WBBSE Solutions For Class 9 Physical Science Chapter 7 Sources Of Sound And Propagation Of Sound

Chapter 7 Sources Of Sound And Propagation Of Sound Synopsis

1. Sound is a type of longitudinal elastic wave which is produced due to the vibration of a body. Sound can propagate through any material medium i.e., solid, liquid or gaseous medium.
2. The sound produced due to the vibration of the two elastic vocal chords present in the larynx of human beings is known as voice. The air coming from the lungs passes through the larynx and generates vibration in the vocal chords. If there is a change in the tension of the vocal chords or a change of air flow during respiration, then different types of sound are produced.
3. Sound waves are elastic waves, hence need a material medium for their propagation.
4. If a body traverses the same path repeatedly at fixed intervals of time or its motion undergoes repetition, then the motion of the body is known as periodic motion.
5. The motion of a particle is called vibration or oscillation when it follows a periodic motion along the same path and in opposite directions, repeatedly.

Some Quantities Related To Vibration:

Amplitude:

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The maximum displacement of a particle in a vibratory motion from its equilibrium position.

Time Period:

The time required to complete one oscillation.

Frequency:

The number of complete oscillations performed per unit of time.

Relation Between Time Period And Frequency:

If time period of a vibrating particle is T s. Then,

Frequency of the particle n = \(\frac{1}{t}\)

or, nT = 1

The turbulence created due to the combined vibration of the particles of a material medium, which transmits energy from one place to another without moving the particles, is known as a mechanical wave.

Production And Propagation Of Mechanical Waves Depend On The Following Three Properties Of The Material Medium:

1. Elasticity,
2. Inertia and
3. Cohesion

Waves Are Of Following Two Types:

1. Longitudinal waves and
2. Transverse waves.

1. If a wave propagates in a direction parallel to the motion of the vibrating particles of a material medium, the wave is known as a longitudinal wave.
2. If a wave propagates in a direction perpendicular to the motion of the vibrating particles of a material medium, the wave is known as a transverse wave.
3. When a wave propagates through a material medium, then the maximum displacement of any vibrating particle of the medium from its position of equilibrium is called its amplitude (a).
4. During propagation of a wave, the distance traversed by the wave during the time when a particle of the medium undergoes a complete vibration is called its
   wavelength (λ).
5. If the velocity, wavelength, and frequency of a wave are V, λ, and n respectively, then the relationship between them is V = nλ.
6. Among solid, liquid, and gaseous mediums, only the solid medium can oppose or resist any change of shape. Hence, transverse waves can propagate only through a solid medium.
7. Among solid, liquid and gaseous mediums, all the three can oppose or resist any change of volume. Hence, longitudinal waves can propagate through all of them.
8. Light wave is an electromagnetic wave. So, a light wave can also propagate through vacuum.
9. Alternate compressions and rarefactions are generated during the propagation of a longitudinal wave.
10. Alternate crests and troughs are generated during the propagation of a transverse wave.
11. In case of longitudinal waves, the summation of one compression and one rarefaction gives us the definition of wavelength. In case of transverse waves, wavelength is defined as the distance between two consecutive crests or two consecutive troughs.
12. In a solid medium, the velocity of a longitudinal wave is greater than the velocity of a transverse wave.

Chapter 7 Sources Of Sound And Propagation Of Sound Short And Long Answer Type Questions

Question 1. What is sound?

Answer:

Sound:

Sound is a type of longitudinal elastic wave which is produced due to the vibration of a body. It can propagate through any material medium, i.e., solid, liquid, or gaseous medium. If the frequency of sound is within the range of 20 Hz, it produces a sensation of nearing in human ears and is known as audible sound.

Question 2. Sound does not propagate in the absence of a material medium. Why?

Answer:

Sound Does Not Propagate In The Absence Of A Material Medium:-

Sound wave is an elastic wave. A material medium is necessary for the propagation of elastic waves. That is why sound does not propagate in the absence of a material medium.

Question 3. Show by experiments that sound produced due to vibration of a body.

Answer:

Sound Produced Due To Vibration Of A Body Is Shown Below:-

Experiment: A bell is hung by a rope. Now, the bell is struck by a hammer.

Observation: A loud sound is heard after the bell is struck by the hammer and the bell starts vibrating. Now if the’ bell is touched slowly, vibration of the bell can be felt and the sound of bell gradually stops.

Conclusion: Sound is emanated from the bell only because it vibrates.

Question 4. Explain the working principle of production of sound in the vocal chords of a human being with examples.

Answer:

Working Principle Of Production Of Sound In The Vocal Chords Of A Human Being With Examples:-

Narrow strips of two thin papers of the same type are taken. By keeping one of the papers on the other, the papers are stretched by both hands and air is blown heavily in the gap between the two papers. It is found that sound is produced.

Sound is produced in our vocal chords in a similar way. The vibration of the elastic vocal chords produces sound which we hear as our voice. Air from the lungs passes through the larynx and produces vibrations in the vocal chords. During each vibration of the vocal chord, free air enters the pharynx and initiates a sound.

As this sound progresses more through the oral cavity, it becomes louder. In this way, we can make different types of sound through our mouth by producing vibration of air.

Question 5. How different types of sound are produced in our vocal chords? Which muscle controls the elasticity of the two vocal chords?

Answer:

1. If there is a change in the tension of the vocal chords or a change of airflow during respiration, then different types of sound are produced.
2. Elasticity of the two vocal chords is controlled by thyroarytenoid muscles.

Question 6. Demonstrate with the help of an experiment that propagation of sound requires a material medium.

Answer:

Propagation of sound requires a material medium

A bell jar is kept on the upper platform of an air- extraction pump. A rubber stopper is fixed in the opening of the bell jar. Two wires are inserted through the stopper and connected to a calling bell in such a way that the bell remains inside the jar.

A battery and a switch is connected with the wires which come out of the rubber stopper. Vaseline is applied at the junction of the bell-jar and the platform to make the jar air tight. The calling bell starts ringing when the switch is turned on.

Now, air is slowly pumped out of the jar through the air-extraction pump and it is observed that the sound of the bell is reduced slowly and finally no sound can be heard anymore.

Again, if air is pumped into the jar slowly and the switch is turned on, sound of the calling bell can be heard again. We may conclude from the experiment that sound requires a material medium for its propagation. Sound cannot propagate through vacuum.

Question 7. Why is a walkie-talkie necessary for two people to talk standing side by side on the surface of the moon?

Answer:

There is no atmosphere on the surface of the moon. Entire space is a vacuum. Sound cannot propagate through vacuum. So, two people cannot hear each other even if they stand side by side. This is the reason why a walkie-talkie is necessary for two people to talk on the surface of the moon.

Question 8. Why does not the sound of an’explosion in the sun reach the earth?

Answer:

There is no material medium in the space between the sun and the earth. It is simply vacuum. As sound requires a material medium for its propagation, sound of an explosion in the sun does not reach the earth.

Question 9. If sound is made at one end of a long iron pipe, two sounds can be heard at the other end. Why?

Answer:

If sound is made at one end of a long iron pipe, two sounds can be heard at the other end.

If sound is made at one end of a long iron pipe, sound reaches the other side through both the mediums, iron and air. Velocity of sound in iron is far greater than the velocity of sound in air.

So, sound takes less time to travel through iron than it takes through air. As a result, the first sound one hears comes through iron and the second one that is heard comes through air.

Question 10. Explain with the help of simple experiments that sound may propagate through solid and liquid mediums.

Answer:

Sound may propagate through solid and liquid mediums

Suppose two boys are sitting on two sides of a table [Fig. 4]. If one of the boys put his ear on the table and the other boy knocks gently on the table at the other end, the first boy hears the sound very loudly. Table, made up of wood is a solid material. Hence, sound is propagated through this solid medium.

Sound can also propagate through liquid medium. A bell is immersed in a big bucket filled with water and vibrated. Now, if the ear is touched very carefully to the surface of the water, the ringing of the bell can be heard very clearly. Here, sound is propagated through water. So, sound can propagate through liquid medium.

Question 11. What is periodic motion? Give examples.

Answer:

Periodic Motion:

1. If a body traverses the same path repeatedly at fixed intervals of time or its motion undergoes repetition, then the motion of the body is known as periodic motion.
2. Motion of the earth around the sun, motion of the hands of a clock, and motion of a simple pendulum are examples of periodic motion.

Question 12. What is vibration? Explain with examples.

Answer:

Vibration

1. When a particle makes very fast movements on the two sides of its normal position, then the particle is said to undergo vibration.
2. A thin string or wire fixed at its two ends and stretched firmly may be considered as a summation of many particles on the same line. If the mid-point of the wire is pulled and then released, it starts vibrating on its two sides.
3. At that time, every point or particle of the wire undergoes vibration. Every particle is also said to undergo periodic motion.
4. Thus the motion of a particle is called vibration or oscillation when it follows a periodic motion along the same path and in opposite directions.

Question 13. What do you mean by the amplitude, time period, and frequency of a vibrating particle?

Answer:

Amplitude, Time Period, And Frequency Of A Vibrating Particle:-

Amplitude: The maximum displacement of a vibrating particle from its position of equilibrium is called its amplitude. Suppose, O is the equilibrium position of a vibrating particle.

The article is vibrating within the straight line AB. Then the maximum displacement of the particle in the right-hand side of its equilibrium position is OB and that in the left-hand side is OA. OA is always equal to OB.

Time period: The total time taken by a vibrating particle to execute one complete oscillation is called its time period. Time period T of the particle means that the particle starts from A reaches B and again comes back to A in T seconds.

Frequency: The frequency of a vibrating particle is the number of complete oscillations that the particle undergoes in one second.

Question 14. Establish a relationship between time period and frequency.

Answer:

Relationship Between Time Period And Frequency:-

If T is the time period of a vibrating particle, then one oscillation of the particle takes place in T seconds.

In one second, the particle undergoes \(\frac{1}{T}\) complete oscillation.

∴ frequency, n = \(\frac{1}{T}\)

Question 15. Explain with an example, what do you mean by waves.

Answer:

Waves:-

Wave is a type of turbulence that transmits energy from one place to another place due to collective vibration of the particles of a material • medium without displacement of the particles.

If a stone is thrown in still water of a pond, it is found that a turbulence is set in the water. This turbulence does not remain restricted to a single point. Centred around the point where the stone is struck, this turbulence spreads in a circular form. This is called a wave.

Question 16. Explain the production of a longitudinal wave by using a spring. What is a longitudinal wave?

Answer:

Longitudinal Wave And Production Of A Longitudinal Wave By Using A Spring

1. A long spring is attached to a hook and a small weight is hung at the lower end. The spring is pulled a little in the downward direction and then released. It is seen that the weight attached to the spring oscillates up and down. As a result, a wave proceeds from top to bottom through the spring by means of contraction and expansion.
2. This wave is longitudinal in nature. At any moment, contraction of the spring is called compression of the medium, and expansion of the spring is called rarefaction of the medium.
3. A longitudinal wave is defined as the wave moving parallel to the motion of the vibrating particles of a material medium.

Question 17. What is a transverse wave? Explain the generation of a transverse wave by using a string.

Answer:

Transverse Wave And The Generation Of A Transverse Wave By Using A String:-

1. If a wave moves in a direction perpendicular to the motion of the vibrating particles of a material medium, the wave is called a transverse wave.
2. One side of a thin and long rope is attached to a firm support and the other side is kept horizontal and is jerked several times at right angle to the rope. It is seen that a wave is produced in the rope. Here, each particle vibrates at right angle to the length of the wire. This type of wave is called transverse wave.

1. A transverse wave produced in the wire at any particular moment is shown. At a particular moment, the displacements of points A and C are highest on the upper side, so these are called crests of the wave. At that moment, displacements of point B and D are lowest on the lower side, so these are called troughs of the wave.

Question 18. Define

1. Amplitude and
2. Wavelength.

Answer:

Amplitude And Wavelength:-

1. When a wave is transmitted through a material medium, then the maximum displacement of any vibrating particle from its equilibrium position is called the amplitude (a) of the wave.
2. During the propagation of a wave, the distance traversed by the wave during the time when a particle of the medium undergoes a complete vibration is called its wavelength (λ).

Question 19. What do you mean by the frequency and velocity of a wave?

Answer:

Frequency And Velocity Of A Wave:-

Frequency: The frequency (n) is the number of complete vibrations made by a particle of the medium in one second during the propagation of a wave.

Velocity Of A Wave: Velocity (V) of a wave is the distance traversed by the wave through the medium in unit time.

Question 20. Establish the equation V = nλ, where V is the velocity of the wave, λ is its wavelength and n is its frequency.

Answer:

Suppose, the wavelength of a wave = λ, frequency = n, and velocity of the wave = V.

So, the wave traverses a distance A during the time when a particle of the medium makes one complete vibration. Therefore, the wave traverses a distance nλ during the time when the particle executes n complete vibrations.

The particle takes one second to complete n number of complete vibrations.

∴ The wave progresses a distance of nλ through the medium in one second.

∴ Velocity of the wave, V = nλ.

Question 21. Longitudinal waves can propagate through solid, liquid and gaseous mediums whereas transverse waves can propagate only through solid medium. Explain with reasons.

Answer:

Longitudinal waves can propagate only through that medium that opposes or resists the change of its volume or stops the change itself. As this property is a common feature of solid, liquid and gaseous mediums, longitudinal waves can propagate through all these three.

Again, transverse waves can propagate through that medium which opposes the change of its shape or stops the change itself. Liquid and gaseous mediums have no shape or form, unlike solid mediums. So, transverse waves can propagate only through a solid medium.

Question 22. Is a light wave a longitudinal wave?

Answer:

Longitudinal wave requires a material medium for its propagation. That is, a longitudinal wave propagates only through a material medium. As a light wave does not require any solid medium for its propagation (light wave can pass through vacuum also), it cannot be called a longitudinal wave. It is a type of electromagnetic wave.

Question 23. What are the differences between a longitudinal wave and a transverse wave?

Answer:

The differences between a longitudinal wave and a transverse wave:

Question 24. Which event proves that sound wave is an elastic wave?

Answer:

An elastic wave requires a material medium for its propagation. Similarly, the propagation of a sound Wave also requires a material medium. This proves that sound wave is an elastic wave.

Question 25. Describe in brief, how sound propagates through air medium.

Answer:

Sound Propagates Through Air Medium

Air medium may be considered as a sum of several equidistant parallel layers in the absence of any sound wave in the medium. The source of sound is one arm of a vibrating tuning fork. The arm of the fork (B) vibrates in the direction AC. When it goes from A to C in its vibrating state, it compresses the layers of air. As a result, that layer condenses and its pressure is increased.

This is called compression. Again, the gaseous medium opposes the change of volume. Due to application of pressure on that layer of air, particles of that region are displaced from their equilibrium position (displacement is different for different positions). As a result of displacement, elastic stress is originated in that area which tries to restore the particles to its to its equilibrium position.

Hence, that layer of air applies pressure on the next layer which is compressed and energy is transmitted. Next, when the arm B of the tuning fork goes from C to A, then pressure on the right-hand side of point C is reduced. So, a partial vacuum is created and expansion of the layer of air at that place takes place. This is called rarefaction.

One full vibration takes place when the arm of the tuning fork goes from A to C and again from C to A. Thus, due to vibration (half-vibration) of the arm of the tuning fork from A to C, a compression of air adjoining the arm is produced and due to vibration (half-vibration) of the arm of the tuning fork from C to A, a rarefaction of air adjoining the arm is produced.

An long as the vibration of the tuning fork continues, compressions and rarefactions are created periodically and these move through the medium. In this way, sound propagates through air.

Chapter 7 Sources Of Sound And Propagation Of Sound Very Short Answer Type Questions Choose The Correct Answer

Question 1. When sound is produced by the mouth, vibration is observed in

1. Vocal chords
2. Pharynx
3. Trachea
4. Cochlea

Answer: 1. Vocal chords

Question 2. If sound is made at one end of a long pipe, how many times is it heard at the other end?

1. Once
2. Twice
3. Thrice
4. Four times

Answer: 2. Twice

Question 3. Sound waves can propagate through

1. Gaseous medium only
2. Solid and liquid mediums only
3. Solid, liquid, and gaseous mediums
4. Vacuum

Answer: 3. Solid, liquid, and gaseous mediums

Question 4. Velocity of a sound wave of wavelength 2 m is 350 m/s. What is its frequency?

1. 170 Hz
2. 175 Hz
3. 700 Hz
4. 350 Hz

Answer: 2. 175 Hz

Question 5. The frequency of a tuning fork is 300 Hz. How much distance does a sound wave traverse during 20 vibrations of the fork? Velocity of sound in air is 333 m/s.

1. 11.1m
2. 33.3 m
3. 44.4 m
4. 22.2 m

Answer: 4. 22.2 m

Question 6. Frequencies of two tuning forks are 100 Hz and 150 Hz respectively. The ratio of the wavelengths of the waves produced by them in air is

1. 2:3
2. 3:2
3. 4:9
4. 9:4

Answer: 2. 3:2

Question 7. Propagation of sound waves through a medium like air may be compared to

1. Compression and rarefaction of a spring
2. Vibration of a string
3. Waves generated in water
4. Propagation of light waves

Answer: 1. Compression and rarefaction of a spring

Question 8. If time period is 0.2 s, what is the corresponding frequency?

1. 4 Hz
2. 5 Hz
3. 10 Hz
4. 20 Hz

Answer: 2. 5 Hz

Question 9. Sound propagates fastest through which of the following mediums?

1. Solid medium
2. Liquid medium
3. Gaseous medium
4. Vacuum

Answer: 1. Solid medium

Question 10. Sound does not propagate at all through which of the following mediums?

1. Solid medium
2. Liquid medium
3. Gaseous medium
4. Vacuum

Answer: 4. Vacuum

Question 11. Sound is

1. Longitudinal elastic wave
2. Transverse elastic wave
3. Electromagnetic wave
4. None of the above

Answer: 1. Longitudinal elastic wave

Question 12. Frequency of sound is measured in the unit

1. watt
2. erg
3. Second
4. Hertz

Answer: 4. Hertz

Question 13. A simple pendulum takes 40 seconds to oscillate 20 times. Time period of oscillation of the pendulum is

1. 1s
2. 2s
3. 3s
4. 4s

Answer: 2. 2s

Question 14. A simple pendulum takes 40 seconds to oscillate 20 times. Frequency of oscillation of the pendulum is

1. 0.1 Hz
2. 0.2 Hz
3. 0.3 Hz
4. 0.5 Hz

Answer: 4. 0.5 Hz

Question 15. Production of sound requires

1. Vibration of the source of sound
2. Solid medium
3. The presence of audience
4. The presence of a reflector

Answer: 1. Vibration of the source of sound

Question 16. Propagation of sound requires

1. Vibration of the source of sound
2. Solid medium
3. The presence of audience
4. The presence of a reflector

Answer: 2. Solid medium

Question 17. It has been proved experimentally that the velocity of sound in air at 0°C is approximately

1. 332 m/s
2. 442 m/s
3. 1080 m/s
4. zero

Answer: 1. 332 m/s

Question 18. When velocity of sound in air at 0°C is 332 m/s, then which of the following cannot be the velocity of sound through a solid medium at the same temperature?

1. 300 m/s
2. 400 m/s
3. 500 m/s
4. 1000 m/s

Answer: 1. 300 m/s

Question 19. Air comes out of lungs and enters larynx through

1. Vocal chords
2. Pharynx
3. Trachea
4. Heart

Answer: 3. Heart

Question 20. Sound wave is a longitudinal wave. So vibration of the source of sound is

1. Longitudinal
2. Transverse
3. Longitudinal or transverse
4. None of the above

Answer: 3. Longitudinal or transverse

Question 21. What is the distance between two consecutive compressions in the air medium, if A is the wavelength of sound?

1. λ
2. 2λ
3. λ/2
4. 2λ/3

Answer: 1. λ

Question 22. If the distance between three consecutive compressions of longitudinal waves created by a spring is 30 cm, what is the wavelength of the wave?

1. 90 cm
2. 60 cm
3. 45 cm
4. 15 cm

Answer: 4. 15 cm

Question 23. When the direction of velocity of a wave produced in a medium is parallel to the vibration of the particles of the medium, then the wave is positively a

1. Transverse wave
2. Longitudinal elastic wave
3. Electromagnetic wave
4. Lightwave

Answer: 2. Longitudinal elastic wave

Question 24. Which is the essential property of a solid medium for the origin and propagation of mechanical waves?

1. Elasticity only
2. Inertia only
3. Cohesion only
4. All of the above

Answer: 4. All of the above

Question 25. What is the velocity of sound through copper’ if its density and Young’s modulus are 8.6 g/cm³ and 11.8 x 10¹¹ dyn/cm³ respectively?

1. 3704 m/s
2. 3604 m/s
3. 3504 m/s
4. 3404 m/s

Answer: 1. 3704 m/s

Chapter 7 Sources Of Sound And Propagation Of Sound Answer In Brief

Question 1. What type of wave is a sound wave?

Answer: Sound wave is a longitudinal elastic wave.

Question 2. Can sound wave propagate through vacuum?

Answer: No, sound wave cannot propagate through vacuum.

Question 3. What is a sonorous body?

Answer: A body which acts as a source of sound is called a sonorous body.

Question 4. How does the sound produced from the vocal chords of a human being change?

Answer: If there is change of tension of the two vocal chords or there is a change in air flow while breathing, then different types of sound are produced.

Question 5. How can one talk on the surface of the moon?

Answer: One has to take the help of walkie-talkie to talk on the surface of the moon.

Question 6. Write down the expression of velocity of sound through a solid medium.

Answer: Velocity of sound through a solid medium,

V = \(\sqrt{\frac{Y}{\rho}}\) where Y is Young’s modulus of the solid and ρ is the density of the solid medium.

Question 7. The velocity of sound is highest in which gaseous medium?

Answer: Velocity of sound is highest in hydrogen gas.

Question 8. How is sound produced in guitar, sitar and violin?

Answer: Sound is produced in these instruments due to vibration of strings.

Question 9. How is sound produced in a flute?

Answer: Sound is produced in a flute due to vibration of air particles inside the flute.

Question 10. How is sound produced in human larynx?

Answer: Sound is produced in human larynx due to vibration of the two elastic vocal chords present in the larynx.

Question 11. Which muscle controls the elasticity of two vocal chords of a human being?

Answer: A muscle named thyroarytenoid controls the elasticity of the vocal chords.

Question 12. Among iron, water, and air, velocity of sound is maximum in which medium?

Answer: Velocity of sound is maximum in iron.

Question 13. Among iron, water, and air, velocity of sound is minimum in which medium?

Answer: Velocity of sound is minimum in air.

Question 14. If a sound is made at one end of a long iron pipe, we hear sound twice at the other end (due to the two mediums, iron, and air). Which sound is heard at first?

Answer: The sound which is transmitted through iron is heard at first.

Question 15. Between the motion of the arm of a clock and the motion of a simple pendulum, which one is a periodic motion but not a linear simple harmonic motion?

Answer: The motion of the arm of a clock is a periodic motion but it is not a simple harmonic motion.

Question 16. A particle is executing a simple harmonic motion between two points A and B. If AB = 10 cm, what is the amplitude of the particle?

Answer: Amplitude of the particle = 10/5 = 5 cm

Question 17. If the time period of an oscillating particle is 0.01 s, then what is its frequency?

Answer: Time period, T = 0.01 s

∴ frequency, n = \(\frac{1}{T}\) = \(\frac{1}{0.01}\) = 100 Hz

Question 18. If the frequency of an oscillating particle is 5 Hz, what is its time period?

Answer: Frequency, n = 5 Hz

∴ time period, T = \(\frac{1}{n}\) = \(\frac{1}{5}\) = 0.2 s

Question 19. Which wave progresses parallel to the movement of particles oscillating in a material medium?

Answer: Longitudinal wave progresses in a direction parallel to the movement of particles oscillating in a material medium.

Question 20. Which wave progresses perpendicular to the movement of particles oscillating in a material medium?

Answer: Transverse wave progresses in a direction perpendicular to the movement of particles oscillating in a material medium.

Question 21. Can longitudinal waves propagate through solid, liquid, and gaseous mediums?

Answer: Yes, longitudinal waves can propagate through solid, liquid, and gaseous mediums.

Question 22. Can transverse waves propagate through solid, liquid, and gaseous mediums?

Answer: No, transverse waves can propagate only through solid medium.

Question 23. Which length is considered as a wavelength in case of a longitudinal wave?

Answer: Total length of one compression and one rarefaction is taken as a wavelength in case of a longitudinal wave.

Question 24. Which length is considered as a wavelength in case of a transverse wave?

Answer: The distance between two successive crests or two successive troughs is taken as a wavelength in case of a transverse wave.

Question 25. What is the relationship between propagation of a sound wave and medium?

Answer: Sound wave can propagate only through an elastic medium.

Question 26. How many types of waves are there? Name them.

Answer: There are two types of waves:

1. Longitudinal wave and
2. Transverse wave.

Chapter 7 Sources Of Sound And Propagation Of Sound Fill In the Blanks

Question 1. Sound is a  ______ elastic wave.

Answer: Longitudinal

Question 2. Any particle moving with linear simple harmonic motion periodically traverses the same path in _______

Answer: Opposite direction

Question 3. Transverse waves propagate only through solid medium because a solid medium can resist the change of its _______

Answer: Shape

Question 4. Light wave can propagate through vacuum because it is a type of ______ wave.

Answer: Electromagnetic

Question 5. The reciprocal of time period is ________

Answer: Frequency

Question 6. ______ is the number of complete oscillations made by an oscillating particle in one second.

Answer: Frequency

Question 7. Velocity of sound in air at 0°C is ______

Answer: 332 m/s

Question 8. During the propagation of ______ waves, the layers of the medium come very close to each other and the phenomenon is known as compression.

Answer: Longitudinal

Question 9. During the propagation of transverse waves, ______ and ______ are produced in a cyclic order.

Answer: Crests, troughs

Question 10. ______ waves propagate in a direction perpendicular to the direction of motion of the particles of the medium.

Answer: Transverse

Question 11. ______ waves propagate in a direction parallel to the direction of motion of the particles of the medium.

Answer: Longitudinal

Question 12. Velocity of sound waves is ______ compared to the velocity of light waves.

Answer: Less

Question 13. The unit of frequency is ______

Answer: Hertz(Hz)

Question 14. If the time period of a sound wave is 0.05s, its frequency is ______

Answer: 20Hz

Chapter 7 Sources Of Sound And Propagation Of Sound State Whether True Or False

Question 1. Sound is a type of transverse elastic wave which is produced due to the vibration of a body.

Answer: False

Question 2. Sound waves do not need any material medium for their propagation.

Answer: False

Question 3. Production and propagation of mechanical waves depend on the cohesive property of the material medium.

Answer: True

Question 4. Longitudinal waves propagate in a direction perpendicular to the motion of the vibrating particles of a material medium.

Answer: False

Question 5. Alternate compressions and rarefactions generate during the propagation of a longitudinal wave.

Answer: True

Question 6. Velocity of sound is highest in gaseous medium.

Answer: False

Question 7. Wavelength of sound wave is much larger than that of light wave.

Answer: True

Question 8. Sound is a kind of electromagnetic wave.

Answer: False

Question 9. Velocity of sound wave does not changes with the change of medium.

Answer: False

Question 10. Speed of light and speed of sound are same in a medium.

Answer: False

Question 11. Sound propagates in air medium in the form of longitudinal waves.

Answer: True

Chapter 7 Sources Of Sound And Propagation Of Sound Numerical Examples

Useful Information

1. A particle executes simple harmonic motion between A and B. If AB = d.
2. The amplitude of this oscillation a = \(\frac{d}{2}\)
3. If time period of oscillation of a particle = T and frequency = n, then, n = \(\frac{1}{T}\).
4. If the frequency and wavelength of a wave in a medium be n and λ respectively, then velocity of the wave V = nλ
5. Velocity of sound in a solid V = \(\sqrt{\frac{Y}{\rho}}\), Y = Young’s modulus, and ρ = density of the solid.

Question 1. The velocity of a sound wave of wavelength 1.7 m is 340 m/s. What is its frequency?

Answer:

Given

Velocity V of the sound wave is 340 m/s and wavelength λ = 1.7 m

Suppose, frequency of the sound wave = n

∴ V = nλ or,n = \(\frac{360}{1.7}\) or, n = 200 Hz

Question 2. The frequency of a tuning for k is 280 Hz. During the time the fork completes 70 full vibrations, a sound wave traverses 80 m through a medium. What is the velocity of sound in the medium?

Answer:

Given

The frequency of a tuning for k is 280 Hz. During the time the fork completes 70 full vibrations, a sound wave traverses 80 m through a medium.

Suppose, the wavelength of the sound produced by this tuning fork = λ, then

70λ = 80 or, λ = \(\frac{80}{70}\) or, λ = \(\frac{8}{7}\) m

Again, frequency, n = 280 Hz

So, velocity of sound, V = nλ

∴ V = 280 Hz x \(\frac{8}{7}\) m = 320 m/s

Question 3. The frequencies of two tuning forks are 200 Hz and 300 Hz respectively. What is the ratio of wavelengths of the waves produced by the forks in air?

Answer:

Given

The frequencies of two tuning forks are 200 Hz and 300 Hz respectively.

Frequency of the first tuning fork, n₁ = 200 Hz

In this case, if the wavelength of the sound produced = λ₁.

Then velocity of sound, V = n₁λ₁….(1)

Again, frequency of second tuning fork, n₂ = 300 Hz.

Here, if the wavelength of the sound produced = λ₂, then velocity of sound is given by, V = n₂λ₂…..(2)

Comparing equations (1) and (2), we get

\(n_1 \lambda_1=n_2 \lambda_2\)

 

or, \(\frac{\lambda_1}{\lambda_2}=\frac{n_2}{n_1}\)

or, \(\frac{\lambda_1}{\lambda_2}=\frac{300}{200}=\frac{3}{2}\)

∴ λ₁ :  λ₂= 3:2

Question 4. Young’s modulus of a metal, Y = 12.8 x 10¹¹ dyn/cm^2, and its density is 8 g/cm³. What is the velocity of sound in the metal?

Answer:

Given

Young’s modulus of a metal, Y = 12.8 x 10¹¹ dyn/cm^2, and its density is 8 g/cm³.

Velocity of sound in the metal, = 4 x 10⁵ cm/s

V = \(\sqrt{\frac{Y}{\rho}}=\sqrt{\frac{12.8 \times 10^{11}}{8}}=4 \times 10^5 \mathrm{~cm} / \mathrm{s}\)

= 4000 m/s = 4 km/s

Question 5. A and B are standing 1 km apart from each other. A fires a shot and B observes a flash of fire immediately but hears the sound of the shot after 2.5 s. What is the velocity of sound in air?

Answer:

Given

A and B are standing 1 km apart from each other. A fires a shot and B observes a flash of fire immediately but hears the sound of the shot after 2.5 s.

Distance between A and B, s = 1 km = 1000 m.

B sees the flash as soon as the shot is fired. Since the flash of fire and sound are produced at the same time as the firing of the shot, time taken by sound wave reaches B from A, t= 2.5 s.

∴ velocity of sound in air, v = \(\frac{s}{t}=\frac{1000}{2.5}=400 \mathrm{~m} / \mathrm{s}\)

Question 6. A man adjusts time in his watch by hearing the sound of siren in a factory. It is later found that the watch is running slow by 3 s. If the velocity of sound in air is 340 m/s, what is the distance of the factory from the man?

Answer:

Given

A man adjusts time in his watch by hearing the sound of siren in a factory. It is later found that the watch is running slow by 3 s. If the velocity of sound in air is 340 m/s,

If the watch is adjusted by hearing the sound of the siren and still it slows down by 3 s.

This means that sound takes 3 s to reacfi him from the factory.

∴ distance of the factory from the man = 340 X 3 = 1020 m

Question 7. The wavelength of a sound wave of frequency 2.5 kHz is 40 cm. How much time does this wave take to traverse a distance of 2 km?

Answer:

Given

The wavelength of a sound wave of frequency 2.5 kHz is 40 cm.

Frequency of sound wave, n = 2.5 kHz = 2500 Hz

Wavelength, λ = 40 cm = 0.4 m

Suppose, velocity of sound wave = V

∴ V = nλ = 2500 x 0.4 = 1000 m/s

So, required time = \(\frac{2000}{1000}\) = 2 s

Question 8. A body vibrating with a definite frequency produces waves of lengths 15 cm and 25 cm in mediums X and Y respectively. What is the ratio of the velocities of the waves in the two mediums?

Answer:

Given

A body vibrating with a definite frequency produces waves of lengths 15 cm and 25 cm in mediums X and Y respectively.

The wavelength of the wave in X medium = 15cm

The wavelength of the wave in Y medium = 25cm

As we know, V = nλ

∴ V_X = nλ_X

and V_Y = nλ_Y [frequency is constant]

So the ratio is given by

\(\frac{V_X}{V_Y}=\frac{n \lambda_X}{n \lambda_Y}=\frac{\lambda_X}{\lambda_Y}=\frac{15}{25}=\frac{3}{5}\)

∴ V_X : V_Y= 3:5

Question 9. The frequency of a tuning fork is 512 Hz. How many complete vibrations does the tuning fork make when sound produced by it traverses 550 m? Assume velocity of sound in air = 320 m/s.

Answer:

Given

The frequency of a tuning fork is 512 Hz.

Velocity of sound in air, V = 320 m/s

Frequency, n = 512 Hz

Wavelength of sound, λ = \(\frac{V}{n}\) = \(\frac{320}{512}\) = 0.625 m

∴ The tuning fork executes \(\frac{550}{0.625}\) = 880 complete

Vibrations when sound travels 550 m.