
9. What is frequency? Write its SI unit. (2073 Sunsari)
Ans: The number of
complete waves in a second is called frequency. Its SI unit is hertz (Hz).
10. What
is wave length? Write its unit. (2073 Siraha)
Ans: The distance between two successive points is known as wave length. Its unit is meter.
11. Why
are electrical devices used to talk in the space? Give suitable reason. (2072
Kanchanpur)
Ans: We know that, moon has no atmosphere. It means that there is no any medium and without medium sound cannot travel. Sound needs medium to travel so electrical devices are used to talk in space
12. Why does louder sound heard in empty room than room with other materials? Ans: In empty room no absorbers are there so that almost all sound is reflected from the wall and heard louder but materials in the room absorb more sound. Hence, sound will be faint.
13. What
are the conditions for echo or reverberation.
Ans:
The following are conditions for echo or reverberation:
§
The
sound should be so loud that after reflection also listener must listen.
§ For an echo distance of reflecting object must be more than 17m and for reveberation distnace must be less tha 17m.
14. Write
any two uses of echo.
Ans: The following are
the uses of echo:
§
It
is used to find the depth of the sea.
§ Bat uses echo to find its path and prey.
15. What
is reflection of sound?
Ans: When a sound strikes on a hard surface, it returns to the same medium. This phenomenon is called reflection of sound.
16. Why
does sound move faster in solid then in liquid or gas?
Ans: Sound is produced due to molecular vibration of a body. Solids have large number of molecules than others. So, vibration will also be large. Hence, sound moves faster in solid than liquid or gas.
Numerical Questions of
DLE || Solved
1. A boy
sounted by sitting at the top of a well and he heard his clear echo after 0.3
second. Calculate the distance between boy and the surface of water in the
well.
Ans:
Given,
Time of echo (t) = 0.3 sec
distance of water surface (d) = ?
velocity of sound in air (v) = 332 m/s
As we know that,
for echo,
v = 2d/t
or, 332 = (2xd)/0.3
or, 332 x 0.3 = 2d
or, 99. 6 = 2d
or, d = 99.6/2
or, d = 49.8m.
Hence, the distance between boy and the surface of water in the well is 49.8m.
2.What is
the wave length of sound which has frequency 100Hz and velocity 250m/2?
Ans:
Given,
frequency (f) = 100 Hz
velocity of sound (v) = 250m/s
wavelength = ?
As we know that,
v = frequency x wavelength
wave length = velocity / frequency
wave lenght = 250/100
wave lenght = 2.5 m
Hence, the wave length of sound is 2.5m.
3. If the
speed of sound 550m/s and its frequency 50 Hz. Calculate the wave lenght of the
sound.
Ans:
Given,
velocity of sound = 550m/s
ferquency of sound = 50 Hz
wave length = ?
As we know that,
velocity = frequency x wave length
wave length = velocity/frequency
wave length = 550/50
wave length = 11m.
Hence, the wave length of the sound is 11m.
4.
Calculate the speed of sound wave having wavelength of 15m and frequency of 100
Hz.
Ans:
Given;
wavelenght = 15m
frequency of sound = 100 Hz
velocity of sound = ?
As we know that;
Velocity = frequency x wavelength
velocity = 100 x 15
velocity = 1500 m/s.
Hence, the speed of sound wave is 1500 m/s.