Selina Concise Class 7 Physics Solution Chapter No. 6- ‘Sound’ For ICSE Board Students.
Selina Concise Physics Solutions for Class 7th Chapter 6 Sound with easy language, expert answers and doubt clearing sections. The book is especially designed keeping in mind the syllabus of ICSE board and the difficulty level faced by the students of ICSE boards class 7.
Selina Class 7 Physics Chapter 6 Solution:
A) Objective Questions :
1.) Write true or false for each statement :
a) Sound can travel in vacuum.
Answer: False.
Medium is necessary for propagation of sound.
b) Sound is a form of energy.
Answer: True.
Sound is a type of energy.
c) Sound can only be produced by vibrating bodies,
Answer: True.
Vibrations must takes place for production of sound.
d) Larger is the amplitude, feeble is the sound.
Answer: False.
The intensity of sound is depend on amplitude.
e) The frequency is measured in hertz.
Answer: True.
The SI unit of frequency is per second of Hertz.
f) Loudness depends on frequency.
Answer: False.
The intensity of sound is depend on amplitude.
g) Waveforms of two different stringed instruments can be the same.
Answer: False.
Waveforms of two different stringed instruments can be different. .
h) Female voice is shriller than the male voice.
Answer: True.
i) A ticking clock sound is heard late when heard through a metal.
Answer: False.
The speed of sound is greater in metals.
2.) Fill in the blanks:
Answer:
a) Sound is produced when a body vibrates.
b) The number of times a body vibrates in one second is called its frequency.
c) The pitch of a sound depends on its frequency.
d) Sound can travel in a medium solid, liquid and gases.
e) We can hear sounds of frequency in the range of 20 Hz to 20,000 Hz.
f) Sound requires a medium for propagation.
g)Sound travels faster in solid than in liquids.
h)The sound heard after reflection is echo.
i) Sound produces sensation in ears.
3.) Match the following :
Column A Column B
(a) Vibrations cause (i) absorb sound
(b) A shriller sound is (ii) second
(c) Unit of frequency (iii) sound
(d) Unit of time period (iv) of high pitch
(e) Curtains (v) hertz
Answer:
Column A Column B
(a) Vibrations cause (iii) sound
(b) A shriller sound is (iv) of high pitch
(c) Unit of frequency (v) hertz
(d) Unit of time period (ii)second
(e) Curtains (i) absorb sound
4.) Select the correct alternative :
a) We can distinguish a shrill sound from a flat sound by its :
(i) amplitude
(ii) loudness
(iii) pitch
(iv) none of the above.
Answer:
(iii) pitch
Shilling sound contains high frequency.
b)We can hear sound of frequency :
(i) 10 Hz
(ii) 500 Hz
(iii) 100,000 Hz
(iv) 50.000 Hz
Answer:
(ii) 500 Hz
We can hear sounds of frequency in the range of 20 Hz to 20,000 Hz
c) Sound cannot travel in
(i) gases
(ii) liquids
(iv) solids
(iv) vacuum.
Answer:
(iv) vacuum.
Medium is necessary for propagation of sound.
d) The minimum distance required between the source and the reflector so as to hear the echo in air is :
(i) 10 m
(ii) 17 m
(ii) 34 m
(iv) 50 m.
Answer:
(ii) 17 m
We know that, if reflection of sound takes place in 0.1 seconds then we can hear echo.
Thus, time = 0.1 second.
Speed of sound in air = 330 m/s.
As,
Speed = distance / time
Distance = speed × time
Distance = 330×0.1
Distance = 33 m.
This is the actual distance travelled by the sound.
But we know that,
Total distance travelled by sound = 2 × Distance between source of sound and echo point.
Thus,
Distance between source of sound and echo point =
(Total distance travelled by sound)/2
Distance between source of sound and echo point = 33/2
Distance between source of sound and echo point = 17.5.
e) Wavelength is measured in
(i) kg
(ii) second
(ii) litre
(iv) metre
Answer: (iv) metre
Wavelength has dimensions of length.
f) The speed of sound in water is :
(i) 332 m/s
ii) 1500 m/s
(ii) 5000 m/s.
(iv) 1000 m /s.
Answer:
ii) 1500 m/s
g) Sound travels the fastest in :
(i) liquids
(ii) solids
(iii) gases
(iv) vacuum
Answer:
(ii) solids
B.) Short/Long Answer Questions :
1) What do you mean by a vibratory motion ?
Answer: The vibratory is a type of oscillatory motion in which a part of object is fixed and another part is allow to oscillate. In this type of motion, objects gain new shape while vibrating.
Example- diaphragm of tabla, vibration of string of guitar etc.
2) What is sound ?
Answer: Sound is a type of energy which gives sensation to our ears. It is produced by Vibrations of the object.
3) How is sound produced ?
Answer: When the objects starts to vibrates then it produces sound. There are so many ways of production of sound.Methods of production of sound-
- An object heat on another object.
- Collision between objects.
- Plucking a string or wire.
4) Describe an experiment to show that each source of sound is a vibrating body.
Answer: Vibrations are necessary for production of sound. We can illustrate this with a simple experiment.
Activity-
- Take a beaker and fill water in it.
- Take a tuning fork.
- Heat A tuning fork on this beaker.
- W you can hear the sound.
- Observe the water in beaker and movement of tuning fork.
- The Vibrations are takes place on tuning fork and surface of water in beaker.
5) Name two sources of sound.
Answer: sources of sound
- Living beings.
- Electrical machines.
- Collision of objects.
- Friction between objects
6) How do we produce sound ?
Answer: We know that, sound produces by Vibrations. Vocal chords in our voice box get vibrates and produces sound.
7) The bees do not have voice-boxes but they still make sound. How ?
Answer: Bees do not have voice boxes. Bees hits wings to air molecules in space thus it start vibrating and produces sound.
8) Can sound travel through a vacuum ? Describe an experiment to explain your answer.
Answer:
- As we know that, sound is the mechanical wave which requires material medium for its propagation.
- The material medium may be solid, liquid or gas.
- The speed of propagation of sound is different in different media.
- To prove sound does not travel through vacuum that means it requires material medium for its propagation we perform an experiment called as bell jar experiment as follows.
- We have taken an electric bell and also the airtight glass bell jar. Now we have suspended the electric bell inside the airtight glass bell jar as shown in figure.
- And the glass bell jar is connected to vacuum pump and if we make switch on then we hear sound of electric bell jar clearly. Now we started the vacuum pump so that there will vacuum is created inside the glass bell jar.
- Due to starring the vacuum pump the air from inside the bell jar is pumping out and in that manner we are listening the sound of decreasing intensity.
- After sometime we observed that there is no sound is hearing although the same current is provided to the electric bell jar
- That means, after sometime there will be no air present inside the glass bell jar that means totally vacuum will be created and due to which we can’t hear the sound of electric bell.
- This experiment proves that, sound wave does not travels through vacuum and it requires material medium for its propagation.
9) Describe an experiment to show that sound can travel in water.
Answer: Medium is required for propagation of longitudinal wave. Sound is an example of longitudinal wave. Thus it can propagate through water also. We can illustrate this with the help of an activity.
Activity-
Take a balloon. Fill water in it. Put a bell on other side of Balloon and listen by putting ear on opposite side of balloon. You can clear listen the voice of bell. This experiment shows that, sound can travels through water also.
10) Describe an experiment to show that sound can travel in a solid.
Answer: Sound is type of longitudinal wave. Medium is necessary for propagation of longitudinal wave. Sound can travels through Solid. We can illustrate this with the help of an activity below.
Activity:
- Take two hollow plastic cups.
- Tia a string to both the cube as shown in figure.
- Straight the string.
- Speak in first cup and say your friend to listen in second cup.
- You can listen the voice of your friend through the string.
- From this activity we conclude that, sound can propagate through the silolid.
11) Can two persons hear each other on moon’s surface ? Give reason to support your answer.
Answer: No, we can not hear sound on surface of moon. We know that, medium is required for propagation of sound and medium is absent on moon.
12) What is a longitudinal wave ?
Answer: The wave in which direction of vibration of molecules is parallel to direction of propagation of energy or wave is called longitudinal wave.
Sound is an example of longitudinal wave.
13) Define the following terms:
Amplitude, Time period, Frequency.
Answer:
Amplitude: The maximum displacement of the particles from it’s mean position in the wave is called amplitude of that wave.
Time period: The time required to complete one oscillations is called time period. It is denoted by T.
SI unit of period is second.
Frequency: The Number of oscillations completed by the object in one second is called frequency of that wave. It is denoted by f.
The SI unit of frequency is per second of Hertz.
14) Write the audible range of frequency for the normal human ear.
Answer: The audible range of frequencies for normal human ear is 20 Hz to 20,000 Hz.
15) What are ultrasonic ? Can you hear the ultrasonic sound ?
Answer: The sound contains frequency above 20,000 Hz is called as ultrasound. This type of sound possesses higher energies.
Human ear can not listen ultrasound. Some animals have ability to listen ultrasound like bat, deer, leopards, dogs, monkeys etc.
16) What are infrasound ? Can you hear them ?
Answer: The sound contains frequency below 20 Hz is called as infrasound. This type of sound possesses lower energy
We can not hear infrasound.
17) How does a bat make use of ultrasonic waves to find its way ?
Answer: Bat produces high frequency ultrasound while flying. The ultrasound wave hits on the surface of object which is on the path of bat and reflect back. Bat can detect the object in the path by listening echo produced by that ultrasound. In this way bat can fly at night without colliding with other objects.
18) Name the two characteristics of sound which differentiate any two sounds from each other.
Answer: We can differentiate any two sounds from each other on the basis of different frequency and amplitude of that sound wave.
19) On what factor does the loudness of a sound depend ?
Answer: Loudness of the sound depends on the amplitude of the sound wave. If amplitude is greater then intensity of sound is greater hence produces load sound.
20) How does the loudness of sound produced depend on the vibrating area of the body ?
Answer: We know that loudness of sound is depend on amplitude of that sound wave. Loudness also depend on area on where sound produced. If the area of that instrument is greater then it produces loud sound.The sound of greater intensity is produced by the drum which has greater area than smaller area.
Example- The sound produced by drum having greater area is louder than the sound produced by drum having smaller area.
21) The outer case of the bell in a temple is made big. Give a reason.
Answer: We know that, loudness of sound is depend on the surface area of vibrating body. If the area is greater then it produces loud sound. If the outer case of the bell I a temple is made big then it produces loud sound
22) State the factors on which the pitch of a sound depends.
Answer: Pitch of the sound is depend of number of oscillations performed by the object in second. It is depend on frequency of that sound wave.
Low frequency sound has low pitch and high frequency sound has high pitch.
23) Differentiate between a high pitch sound and a low pitch sound.
Answer:
High pitch sound-
- High pitch sound contains high frequency.
- High pitch sound is shriller.
- The voice of girl is shriller than a boy.
Low pitch sound-
- Low pitch sound contains low frequency.
- Low pitch sound is flat or grave.
- The voice of girl is shriller than a boy.
- The sound of child is shriller than adult.
24) How does a man’s voice differ from a woman’s voice ?
Answer: We know that, pitch of sound is depend on the frequency. Woman’s voice is shriller because it contains higher frequency and man’s sound is flat or grave because it contains lower frequency.
25) Name the characteristic which differentiates two sounds of the same pitch and same loudness.
Answer: The characteristic which differentiates two sounds of the same pitch and same amplitude is quality of sound. Quality of sound depends on shape of the sound wave.
26) You recognize your friend by hearing his voice on a telephone. Explain.
Answer: We can distinguish between two sounds because of pitch and shape of sound wave. Each sound wave has definitely quality and pitch. We know that pitch or quality of our friend. Thus we can recognize our friend by hearing his voice on telephone.
27) A musician recognizes the musical instrument by hearing the sound produced by it, even without seeing the instrument. Which characteristic Of sound makes this possible ?
Answer: We can distinguish between two sounds because of pitch and shape of sound wave. Each sound wave has definitely quality and pitch. Magician knows the pitch and quality of each sound of musical instruments. Therefore he identify that instruments.
28) Describe an experiment to show the production of sound having low and high pitch.
Answer: We can illustrate that production of sound having low and high pitch with the help of following activity.
Activity:
- Take three beakers of equal size.
- Add water in it upto different level as show in Fig.
- Blow air in each beaker by our mouth.
- Observe the sound produced in each case.
- The sound produced in beaker A is flat and becomes more and more when the level of water increases.
- The frequency of sound produced in beaker A is less than beaker B and beaker C.
29) How does a musician playing on a flute change the pitch of sound produced by it ?
Answer: Flute has several holes. Musician change the length of the flute by closing the different holes of air columns. In this way by changing the frequency of sound musician playing on a flute change pitch of sound produced by it.
30) Why are musical instruments provided with more than one string.
Answer:
Some musical instruments has multiple strings on it. Each string of that musical instrument has different thickness. Thus each string produces it’s own frequency. In this way musician produces pleasant melody.
31) How can the pitch of sound produced in a piano be changed ?
Answer: The pitch of sound is depend on frequency. Also we know that frequency is depend on the tension in string which produces sound. Musician changes the frequency by changing the tension in string. In this waypitch of sound produced in a piano be changed.
32) Explain why you can predict the arrival of a train by placing your ear on the rails without seeing it.
Answer: Medium is necessary for propagation of sound. Also speed of sound is depend on medium. If the medium is denser then speed of sound is also greater. So speed of is as
Solid > liquid > gases.
Thus speed of sound is greater in railway lines than air. Therefore When we put our ear to a railway line, we can hear the sound of an approaching train even when the train is far off but its sound cannot be heard through the air.
33) Write the approximate speed of sound in (i) air. (ii) water and (iii) steel.
Answer:
Sound can travels through air, water and steel also.
The speed of sound in these medium are as,
Air- 330 m/s.
water-1500 m/s.
Steel- 5960 m/s.
34) During a thunderstorm, the sound of a thunder is heard after the lightning is seen. Why ?
Answer: We know that speed of sound in air is 344 m/s and speed of light is 3 × 10⁸ m/s. The flash of lightning reaches us first and the sound of thunder is heard a little later because speed of light greater than speed of sound.
35) Describe an experiment to estimate the speed of sound in air.
Answer: We can calculate speed of sound in air with the help of an example.
Activity:
Stand two boys on a the top of two hills. Suppose the distance between these two boys is 1700 m.
A boy who stand on top of first hill fire a gun in upwards direction. The boy who stand on top of second hill listen the sound of firing after 5 seconds.
We can calculate speed of sound in air by solving this problem.
The distance = 1700 m,
Time = 5 seconds.
As we know that,
Speed = distance /time
Speed = 1700/5
Speed = 340 m/s. The speed of sound is 340 m/s.
In this way, we cam calculate speed of sound in air.
36) Can sound travel through solids and liquids ? In which of these two does it travel faster ?
Answer: Sound is wave thus it can propagate through Solid and liquid also.
The speed of sound is greater in solids than liquids.
37) What do you mean by reflection of sound ?
Answer: Sound is a wave thus it exhibit phenomenon of reflection.
When sound wave falls on surface and goes back in same medium is called reflection of sound. Bending of sound wave in same medium is called reflection of sound.
38) State one use of reflection of sound.
Answer:
Uses of reflection of sound.
- Musical instruments like megaphone, flue are based on principle of reflection of sound.
- We can hear echo because of reflection of sound.
39) What is an echo ?
Answer: When a sound wave strikes on the surface and reflect backs, this repetition of sound us called echo.
We can hear echo at point. We can hear echo if the minimum distance between source of sound and echo point is approximately 17 m.
40) What minimum distance is required between the source of sound and the reflecting surface to hear an echo ? Give reason.
Answer:
The minimum distance between source and echo point is approximately 17 m.
We know that, if reflection of sound takes place in 0.1 seconds then we can hear echo.
Thus, time = 0.1 second.
Speed of sound in air = 330 m/s.
As,
Speed = distance / time
Distance = speed × time
Distance = 330×0.1
Distance = 33 m.
This is the actual distance travelled by the sound.
But we know that,
Total distance travelled by sound = 2 × Distance between source of sound and echo point.
Thus,
Distance between source of sound and echo point =
(Total distance travelled by sound)/2
Distance between source of sound and echo point = 33/2
Distance between source of sound and echo point = 17.5.
The minimum distance for echo is approximately 17 m.
41) List four substances which are good absorbers of sound.
Answer:
The good sound absorbers are-
- Wood
- Furniture
- Plaster of Paris
- Curtains
- Carpets
42) List the measures that you will take when designing a sound-proof room.
Answer: We take a measures while designing sound- proof room are-
- Plaster of Paris is a good sound absorbing material. Enclose the roof of room by Plaster of Paris.
- We can enclose wall by sound absorbing materials like wood, Plaster of Paris etc.
- Carpets are also good sound absorbers. So we can use it sound proof rooms.
- Smooth surfaces of fan electronic machines reflect sound. Therefore use them in outer part of the room as possible.
- Doors, windows are also act as sound reflecting surface. Cover them by sound absorbing materials.
Numericals
1) A boy fires a gun and another boy at a distance of 1020 m hears the sound of firing the gun 3 s after seeing its smoke. Find the speed of sound.
Answer: Given,
Distance between two boys= 1020 m,
Second boy listen sound of firing after 3 s. Time = 3 s.
As we know that,
Speed = distance /time
Speed = 1020/3
Speed = 340 m/s. The speed of sound is 340 m/s.
2) A boy on a hill A fires a gun. Another boy on hill B hears the sound after 4 s. If the speed of sound is 330 m/, find the distance between the two hills.
Answer: Given,
Time = 4 s,
Speed of sound = 330 m/s,
As we know that,
Speed = distance/ time
Distance = speed × time
Distance = 330 × 4
Distance = 1320 m
The distance between two hills is 1320 m.