Chapter : SOUND
NCERT Solutions:
1. How does the sound produced by a vibrating object in a medium reach your ear?
Solution: When an object vibrates, it necessitates the surrounding particles of the medium to vibrate.
The particles that are adjacent to vibrating particles are forced to vibrate. Hence, the sound produced
by a vibrating object in a medium is transferred from particle to particle till it reaches your ear.
1. Explain how sound is produced by your school bell.
Solution: When the school bell is hit with a hammer, it moves forward and backwards, producing
compression and rarefaction due to vibrations. This is how sound is produced by the school bell.
2. Why are sound waves called mechanical waves?
Solution: Sound waves require a medium to propagate to interact with the particles present in them.
Therefore, sound waves are called mechanical waves.
3. Suppose you and your friend are on the moon. Will you be able to hear any sound produced
by your friend?
Solution: No. Sound waves require a medium to propagate. Due to the absence of an atmosphere on
the moon and since sound cannot travel in a vacuum, I will not be able to hear any sound produced by
my friend.
1. Which wave property determines (a) loudness, (b) pitch?
Solution:
(a) Amplitude – The loudness of the sound and its amplitude is directly related to each other. The
larger the amplitude, the louder the sound.
(b) Frequency – The pitch of the sound and its frequency is directly related to each other. If the pitch
is high, then the frequency of sound is also high.
2. Guess which sound has a higher pitch: guitar or car horn?
Solution: The pitch of a sound is directly proportional to its frequency. Therefore, the guitar has a
higher pitch when compared to a car horn.
3. What are the wavelength, frequency, time period and amplitude of a sound wave?
Solution:
(a) Wavelength – Wavelength can be defined as the distance between two consecutive rarefactions or
two consecutive compressions. The SI unit of wavelength is metre (m).
(b) Frequency – Frequency is defined as the number of oscillations per second. The SI unit of
frequency is hertz (Hz).
(c) Amplitude – Amplitude can be defined as the maximum height reached by the trough or crest of a
sound wave.
(d) Time period – The time period is defined as the time required to produce one complete cycle of a
sound wave.
4. How are the wavelength and frequency of a sound wave related to its speed?
Solution:
Wavelength, speed, and frequency are related in the following way:
Speed = Wavelength x Frequency
v=λν
, 5. Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m/s in
a given medium.
Solution: Given that,
Frequency of sound wave = 220 Hz
Speed of sound wave = 440 m/s
Calculate wavelength.
We know that
Speed = Wavelength × Frequency
v=λν
440 = Wavelength × 220
Wavelength = 440/220
Wavelength = 2
Therefore, the wavelength of the sound wave = 2 metres
6. 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?
Solution: The time interval between successive compressions from the source is equal to the time
period, and the time period is reciprocal to the frequency. Therefore, it can be calculated as follows:
T= 1/F
T= 1/500
T = 0.002 s
7. Distinguish between loudness and intensity of sound.
Solution: The amount of sound energy passing through an area every second is called the intensity of
a sound wave. Loudness is defined by its amplitude.
1. In which of the three media, air, water or iron, does sound travel the fastest at a particular
temperature?
Solution: Sound travels faster in solids when compared to any other medium. Therefore, at a
particular temperature, sound travels fastest in iron and slowest in gas.
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 ms-1?
Solution: Speed of sound (v) = 342 ms-1
Echo returns in time (t) = 3 s
Distance travelled by sound = v × t = 342 × 3 = 1026 m
In the given interval of time, sound must travel a distance which is twice the distance between the
reflecting surface and the source.
Therefore, the distance of the reflecting surface from the source =1026/2 = 513 m
1. Why are the ceilings of concert halls curved?
Solution: The ceilings of concert halls are curved to spread sound uniformly in all directions after
reflecting from the walls.
NCERT Solutions:
1. How does the sound produced by a vibrating object in a medium reach your ear?
Solution: When an object vibrates, it necessitates the surrounding particles of the medium to vibrate.
The particles that are adjacent to vibrating particles are forced to vibrate. Hence, the sound produced
by a vibrating object in a medium is transferred from particle to particle till it reaches your ear.
1. Explain how sound is produced by your school bell.
Solution: When the school bell is hit with a hammer, it moves forward and backwards, producing
compression and rarefaction due to vibrations. This is how sound is produced by the school bell.
2. Why are sound waves called mechanical waves?
Solution: Sound waves require a medium to propagate to interact with the particles present in them.
Therefore, sound waves are called mechanical waves.
3. Suppose you and your friend are on the moon. Will you be able to hear any sound produced
by your friend?
Solution: No. Sound waves require a medium to propagate. Due to the absence of an atmosphere on
the moon and since sound cannot travel in a vacuum, I will not be able to hear any sound produced by
my friend.
1. Which wave property determines (a) loudness, (b) pitch?
Solution:
(a) Amplitude – The loudness of the sound and its amplitude is directly related to each other. The
larger the amplitude, the louder the sound.
(b) Frequency – The pitch of the sound and its frequency is directly related to each other. If the pitch
is high, then the frequency of sound is also high.
2. Guess which sound has a higher pitch: guitar or car horn?
Solution: The pitch of a sound is directly proportional to its frequency. Therefore, the guitar has a
higher pitch when compared to a car horn.
3. What are the wavelength, frequency, time period and amplitude of a sound wave?
Solution:
(a) Wavelength – Wavelength can be defined as the distance between two consecutive rarefactions or
two consecutive compressions. The SI unit of wavelength is metre (m).
(b) Frequency – Frequency is defined as the number of oscillations per second. The SI unit of
frequency is hertz (Hz).
(c) Amplitude – Amplitude can be defined as the maximum height reached by the trough or crest of a
sound wave.
(d) Time period – The time period is defined as the time required to produce one complete cycle of a
sound wave.
4. How are the wavelength and frequency of a sound wave related to its speed?
Solution:
Wavelength, speed, and frequency are related in the following way:
Speed = Wavelength x Frequency
v=λν
, 5. Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m/s in
a given medium.
Solution: Given that,
Frequency of sound wave = 220 Hz
Speed of sound wave = 440 m/s
Calculate wavelength.
We know that
Speed = Wavelength × Frequency
v=λν
440 = Wavelength × 220
Wavelength = 440/220
Wavelength = 2
Therefore, the wavelength of the sound wave = 2 metres
6. 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?
Solution: The time interval between successive compressions from the source is equal to the time
period, and the time period is reciprocal to the frequency. Therefore, it can be calculated as follows:
T= 1/F
T= 1/500
T = 0.002 s
7. Distinguish between loudness and intensity of sound.
Solution: The amount of sound energy passing through an area every second is called the intensity of
a sound wave. Loudness is defined by its amplitude.
1. In which of the three media, air, water or iron, does sound travel the fastest at a particular
temperature?
Solution: Sound travels faster in solids when compared to any other medium. Therefore, at a
particular temperature, sound travels fastest in iron and slowest in gas.
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 ms-1?
Solution: Speed of sound (v) = 342 ms-1
Echo returns in time (t) = 3 s
Distance travelled by sound = v × t = 342 × 3 = 1026 m
In the given interval of time, sound must travel a distance which is twice the distance between the
reflecting surface and the source.
Therefore, the distance of the reflecting surface from the source =1026/2 = 513 m
1. Why are the ceilings of concert halls curved?
Solution: The ceilings of concert halls are curved to spread sound uniformly in all directions after
reflecting from the walls.
