Sound QA ICSE 9

 

(Q.1)   Why the two astronauts are not able to talk on the moon’s surface? (Ans) There is no atmosphere on the moon’s surface and sound need a material medium for its propagation. Therefore, two astronauts are not able to talk on the surface of moon.	(1Mark)

(Q.2)   What are the thermal changes in air as the sound propagates through it? the sound wave propagates through the air, there are changes in pressure and volume which is adiabatic, i.e., the temperature increases in the region of compression and it decreases in the region of rarefaction.	(1Mark)

(Q.3)   What do you understand by ‘Persistence of hearing’? (Ans) The sensation of any sound persists in our ear for nearly 0.1s, after this the original sound heard dies off. This is known as ‘Persistence of hearing’.	(1Mark)

(Q.4)  What is sound?	(1Mark)

(Ans)

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

(Q.5)  State a difference between infrasonic and ultrasonic vibrations.	(1Mark)

(Ans)

Infrasonic vibrations are those vibrations which have frequency less than 20 Hz whereas ultrasonic vibrations are those vibrations which have frequency more than 20 KHz.

(Q.6)  Define wave.	(1Mark)

(Ans)

The movement of the disturbance through a medium due to the repeated periodic motion of the particles of the medium about their mean position is known as wave.

(Q.7)  What is the relation between frequency and wavelength?	(1Mark)

(Ans)

(Q.8)  Define time period of a wave. Give its SI unit.	(1Mark)

(Ans)

The time required by a wave to complete one oscillation is called its time period. It is represented by the letter ‘T’. Its SI unit is second (s).

(Q.9)  Define wave velocity. Give its SI unit.	(1Mark)

(Ans)

In a particular medium, the distance covered by the disturbance in one second is called its wave velocity. It is represented by the letter ‘v’. Its SI unit is meter/second.

(Q.134)  What is intensity of sound?	(1Mark)

Ans) The amount of sound energy passing each second through unit area is called the intensity of sound.

(Q.135)  Why sound wave is called longitudinal wave?	(1Mark)

(Ans) Sound wave is called longitudinal wave because disturbances propagate in the medium as a series of compressions and rarefactions.

(Q.136)  What is a sound? (Ans) Sound is a form of energy which produces a sensation of hearing in ears.	(1Mark)

(Q.137)  Why sound waves are called mechanical waves?	(1Mark)

(Ans) Sound waves are called mechanical waves because they need material medium to propagate.

(Q.138)  What is a crest and a trough in a wave?	(1Mark)

(Ans) A peak is called the crest and a valley is called the trough of a wave.

(Q.139)  �Can sound wave travel through vacuum?	(1Mark)

(Ans) No. Sound wave can not travel through vacuum.

(Q.140)   The frequency of the tuning fork is 450 Hz. Calculate the distance travelled by sound when the fork makes 50 vibrations. Take the speed of sound in air = 320 ms-1. (Ans)	(2Marks)

(Q.141)   When vibration is a necessary condition for a production of sound, then why the vibrating pendulum does not produce sound? (Ans) The audible range of normal human ear is 20Hz to 20,000Hz. A vibrating pendulum produces sound but the frequency of the vibrating pendulum does not lie within the audible range and hence, it does not produce audible sound.	(2Marks)

(Q.142)  State three necessary conditions required for hearing sound.	(2Marks)

(Ans)

Three necessary conditions required for hearing sound are:

(i) Sound needs a medium to travel which pick up the energy and propagate it in the forward direction. It can travel through solids, liquids and gases but not through vacuum.

(ii) The vibrating body should be present which transfer its energy to the surroundings.

(iii) The receiver must be there to receive the vibrations of sound which are then forwarded to the brain.����������

(Q.143)  What are sonic vibrations? Give two examples of sonic vibrations.	(2Marks)

(Ans)

Sonic vibrations are also known as audio vibrations. These vibrations have the frequency range from 20 Hz to 20,000 Hz which can be perceived by the human ear. Two examples of sonic vibrations are:

(i) Vibrations produced by the flute.

(ii) Vibrations produced by the drum.

(Q.144)  What is Galton’s whistle? What is its use?	(2Marks)

(Ans)

Galton’s whistle is a special kind of whistle which can produce sounds of more than 20,000 Hz.�

Use: It enables the owner of a dog to summon it without disturbing his/her neighbours.

(Q.145)  Why does the flash seen first and a thunder heard a little later when the gun is fired?	(2Marks)

(Ans)

The speed of light is more as compared to speed of sound in air. The speed of light is 3 x 10⁸ m/s and the speed of the sound in air is 332 m/s. Due to this, when the gun is fired, the flash is seen first and a thunder is heard a little later.

(Q.146)  Perform an activity to show that sound is produced by the tuning fork.	(2Marks)

(Ans)

Activity: Sound produced by tuning fork:
1. Take a tuning fork and hold it from its stem.
2. Take a rubber pad and strike the tuning fork from its stem.

Observation: The prongs of the tuning fork vibrate.
Result: Due to vibrations, sound is produced.��

(Q.147)  Sound can travel through solids and liquids. Give one example of each.	(2Marks)

(Ans)

1.The diver under the water can hear the words spoken on the bank of the river. This shows that sound travels through liquids.
2. We can hear the sound of moving train at a distance when we keep our ears on the rails. This shows that sound travels through solids.

(Q.148)   A bat can hear sounds upto frequencies of 120kHz. Find the wavelength of sound in the air at this frequency. (Speed of sound in air = 344 m/s) Q 148   A bat can hear sounds upto frequencies of 120kHz. Find the wavelength of sound in the air at this frequency. (Speed of sound in air = 344 m/s) (2Marks) (Ans) Frequency = 120kHz; Velocity of sound in the air = 344m/s Wavelength= (Wave velocity)/(Frequency) ���������������� = 344ms-1/120kHz ���������������� = 344ms-1/(120 x 103 s-1) ���������������� = 0.29 cm

(2Marks)

(Q.149)  What is the frequency of an oscillating body?� The frequency of a source is 80Hz. Find the number of times it vibrates in a minute.	(2Marks)

(Ans)

Frequency of an oscillating body is the number of complete oscillations made by it in one second.

The frequency of a source is 80Hz. It means that it makes 80 complete oscillations in one second.

Hence, in one minute (60 seconds), the number of oscillations it will make = 80Hz x 60s

�������������������������������� = 4800

(Q.150)  A sound wave has a frequency of 1500Hz and wavelength 25cm. How long will it take to travel 3 km?	(2Marks)

(Ans) Given, frequency =1500Hz .

(Q.151)  Calculate the time in which a tuning fork of frequency 234 Hz completes 26 vibrations.	(2Marks)

(Ans) Given , frequency = 234 Hz�
Time period (T) = 1/frequency
������������������������� = 1/234 s
Thus, Time taken to complete 26 vibrations = T x 26
�������������������������������������������������������������������� = 26/234
�������������������������������������������������������������������� = 1/9 s

���

(Q.152)  We hear thunder sound a few seconds after the flash is seen even though they occur simultaneously. Why?	(2Marks)

(Ans)

Sound travels a million times slower than light in air.

Speed of sound in air = 346 m/s

Velocity of light in air = 3x10⁸ m/s

Therefore, thunder sound is heard a few seconds after the flash is seen.

(Q.153)   A source of sound is placed at one end of an iron rod which is one kilometer long. After an interval of 1 second, two sounds are heard at the other end of an iron rod. Calculate the speed of sound in iron assuming the speed of sound in air is 340ms-1.� Why two sounds are heard?	(3Marks)

(Ans)

Two sounds are heard as the sound travels through air as well through iron.���

Speed of sound in air = 340 ms^-1

Length of the rod = 1000 m

Time taken by sound to travel in air,

t₁ = distance/speed

�� = 1000m/340 ms^-1

�� = 2.94 s^{���������������������}

The time interval between two sounds = 1s

Thus, the time taken by the sound to travel one kilometer long in an iron rod,

t₂ = 2.94 s – 1s = 1.94 s�

So, the speed of sound in iron = distance/time

������������������������������������ �������= 1000m/1.94s

������������������ �������������������������= 515.46 ms^-1

(Q.154)  The red light has a wavelength of 65 x 10-10m. Calculate the frequency and time period if the velocity of light is 3 x 108ms-1.	(3Marks)

(Ans)

(Q.155)  Explain why the jet plane when passes over, shatters the glasses of the window of the house.	(3Marks)

(Ans)

A jet plane travels with supersonic speed (speed of a jet exceeds the speed of sound). Hence, it produces shock waves in the air. These shock waves carry a large amount of energy. The air pressure variation associated with such waves produces sharp and loud sound called “sonic boom”. These shock waves have enough energy to shatter glasses of the window of the house.

(Q.156)  What are elastic waves? Give two examples of elastic waves.	(3Marks)

(Ans)

The motion in a medium in which, when particles are displaced, a force proportional to the displacement acts on the particles. This force restores them to their original position. If a material possess the property of elasticity and the particles in a certain region are set in vibratory motion, an elastic wave will be propagated.

Two examples of elastic waves are:
i.������� Waves produced in the stretched string.�

ii.������� Waves on the surface of water.

(Q.157)  Define the following terms and give their SI units:�������������������������������������������������������������������������������������� (i) Wavelength (ii) Amplitude (iii) Frequency	(3Marks)

(Ans)

(i) Wavelength: The linear distance between two consecutive vibrating particles of the medium in the same phase is called its wavelength. It is represented by . Its SI unit is metre (m).�

(ii) Amplitude: The maximum displacement of a vibrating particle about its mean position is known as amplitude.�

�AS, BR, CQ and DP are the displacement of the particles A, B, C and D about their mean position. They represent the amplitude. Its SI unit is metre (m).�

(iii) Frequency: The number of complete vibrations made by the vibrating particle about its mean position in one second. It is represented by letter ‘f’ or_� . Its SI unit is Hertz (Hz) or s^-1.

(Q.158)   Give three differences between light waves and sound waves. (Ans) ���������� S.No. Light Waves Sound Waves 1. These waves are produced from the electrons present in the excited state. These waves are produced due to vibrations of different objects in a material medium.� 2. These waves can be propagated through vacuum. They can propagate through material medium (solids, liquids and gases) and cannot travel through vacuum.� 3. They travel with the speed of 3 x 108 m/s in air. They travel with the speed of 332 m/s at 20 oC.

(3Marks)

(Q.159)  What will happen to the loudness of a sound wave if its amplitude is doubled? Name a unit of sound. Is loudness affected by change in frequency?	(3Marks)

(Ans)

The loudness of sound is proportional to the square of the amplitude of the vibration of the sound wave. Therefore, the new sound will be four times louder than the original one.
The loudness is expressed in decibel(dB). No, loudness of a sound will not be affected by change in its frequency even though its pitch will change.

(Q.160)  �Find the distance travelled by a sound wave of frequency 20 KHz, wavelength 1.7 cm in 10 s.�	(3Marks)

(Q.161)  Define frequency of a sound wave. What is its SI unit and how is it related to pitch? What is the frequency of the wave shown below?��� ���������������������� �	(3Marks)

(Ans)

(Ans) The number of complete periodic oscillations of a sound wave per unit time is called the frequency of the sound wave.�
The SI unit of frequency is hertz (Hz). Higher the frequency of a sound wave, higher is its pitch and vice-versa.�
Number of complete oscillations in one second = 4.
Frequency of the wave = 4 Hz.

(Q.162)  Explain that sound is a mechanical wave. How do sound waves travel in different media? What is the approximate speed of sound in air?	(3Marks)

(Ans) A mechanical wave is the wave which requires a medium to travel. Mechanical waves cannot pass through vacuum. Since sound wave requires a medium to travel and it cannot pass through the vacuum, it is a mechanical wave.

Sound is produced by vibrating objects. When a vibrating object moves forward, it pushes and compresses the air in front of it creating a region of high pressure. The region is called a compression. When the vibrating object moves backward, it creates a region of low pressure called rarefaction. The object moves back and forth and creates a series of compression and rarefaction. Thus, sound propagates in the medium. The process is explained in the diagram below:�

The approximate speed of sound in air at 293 K is 343 m/s.

(Q.163)  What is the relevance of amplitude and of frequency of a vibrating body to sound produced by it?	(3Marks)

(Ans)

The loudness of a sound is determined by the amplitude of vibration of the vibrating body producing it. An increase in the amplitude of vibration results in making sound louder and vice-versa.

The frequency of vibration of a vibrating body controls the pitch of the sound produced by it. If we increase the frequency of vibration, the sound produced becomes sharper and shriller. However, if we decrease the frequency, the sound becomes flat.

(Q.164)  Define speed of sound. Name the factors on which the speed of sound depends. Why the intensity of sound of a speaker is larger immediately after rain?	(3Marks)

(Ans)

Speed of sound tells us about the rate at which the sound travels from the sound producing body to our ears.

The speed of sound depends on –

1. Nature of the medium

2. Temperature of the medium

3. Humidity of the medium

Immediately after the rain, there is an abundance of water vapours in the air. Velocity of sound in moist air is greater than the velocity of sound in dry air. Therefore, sound will take lesser time to reach its destination when it travels through the moist air. So, there will be lesser decrease in its amplitude due to air resistance. As a result, intensity of sound of a speaker will be larger immediately after the rain.

(Q.165)  Are the particles of the medium dragged along with the wave as sound wave propagates through the medium? In which medium—solid, liquid or gas, does the sound wave travel fastest?	(3Marks)

(Ans)

No, the particles of the medium (material particles) are not dragged along with the wave. The particles of the medium simply oscillate about their mean positions at their fixed locations. And the disturbance is handed over from one particle to the next particle in contact.

Sound travels fastest in solid.

(Q.166)  Explain, how compressions and rarefactions are produced in air near a source of sound.	(3Marks)

(Ans) When a vibrating object moves forward, it pushes and compresses the air in front of it creating a region of high pressure. The region is called a compression. When the vibrating object moves backward, it creates a region of low pressure called rarefaction. The object moves back and forth and creates the series of compression and rarefaction. Thus sound propagates in the medium. The process is explained in the diagram below: �

(Q.167)  How do sound waves propagate?	(3Marks)

(Ans) When a vibrating object moves forward, it compresses the air in front of it creating a region of high pressure called compression. When it moves backwards, it creates a region of low pressure called rarefaction. As the object moves back and forth rapidly, a series of compressions and rarefactions is created in air which constitutes sound waves. Thus, sound waves propagate as longitudinal waves.

(Q.168)  What do you understand by frequency of a sound wave?� What is the relationship between velocity of sound wave and its frequency? The wavelength of sound emitted by a source is 1.25 � 10–2 m. If the velocity of the sound is 342 m/s, what is the frequency of the sound?	(3Marks)

(Ans)

Frequency of sound wave is defined as the number of complete waves produced per second. The relationship between velocity and frequency of sound wave is given as,
Here is the wavelength of the sound wave, is the frequency of the sound wave and is the velocity of sound wave.

(Q.169)  Define time period of sound wave. What is its SI unit? How is it related with frequency?	(3Marks)

(Ans)

The time taken by sound wave to complete one oscillation in a medium is known as time period.

The SI unit of time period is second (s).

Time period is the reciprocal of frequency.

(Q.170)  Give five differences between light waves and sound waves.	(5Marks)

(Ans)

���������� S.No.	Light Waves	� Sound Waves
1.	These waves are produced from the electrons present in the excited state.	These waves are produced due to vibrations of different objects in a material medium.�
2.	These waves can be propagated through vacuum.	They can propagate through material medium (solids, liquids and gases) and cannot travel through vacuum.�
3.	They travel with the speed of 3 x 108 m/s in air.	They travel with the speed of 332 m/s at 20 oC.
4.	Their velocity does not change with change in �temperature, humidity, etc.	Their velocity changes with change in temperature, humidity, etc.
5.	These waves produce the sensation of vision.	These waves produce the sensation of hearing.

(Q.171)  Describe with the help of diagram to prove that material medium is necessary for the propagation of sound.	(5Marks)

(Ans)

Material medium is necessary for the propagation of sound

An electric circuit having a battery, a switch, and an electric bell is arranged inside a bell jar which is kept on an evacuated pump.

As the switch is brought into ON position, the sound is heard when there is an air within the bell jar. With the help of evacuated pump, the air is gradually removed out from the bell jar.�

The intensity of the sound decreases and no sound is heard when the air is completely removed. This is because air acts as a medium for the sound waves which is no longer present.

In order that sound should be heard from the vibrating body, some medium around that body is necessary so that sound can reach the ears. This medium is normally air but sound can travel through solids, liquids and other gases also.

Conclusions:

(i) Some medium is required around the vibrating body through which the sound can travel.

(ii) Medium can be solid, liquid and gases, but it cannot be vacuum.

(Q.172)  Priya used a vibrating tuning fork to produce 2,500 sound waves in 5 seconds. She found that the distance covered by a compression and an adjacent rarefaction was 68 cm.� Can you help her to find the (i) frequency,� (ii) wavelength and� (iii) velocity of the sound wave produced by this tuning fork?	(5Marks)

(Ans)

(i)Frequency(f):
It is the number of oscillations produced in one second.
No. of waves produced in 5 seconds = 2,500
So, the no. of waves produced in 1 second = 2,500/5= 500.

Hence, frequency of this sound wave is 500 Hz.

(iii) Velocity(v) of a sound wave is

(Q.173)  Rajan and Mohan are standing at opposite ends of an aluminium rod. Rajan strikes the end of the rod with a stone. Calculate the ratio of times taken by the sound waves in air and in aluminium to reach Mohan. [Velocity in aluminium = 6420m/s]	(5Marks)

(Ans)

Let the length of the rod be l.

Time taken by the rod to travel distance l in aluminium rod, t₁ = distance/(speed of sound in rod)

��������� =l/v_al

Time taken by sound to travel distance l in air –

t₂ = distance/(speed of sound in air)

��� = l/v_a

(Q.174)  Explain the statement ‘Wave transfers energy and not the matter’ with the help of an activity.	(5Marks)

(Ans) 1. Take a coin and throw it in a container filled with water.�
2. Now, place a wooden cork on the surface of the water.

Observation:
1. As the coin is thrown into the water container, ripples move outward on the surface of the water.
2. When the cork is placed on the surface of the water, it begins to move up and down, as the ripples reach the wave.

Conclusion:
1. Energy is transferred by the wave from one position to another.
2. Matter is not transported as the wave moves through the medium.

Explanation: When the still water is disturbed by the coin, then at the point of disturbance, its particles begin to vibrate about their mean positions. These particles set their neighbouring particles into vibration by imparting some of their energy to them. The neighbouring particles, further, set their neighbouring particles into vibration by transferring energy to them. The process continues. So, the energy gets transferred from one place to another. But, in this process, the cork vibrates at its place and so, we can say that matter does not get transferred in the process.