Class 7 ICSE Solutions for Concise Physics Selina: Sound

SOUND

OBJECTIVE QUESTIONS:

Question 1: Write true or false for each statement:

(a) Sound can travel in vacuum: False

(b) Sound is a form of energy: True

(c) Sound can only be produced by vibrating bodies: True

(d) Larger is the amplitude, feeble is the sound: False

(e) The frequency is measured in hertz: True

(f) Loudness depends on frequency: False

(g) Waveforms of two different stringed instruments can be the same: False

(h) Female voice is shriller than the male voice: True

(i) A ticking clock sound is heard late when heard through a metal: False

Question 2: Fill in the blanks:

(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.

Question 3: Match the following:

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

Question 4: Choose the correct options:

Question 4a: We can distinguish a shrill sound from a flat sound by its:

• amplitude
• loudness
• pitch
• none of the above

Question 4b: We can hear sound of frequency:

• 10 Hz
• 500 Hz
• 100,000 Hz
• 50,000 Hz

Question 4c: Sound cannot travel in:

• gases
• liquids
• solids
• vacuum

Question 4d: The minimum distance required between the source and the reflector so as to hear the echo in air is:

• 10 m
• 17 m
• 34 m
• 50 m

Question 4e: Wavelength is measured in:

• kg
• second
• liter
• metre

Question 4f: The speed of sound in water is:

• 332 m /s
• 1500 m/s
• 5000 m /s
• 1000 m /s

Question 4g: Sound travels the fastest in:

• liquids
• solids
• gases
• vacuum

SHORT/LONG ANSWER TYPE:

Question 1: What do you mean by a vibratory motion?

Answer: The oscillatory (to and fro) motion in which the body assumes a new shape during its motion

is called vibratory motion. In Vibratory motion, a part of the body oscillates keeping its other part fixed.

Question 2: What is sound?

Answer: Sound is a form of energy that produces the sensation of hearing in our ears.

Question 3: How is sound produced?

Answer: Sound is produced when a body vibrates.

Question 4: Describe an experiment to show that each source of sound is a vibrating body.

Answer: The following experiment shows that each source of sound is a vibrating body:

• Take a ruler.
• Press its one end on the table with the left hand as shown in the figure below.
• Pull down the other end of the ruler with the right hand and then leave it.

We will notice that the ruler vibrates i.e., the ruler moves to and fro and a humming sound is heard. After sometime 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. Hence, we can say that each source of sound is a vibrating body.

Question 5: Name two sources of sound.

Answer: The two sources of sound are:

• Tuning fork
• Musical instruments

Question 6: How do we produce sound?

Answer: We produce sound when our vocal cords vibrate on blowing air through them by our lungs.

Question 7: The bees do not have voice-boxes but they still make sound. How?

Answer: Bees make sound by moving their wings up and down very fast.

Question 8: Can sound travel through a vacuum? Describe an experiment to explain your answer.

Answer: No, sound cannot travel through a vacuum.

It is shown by the following experiment:

• Take an electric bell and an air-tight glass jar connected to a vacuum pump. Suspend the electric bell inside the jar.
• Connect the bell to a battery through a switch.
• On pressing the switch bell starts ringing and a sound is heard.
• Now start the vacuum pump. It withdraws the air from the jar.

We will notice that as the air from the jar is evacuated the sound gradually becomes more and more feeble. After sometime when no air is left within the jar, no sound is heard. But we can notice the hammer of the electric bell is striking the gong. This shows that sound cannot travel in vacuum because when there is air in the jar, we can hear the sound but when air is evacuated from the jar through vacuum pump, we cannot hear the sound although the bell is vibrating.

Question 9: Describe an experiment to show that sound can travel in water.

Answer: The following experiment shows that sound can travel in water:

• Fill a balloon with water and hold it near your ear.
• Now keep a watch gently to the other side of the balloon.

We will hear the ticking sound of the watch. This shows that sound can travel in water.

Question 10: Describe an experiment to show that sound can travel in a solid.

Answer: The following experiment show that sound can travel in a solid:

• Take two empty ice-cream cups. Make a small hole at the bottom of each cup and pass a long thread 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 telephone as shown in the below figure and talk to your friend.

You will be able to hear the sound of your friend. This shows that sound can travel through solid as sound travels through the thread and reaches the ear.

Question 11: Can two persons hear each other on moon's surface? Give reason to support your answer.

Answer: No, two persons cannot hear each other on moon's surface because sound needs a medium for its propagation but on moon's surface there is vacuum, so sound cannot propagates through it.

Question 12; What is a longitudinal wave?

Answer: The wave in which the particles of the medium vibrate about their mean positions, in the direction of propagation of sound is called longitudinal wave.

Question 13:Define the following terms:

• Amplitude
• Time period
• Frequency

Answer:

• Amplitude (a): The maximum displacement of a wave on either side of its mean position, is called the amplitude of wave.

• Time period (T): The time taken by a wave to complete its one vibration is called the time period of the wave.

• Frequency (f or n): The number of vibrations produced by a source of sound wave in one second is called the frequency of the wave.

Question 14: Write the audible range of frequency for the normal human ear.

Answer: The audible range of frequency for the normal human ear is 20 Hz to 20,000 Hz.

Question 15: What are ultrasonics? Can you hear the ultrasonic sound?

Answer: Sounds of frequencies higher than 20,000 Hz are called ultrasonic sounds. We cannot hear ultrasonic sound.

Question 16: What are infrasonic? Can you hear them?

Answer: Sounds of frequency lower than 20 Hz are called infrasonic sounds. We cannot hear the infrasonic sounds.

Question 17: How does a bat make use of ultrasonic waves to find its way?

Answer: Bat produces ultrasonic sound while flying. Ultrasonic waves reflect back after striking any obstacles in their way and bats hear it. Thus, they detect the obstacles in their path and change their path without colliding with the obstacle.

Question 18: Name the two characteristics of sound which differentiate any two sounds from each other.

Answer: Loudness and pitch

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.
• The area of the vibrating body.

Question 20: How does the loudness of sound produced depend on the vibrating area of the body?

Answer: The loudness of sound produced is directly proportional to the vibrating area of the body. 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 increase the surface area of the vibrating body. It helps in producing a louder sound as greater the area of the vibrating body, louder is the sound produced, which can be heard to a longer distance.

Question 22: State the factors on which the pitch of a sound depend?

Answer: The pitch of a sound depends on its frequency.

Question 23: Differentiate between a high pitch sound and a low pitch sound.

Answer:

High pitch sound                                 Low pitch sound

High pitch sound has high frequency. Low pitch sound have low frequency.

If pitch is high, sound is shrill.         If pitch is low, sound is flat or grave.

Question 24: How does a man's voice differ from a woman's voice?

Answer: The voice of a woman is shriller than that of a man because the voice of a woman has a higher pitch.

Question 25: Name the characteristic which differentiates two sounds of the same pitch and same loudness.

Answer: Quality is the characteristic which differentiates two sounds of the same pitch and same loudness.

Question 26: You recognize your friend by hearing his voice on a telephone. Explain.

Answer: Quality of sound makes it possible to recognize a person by hearing his voice on a telephone without seeing him. The vibrations produced by the vocal cord of each person have a characteristic wave form which is different for different persons.

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: Quality of sound makes it possible for the musician to 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: The following experiment shows the production of sound having low and high pitch:

• Take four test tubes. In first test tube take a little amount of water in it.
• Blow air in the tube by placing your lip on the mouth of the test tube.

You will hear a flat sound i.e. sound of low pitch.

• Now add more and more water in the remaining three test tubes as shown in below figure.
• Again blow air in each of these three test tubes.

You will notice sound becomes more and more shrill i.e. high pitch.

In first tube water is very less, so flat sound (low pitch) is produced. As we add more and more water in the tubes, the length of vibrating air column above water level decreases, so sound becomes more shriller (high 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 changing the length of vibrating air column on closing the different holes present in it.

Question 30: Why are musical instruments provided with more than one string?

Answer: Musical instruments are provided with more than one string to produce sound of different pitch. Musical instruments are provided with 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: The pitch of sound produced in a piano can be changed by the following ways:

• By changing the place of plucking on the string.
• By changing the tension and thickness of the string.

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 much faster in solids as compared to liquids and gases. The speed of sound in steel is 5960 m /s. So, sound produced by the moving wheels of train travels much faster through the track than air. So you can predict the arrival of a train by placing your ear on the rails without seeing it.

Question 33: Write the approximate speed of sound in:

(i) air

(ii) water

(iii) steel

Answer:

(i) 330 m/s

(ii) 1500 m /s

(iii) 5960 m /s

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. So it takes negligible time for light to reach us as compared to sound. So during a thunderstorm, the sound of a thunder is heard after the lightning is seen.

Question 35: Describe an experiment to estimate the speed of sound in air.

Answer: Estimation of speed of sound in air:

• Suppose we choose two hills A and B about a kilometer apart.
• A person at hill A fires a gun.
• Another person at hill B starts a stop watch as he sees the flash of fire and stops it on hearing the sound.

He measures the time interval between the sight of flash and hearing the sound. Let it be t second.

Measure the distance between the hills A and B. Let it be d metre.

So speed of sound V = ${\displaystyle \frac{\text{distance (d)}}{\text{time (t)}}}$ = ${\displaystyle \frac{\text{d}}{\text{t}}}$ m /s

Experimentally it is found that speed of sound in air is 330 m /s

Question 36: Can sound travel through solids and liquids? In which of these two does it travel faster?

Answer: Yes, sound can travel through solids and liquids. It travels faster in solids as compared to liquids.

Question 37: What do you mean by reflection of sound?

Answer: The return of sound after striking a surface in the same medium is called reflection of sound.

Question 38: State one use of reflection of sound.

Answer: The reflection of sound is used in making the trumpet, sound board, megaphone.

Question 39: What is an echo?

Answer: Echo is the distinct sound heard after reflection from a distant 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 required between the source of sound and the reflecting surface to hear an echo is 16.5 m ( about 17  m).

Speed of sound in air is 330 m /s and the time interval needed to identify two different sounds is 0.1 s, so sound will travel a distance of 330 x 0.1 = 33 m. Since sound has to travel an equal distance in going up to the reflecting surface and in coming back from the reflecting surface, so it must travel nearly 33/2= 16.5 m either.

Question 41: List four substances which are good absorbers of sound.

Answer: Clothes, paper, thermocol, curtains are good absorbers of sound.

Question 42: List the measures that you will take when designing a sound-proof room.

Answer: Following measures should be taken when designing a sound-proof room:

• The roof of the enclosure must be covered by plaster of Paris after putting the sheets of thermocol.
• The wall of the enclosure should be covered by wooden strips.
• The floor must be laid down by thick carpets.
• Thick curtains should be used to cover the doors and keep them closed.
• Thick stripping must be used to cover the openings of doors and windows.
• The machine parts of all the electrical equipment's like fans, air conditioners etc. must be placed outside the enclosure.

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: Distance (d) = 1020 m

time (t) = 3 s

Speed of sound = ${\displaystyle \frac{\text{distance}}{\text{time}}}$ = ${\displaystyle \frac{\text{1020}}{\text{3}}}$ = 340 m /s

So, speed of sound = 340 m /s

Question 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 /s, find the distance between the two hills.

Answer: Speed of sound = 330 m /s

time (t) = 4 s

Speed of sound = ${\displaystyle \frac{\text{distance}}{\text{time}}}$

distance = speed x time = 330 x 4 = 1320 m

So, distance between two hills = 1320 m.

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