Class 9th Science Chapter-11 In-Text Questions

Chapter 11 – Sound

Page 129

Questions with Answers

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Question 1

How does the sound produced by a vibrating object in a medium reach your ear?

Answer:
The vibrating object sets the particles of the medium around it into vibration. These vibrations are passed on from one particle to the next in the form of compressions and rarefactions. In this way, the disturbance travels through the medium and reaches the ear, producing the sensation of sound.

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Question 2

Explain how sound is produced by your school bell.

Answer:
When the school bell is struck, it starts vibrating. These vibrations produce compressions and rarefactions in the surrounding air. The sound thus produced travels through air and reaches our ears.

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Question 3

Why are sound waves called mechanical waves?

Answer:
Sound waves are called mechanical waves because they require a material medium for their propagation and are produced due to the vibrations of particles of the medium.

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Question 4

Suppose you and your friend are on the moon. Will you be able to hear any sound produced by your friend?

Answer:
No, sound cannot be heard on the moon because there is no medium like air to transmit sound waves.

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Page 132

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Question 1

Which wave property determines (a) loudness, (b) pitch?

Answer:
(a) Loudness depends on the amplitude of the sound wave.
(b) Pitch depends on the frequency of the sound wave.

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Question 2

Guess which sound has a higher pitch: guitar or car horn?

Answer:
The guitar produces a sound of higher pitch than a car horn.

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Question 1

What are wavelength, frequency, time period and amplitude of a sound wave?

Answer:

• Wavelength (λ): The distance between two consecutive compressions or two consecutive rarefactions.

• Frequency (ν): The number of oscillations per second.

• Time period (T): The time taken for one complete oscillation.

• Amplitude (A): The maximum displacement of particles of the medium from their mean position.

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Question 2

How are the wavelength and frequency of a sound wave related to its speed?

Answer:
The speed of sound is given by:

$$v=\lambda \nu$$

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Question 3

Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m s⁻¹ in a given medium.

Answer:

$$\lambda =\frac{v}{\nu }=\frac{440}{220}=2m$$

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Question 4

A person is listening to a tone of 500 Hz sitting at a distance of 450 m from the source of the sound. What is the time interval between successive compressions from the source?

Answer:
Time interval between successive compressions equals the time period.

$$T=\frac{1}{\nu }=\frac{1}{500}=0.002s$$

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Page 133

Sound

Questions with Answers

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Set – 1

Question 1

Distinguish between loudness and intensity of sound.

Answer:

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Set – 2

Question 1

In which of the three media, air, water or iron, does sound travel the fastest at a particular temperature?

Answer:
Sound travels fastest in iron, then in water, and slowest in air.

Explanation:
The speed of sound depends on the nature of the medium. In solids like iron, the particles are closely packed and have strong intermolecular forces. When sound is produced, vibrations are transferred more quickly from one particle to the next.

In liquids, particles are less closely packed than in solids, so sound travels slower than in solids but faster than in gases.
In gases like air, particles are far apart, so vibrations take more time to pass from one particle to another, making sound travel slowest.

Hence, sound travels fastest in iron, followed by water, and slowest in air.

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Page 134

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Question 1

An echo is heard in 3 s. What is the distance of the reflecting surface from the source, given that the speed of sound is 342 m s⁻¹?

Answer:

Speed of sound, $v=342m{s}^{-1}$
Time for echo, $t=3s$

Distance travelled by sound:

$$\text{Distance}=v\times t=342\times 3=1026m$$

Since the sound travels to the reflecting surface and back, the distance of the reflecting surface is:

$$\frac{1026}{2}=513m$$

Distance of the reflecting surface = 513 m

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Page – 135

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Question 1

Why are the ceilings of concert halls curved?

Answer:
The ceilings of concert halls are curved so that sound waves, after reflection, spread uniformly in all directions. This helps the sound to reach all corners of the hall clearly, improving audibility for the audience.

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Page 136

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Question 1

What is the audible range of the average human ear?

Answer:
The audible range of the average human ear is from 20 Hz to 20 kHz.

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Question 2

What is the range of frequencies associated with
(a) Infrasound?
(b) Ultrasound?

Answer:
(a) Infrasound: Frequencies below 20 Hz
(b) Ultrasound: Frequencies above 20 kHz