Sound: Selina Concise Physics Class 7 :Question and Answers
**No error in answers- unlike the other sites which are full of them. This article has been cross-checked and proofread by three Physics teachers simultaneously.
Objective Questions and Answers:
1. Write true or false for each statement:-
(a) Sound can travel in vacuum.
Answer. False.
(b) Sound is a form of energy.
Answer. True.
(c) Sound can only be produced by vibrating bodies.
Answer. True.
(d) Larger is the amplitude, feeble is the sound.
Answer.False.
(e) The frequency is measured in hertz.
Answer. True.
(f) Loudness depends on frequency.
Answer. False.
(g) Waveforms of two different stringed instruments can be the same.
Answer. False.
(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.
2. Fill in the blanks(Answer in bold)
(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 or gas.
(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 solids than in liquids.
(h) The sound heard after reflection is echo.
(i) Sound produces sensation in ears.
3. Match the following
4. Select the correct alternative (Answers in bold)
(a) We can distinguish a shrill sound from a flat sound by its
1. amplitude
2. loudness
3. pitch
4. none of the above.
(b) We can hear sound of frequency
1. 10 Hz
2. 500 Hz
3. 100,000 Hz
4. 50,000 Hz
(c) Sound cannot travel in
1. gases
2. liquids
3. solids
4. vacuum
(d) The minimum distance required between the source and the reflector so as to hear the echo in air is
1. 10m
2. 17m
3. 34 m
4. 50 m
(e) Wavelength is measured in
1. kg
2. second
3. litre
4. metre
(f) The speed of sound in water is
1. 332 m
2. 1500 m
3. 5000 m s
4. 1000 m s
(g) Sound travels the fastest in
1. liquids
2. solids
3. gases
4. vacuum
B. Short/Long Answer Questions
Question 1.
What do you mean by a vibratory motion ? Illustrate with an example.
Answer:
The motion in which certain part or parts of a body always remain fixed and do not move while the rest of the body moves to and fro in a definite style is called vibratory motion.
For example, when the string of a guitar of Veena is plucked, the whole guitar or Veena does not move but only the string vibrates (moves).
Question 2.
What is sound ?
Answer:
Sound is a form of energy which produces the sensation of hearing.
Question 3.
How is sound produced ?
Answer:
Sound is produced by vibrating bodies.
Question 4.
Describe an experiment to show that each source of sound is a vibrating body.
Answer:
Sound is produced when a body vibrates. In other words, each source of sound is a vibrating body. This can be demonstrated by the following experiment.
Take a ruler. Press its one end on the table with the left hand as shown in figure. Pull down the other end of the ruler with the right hand and then leave it.
You will notice that the ruler vibrates i.e., the ruler moves to and fro and a humming sound is heard.
After some time, the ruler stops vibrating. No sound is then heard.
This shows that the humming sound is produced only because of the vibrations of the ruler.
Question 5.
Name two sources of sound.
Answer:
Musical instruments, Human beings, animals, tuning fork, metal bell etc.
Question 6.
How do we produce sound ?
Answer:
Our throat has a larynx which is also called the voice box and it has vocal cords. When we breathe, the vocal cords become loose and the gap between them increases. When we talk, shout or sing, the cords become tight and hence they vibrate, thus producing sound.
Question 7.
Give Reason: The bees do not have voice-boxes but still produce sound.
Answer:
The bees do not have the voice-boxes. Still they produce sound because of the vibrations produced by the quick movement of their wings.
Question 8.
Can sound travel through a vacuum ? Describe an experiment to explain your answer.
Answer:
Sound cannot travel through vacuum.
The following experiment proves this. Arrange an electric bell, a glass bell jar, a vacuum pump, a battery and a switch as shown in the figure. When the circuit is closed by pressing the switch, the bell starts ringing and sound can be heard. Now remove the air from the jar with the help of vacuum pump. The loudness of the sound gradually decreases and a stage comes when no sound is heard.
Question 9.
Describe an experiment to show that sound can travel in water.
Answer:
Take a tub filled with water. Hold a bell in one hand and dip it in water. Now ring the bell inside water. You will be able to hear the sound clearly. This shows that sound can travel through liquids.
Question 10.
Describe an experiment to show that sound can travel in a solid.
Answer:
Make a toy – telephone using two plastic cups and a thread as shown in the figure. Now use the toy-telephone as shown in figure and 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.
Question 11.
Can two person hear each other on moon’s surface ? Give reason to support your answer.
Answer:
No, we cannot hear each other on moon because it has no atmosphere unlike the earth. Since sound requires medium for transmission. It cannot travel through vacuum.
Question 12.
What is a longitudinal wave ?
Answer:
In a longitudinal wave, the particles of air vibrate to and fro about their mean positions in the direction of the propagation of the wave. Sound waves are example of longitudinal waves.
Question 13.
Define the following terms :
Amplitude, Time period, Frequency.
Answer:
(a) Amplitude (A) : The maximum displacement of a wave on either side of its mean position is called Amplitude.
(b) Time Period (T) : Time taken to complete one vibration is called Time Period.
(c) Frequency (f) :Number of oscillations made by a wave in one second is known as its frequency.
Question 14.
Write the audible range of frequency for the normal human ear.
Answer:
The range of frequency from 20 Hz to 20,000 Hz is called the audible range for the normal human ear.
Question 15.
What are ultrasonics ? Can you hear the ultrasonic sound ?
Answer:
Sounds of frequency higher than 20,000 Hz are called the ultrasonics. We cannot hear the ultrasonic sounds.
Question 16.
What are infrasonics ?Can you hear them ?
Answer:
Sounds of frequency lower than 20 Hz are called the infrasonics. We cannot hear the infrasonic sounds.
Question 17.
How does a bat make use of ultrasonics waves to find its way?
Answer:
Bats use the technique of echolocation by producing ultrasonic sound as they fly. When this ultrasonic sound comes back after reflection from any object (or obstacle) in their way, they hear it and thus they detect the presence of the object (or obstacle).
Question 18.
Name the two characteristics of sound which differentiate two sounds from each other.
Answer:
Depending upon the amplitude and frequency of the sound wave, the following two characteristics of sound :
(1) Loudness, and (2) Pitch, differentiate two sounds from each other.
Question 19.
On what factor does the loudness of a sound depend ?
Answer:
The loudness of a sound depends on the amplitude of vibration of the vibrating body producing the sound.
Question 20.
How does the loudness of sound produced depend on the vibrating area of the body ?
Answer:
Greater the area of the vibrating body, louder is the sound produced.
Question 21.
The outer case of the bell in a temple is made big. Give a reason.
Answer:
The outer case of the bell in a temple is made big to cause multiple reflections of sound and the sound can be amplified to increase its loudness.
Question 22.
State the factor on which the pitch of a sound depends.
Answer:
The pitch of a sound depends on its frequency: higher the frequency, higher is the pitch and vice-versa.
Question 23.
Differentiate between a high pitch sound and a low pitch sound.
Answer:
Higher the pitch, the shriller is the sound. Lower the pitch, the flatter or graver is the sound.
Question 24.
How does a man’s voice differ from a woman’s voice ?
Answer:
A woman's voice is shriller than a male's voice because of higher frequency. Higher is the frequency, shriller is the sound.
Question 25.
Name the characteristic which differentiates two sounds of the same pitch and same loudness.
Answer:
The quality of the sound distinguishes two sounds of the same pitch and same loudness.
Question 26.
You recognize your friend by hearing his voice on a telephone. Explain.
Answer:
We can recognize our friend by hearing his voice on a telephone due to quality of sound and pitch of sound.
Question 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:
It is the pitch and quality that helps a musician recognize the musical instrument by hearing the sound produced by it, even without seeing the instrument.
Question 28.
Describe an experiment to show the production of sound having low and high pitch.
Answer:
Take few rubber bands some thicker and longer, few thinner and of shorter length. Stretch these rubber bands by fastening the ends on both sides. Now pluck these rubber bands one by one. The rubber bands thicker and longer will produce sound with a lower pitch. The rubber bands thinner and shorter will produce sound with a higher pitch.
Question 29.
How does a musician playing on a flute change the pitch of sound produced by it ?
Answer:
A musician playing on a flute changes the pitch of sound produced by it by closing different holes on the flute which essentially changes the length of the vibrating air column.
Question 30.
Why are musical instruments provided with more than one string ?
Answer:
The stringed instruments are provided with a number of strings of different thickness and under different tensions so that each string produces sound of a different pitch.
Question 31.
How can the pitch of sound produced in a piano be changed ?
Answer:
In a piano, the pitch of sound produced can be changed by stretching or loosening the strings of the piano.
Question 32.
Explain why you can predict the arrival of a train by placing your ear on the rails without seeing it.
Answer:
Sound travels faster in solids than in gas. The sound produced by the moving wheels of train travels much faster through the track which is a solid medium than through the air. Therefore we can hear through the track much before it is heard through air.
Question 33.
Write the approximate speed of sound in (i) air, (ii) water and (iii) steel.
Answer:
Question 34.
During a thunderstorm, the sound of a thunder is heard after the lightning is seen. Why ?
Answer:
Light travels much faster than sound. The velocity of light is 3 × 10^8 m/s whereas the velocity of sound is 332 m/s. Therefore, we first see the flash of light and then we hear the thunder.
Question 35.
Describe an experiment to estimate the speed of sound in air.
Answer:
To estimate the speed of sound in air suppose we choose two hills A and B about a kilometer apart. A person at the hill A fires a gun. Another person at the hill B starts a stop watch as he sees the flash of the fire and stops it on hearing the sound. Thus, he measures the time interval between the seeing of flash and hearing of the sound. Let it be t second. Then measure the distance between the hills A and B. Let it be S metre. Then
Speed = Distance/Time
=S/t m/s
Question 36.
Can sound travel through solids and liquids ? In which of these two does it travel faster ?
Answer:
Sound can travel through both solids and liquids.
Sound travels faster in solids than liquids.
Question 37.
What do you mean by reflection of sound ?
Answer:
Bouncing back of sound waves from the surface is called reflection of sound or we can say that when sound travels in a given medium it strikes the surface of another medium so that it returns in some other direction, this phenomenon is called reflection of sound.
Question 38.
State one use of reflection of sound.
Answer:
The reflection of sound is used in making the speaking tube (or megaphone), sound board and trumpet.
Question 39.
What is echo ?
Answer:
Echo is the sound heard after reflection from a rigid surface such as a cliff, a hillside, the wall of a building etc.
Question 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 is required between the source of sound and the reflecting surface to hear an echo is 17 m.
Since the sound speed is 340 m / s the sound traveling distance is 34 m in 0.1 second. ... So, if the obstacle is at least 17 m away, then after 0.1 second, the reflected sound or the echo is clearly detected. Hence, to hear a clear echo, the minimum distance between the reflecting surface and the observer is 17 m.
Question 41.
List four substances which are good absorbers of sound.
Answer:
Sofa, fluffs and light substances such as clothes, papers, thermocol, coating of plaster of paris, carpets, curtains, furniture, wood etc. are good absorbers of sound.
Question 42.
List the measures that you will take when designing a sound-proof room.
Answer:
In order to design such a sound proof room we take the following measures
(1) The roof of the enclosure must be covered by plaster of paris after putting the sheets of thermocol.
(2) The walls of the enclosure should be covered by the wooden strips.
(3) The floor must be laid down by thick carpets.
(4) The machine parts of all the electrical equipments such as fan, air conditioner etc. must be placed outside the enclosure.
(5) Thick curtains should be used to cover the doors and keep them closed.
(6) Thick stripping must be used to cover the openings of doors and windows.
C. Numericals
Question 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:
Speed= Distance/Time
= 1020/3 m/s
= 340 m/s
Question 2.
A boy on a hill A fires a gun. The other boy on hill B hears the sound after 4 s. If the speed of sound is 330 m s-1, find the distance between the two hills.
Answer:
Speed, v = 330 m s-1
t = 4s
Distances = v × t
= 330m/s × 4s = 1320 m Ans.
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