Selina Textbook Solutions: Sound | Physics Class 8 ICSE PDF Download

 Page 1


7. Sound 
? SOUND: “Is energy which produces in us the sensation of hearing.” It is produced 
by vibration of body. 
? Sound needs a medium for its propagation. Sound cannot travel in vacuum. 
? Speed of sound is maximum in solids. 5000 ms
-1
 in steel, in water 1500 ms
-1
 and in 
air it is least 330 ms
-1
 nearly. 
? When a body vibrates, the particles of medium also start vibrating and K.E. of 
particles changes into potential energy and P.E. into 
K.E. This is why sound in energy. 
? Sound travels in a medium in the form of wave. 
 
? Longitudinal wave : When the particles of medium move in the direction of motion 
of wave by forming compression and rarefaction. 
 
? AMPLITUDE : “The maximum displacement of the particle of medium on either 
side of mean position.” 
 
? TIME PERIOD: “The time taken by a particle of medium to complete its one 
vibration” “t” 
?  FREQUENCY: “The number of vibrations made by a particle of 
the medium in one second. ƒ measured in Hertz (Hz) 
?  FREQUENCY  ƒ = 1/ t or t = 1 / ƒ 
?  WAVE LENGTH: “The distance travelled by the wave in one one time period of 
vibration of particle of medium.” 
Or 
“The distance between two consecutive compressions or between two 
consecutive rarefactions.” It is denoted by ‘ ? ’ and S.I. unit of wave length is metre 
Page 2


7. Sound 
? SOUND: “Is energy which produces in us the sensation of hearing.” It is produced 
by vibration of body. 
? Sound needs a medium for its propagation. Sound cannot travel in vacuum. 
? Speed of sound is maximum in solids. 5000 ms
-1
 in steel, in water 1500 ms
-1
 and in 
air it is least 330 ms
-1
 nearly. 
? When a body vibrates, the particles of medium also start vibrating and K.E. of 
particles changes into potential energy and P.E. into 
K.E. This is why sound in energy. 
? Sound travels in a medium in the form of wave. 
 
? Longitudinal wave : When the particles of medium move in the direction of motion 
of wave by forming compression and rarefaction. 
 
? AMPLITUDE : “The maximum displacement of the particle of medium on either 
side of mean position.” 
 
? TIME PERIOD: “The time taken by a particle of medium to complete its one 
vibration” “t” 
?  FREQUENCY: “The number of vibrations made by a particle of 
the medium in one second. ƒ measured in Hertz (Hz) 
?  FREQUENCY  ƒ = 1/ t or t = 1 / ƒ 
?  WAVE LENGTH: “The distance travelled by the wave in one one time period of 
vibration of particle of medium.” 
Or 
“The distance between two consecutive compressions or between two 
consecutive rarefactions.” It is denoted by ‘ ? ’ and S.I. unit of wave length is metre 
(m). 
 
?  CHARACTERISTICS OF SOUND : 
(i) Loudness. 
(ii) Pitch (or shrillness). 
(iii) Quality (or timbre or wave form). 
LOUDNESS : is the characteristic of sound by virtue of which a loud sound can be 
distinguished from a faint sound, both having same frequency and same wave 
form. 
?  It depend on: (i) Amplitude of wave (ii) Surface area of vibrating body (ii) Distance 
from the source of sound (iv) Sensitivity of listener: Unit of loudness is (dB) 
decibel. 
?  PITCH: It depends on number of vibrations per second or frequency : more 
frequency is high pitch shrilled sound and low frequency is flat sound. 
?  QUALITY: is the characteristic which distinguishes two sounds’of the same pitch 
and same loudness. It depends on wave form. 
Test yourself 
A. Objective Questions 
1. Write true or false for each statement 
(a) When sound propagates in air, it does not carry energy with it. 
Answer. False. 
(b) In a longitudinal wave, compression and rarefaction are formed. 
Answer. True. 
(c) The distance from one compression to nearest rarefaction is called wavelength. 
Answer. False. 
(d) The frequency is measured in second. 
Answer. False. 
(e) The quality of a sound depends on the amplitude of wave. 
Answer. False. 
Page 3


7. Sound 
? SOUND: “Is energy which produces in us the sensation of hearing.” It is produced 
by vibration of body. 
? Sound needs a medium for its propagation. Sound cannot travel in vacuum. 
? Speed of sound is maximum in solids. 5000 ms
-1
 in steel, in water 1500 ms
-1
 and in 
air it is least 330 ms
-1
 nearly. 
? When a body vibrates, the particles of medium also start vibrating and K.E. of 
particles changes into potential energy and P.E. into 
K.E. This is why sound in energy. 
? Sound travels in a medium in the form of wave. 
 
? Longitudinal wave : When the particles of medium move in the direction of motion 
of wave by forming compression and rarefaction. 
 
? AMPLITUDE : “The maximum displacement of the particle of medium on either 
side of mean position.” 
 
? TIME PERIOD: “The time taken by a particle of medium to complete its one 
vibration” “t” 
?  FREQUENCY: “The number of vibrations made by a particle of 
the medium in one second. ƒ measured in Hertz (Hz) 
?  FREQUENCY  ƒ = 1/ t or t = 1 / ƒ 
?  WAVE LENGTH: “The distance travelled by the wave in one one time period of 
vibration of particle of medium.” 
Or 
“The distance between two consecutive compressions or between two 
consecutive rarefactions.” It is denoted by ‘ ? ’ and S.I. unit of wave length is metre 
(m). 
 
?  CHARACTERISTICS OF SOUND : 
(i) Loudness. 
(ii) Pitch (or shrillness). 
(iii) Quality (or timbre or wave form). 
LOUDNESS : is the characteristic of sound by virtue of which a loud sound can be 
distinguished from a faint sound, both having same frequency and same wave 
form. 
?  It depend on: (i) Amplitude of wave (ii) Surface area of vibrating body (ii) Distance 
from the source of sound (iv) Sensitivity of listener: Unit of loudness is (dB) 
decibel. 
?  PITCH: It depends on number of vibrations per second or frequency : more 
frequency is high pitch shrilled sound and low frequency is flat sound. 
?  QUALITY: is the characteristic which distinguishes two sounds’of the same pitch 
and same loudness. It depends on wave form. 
Test yourself 
A. Objective Questions 
1. Write true or false for each statement 
(a) When sound propagates in air, it does not carry energy with it. 
Answer. False. 
(b) In a longitudinal wave, compression and rarefaction are formed. 
Answer. True. 
(c) The distance from one compression to nearest rarefaction is called wavelength. 
Answer. False. 
(d) The frequency is measured in second. 
Answer. False. 
(e) The quality of a sound depends on the amplitude of wave. 
Answer. False. 
(f) The pitch of sound depends on frequency. 
Answer. True. 
(g) Decibel is the unit of pitch of a sound. 
Answer. False. 
2. Fill in the blanks 
(a) The time period of a wave is 2 s. Its frequency is 0.5 S
-1
. 
(b) The pitch of a stringed instrument is increased by increasing tension in string. 
(c) The pitch of a flute is decreased by increasing length of air column. 
(d) Smaller the membrane, higher is the pitch. 
(e) If a drum is beaten hard, its loudness increases. 
(f) A tuning fork produces sound of single frequency. 
 
3. Match the following 
 
4. Select the correct alternative 
Page 4


7. Sound 
? SOUND: “Is energy which produces in us the sensation of hearing.” It is produced 
by vibration of body. 
? Sound needs a medium for its propagation. Sound cannot travel in vacuum. 
? Speed of sound is maximum in solids. 5000 ms
-1
 in steel, in water 1500 ms
-1
 and in 
air it is least 330 ms
-1
 nearly. 
? When a body vibrates, the particles of medium also start vibrating and K.E. of 
particles changes into potential energy and P.E. into 
K.E. This is why sound in energy. 
? Sound travels in a medium in the form of wave. 
 
? Longitudinal wave : When the particles of medium move in the direction of motion 
of wave by forming compression and rarefaction. 
 
? AMPLITUDE : “The maximum displacement of the particle of medium on either 
side of mean position.” 
 
? TIME PERIOD: “The time taken by a particle of medium to complete its one 
vibration” “t” 
?  FREQUENCY: “The number of vibrations made by a particle of 
the medium in one second. ƒ measured in Hertz (Hz) 
?  FREQUENCY  ƒ = 1/ t or t = 1 / ƒ 
?  WAVE LENGTH: “The distance travelled by the wave in one one time period of 
vibration of particle of medium.” 
Or 
“The distance between two consecutive compressions or between two 
consecutive rarefactions.” It is denoted by ‘ ? ’ and S.I. unit of wave length is metre 
(m). 
 
?  CHARACTERISTICS OF SOUND : 
(i) Loudness. 
(ii) Pitch (or shrillness). 
(iii) Quality (or timbre or wave form). 
LOUDNESS : is the characteristic of sound by virtue of which a loud sound can be 
distinguished from a faint sound, both having same frequency and same wave 
form. 
?  It depend on: (i) Amplitude of wave (ii) Surface area of vibrating body (ii) Distance 
from the source of sound (iv) Sensitivity of listener: Unit of loudness is (dB) 
decibel. 
?  PITCH: It depends on number of vibrations per second or frequency : more 
frequency is high pitch shrilled sound and low frequency is flat sound. 
?  QUALITY: is the characteristic which distinguishes two sounds’of the same pitch 
and same loudness. It depends on wave form. 
Test yourself 
A. Objective Questions 
1. Write true or false for each statement 
(a) When sound propagates in air, it does not carry energy with it. 
Answer. False. 
(b) In a longitudinal wave, compression and rarefaction are formed. 
Answer. True. 
(c) The distance from one compression to nearest rarefaction is called wavelength. 
Answer. False. 
(d) The frequency is measured in second. 
Answer. False. 
(e) The quality of a sound depends on the amplitude of wave. 
Answer. False. 
(f) The pitch of sound depends on frequency. 
Answer. True. 
(g) Decibel is the unit of pitch of a sound. 
Answer. False. 
2. Fill in the blanks 
(a) The time period of a wave is 2 s. Its frequency is 0.5 S
-1
. 
(b) The pitch of a stringed instrument is increased by increasing tension in string. 
(c) The pitch of a flute is decreased by increasing length of air column. 
(d) Smaller the membrane, higher is the pitch. 
(e) If a drum is beaten hard, its loudness increases. 
(f) A tuning fork produces sound of single frequency. 
 
3. Match the following 
 
4. Select the correct alternative 
(a) Sound can not travel in 
1.  solid 
2.  liquid 
3.  gas 
4.  vacuum 
(b) When sound travels in form of a wave 
1. the particles of medium move from the source to the listener 
2. the particles of medium remains stationary 
3. the particles of medium start vibrating up and down 
4. the particles of medium transfer energy without leaving their mean 
positions. 
(c) The safe limit of loudness of audible sound is 
1.  0 to 80 dB 
2.  above 80 dB 
3.  120 dB 
4.  above 120 dB 
(d) The unit of loudness is 
1.  cm 
2.  second 
3.  hertz 
4.  decibel 
(e) In a piano, pitch is decreased by 
1.  using thicker string 
2.  increasing tension 
3.  reducing length of string 
4.  striking it hard Ans. 
B. Short/Long Answer Questions 
Question 1. 
How does sound travel in air ? 
Answer: 
A periodic disturbance in the medium (Air) is created by the vibration of sound and the 
particles of the medium vibrate about their mean position and transfer of energy in the 
form of sound waves takes place, i.e. in the LONGITUDINAL WAVES. 
Page 5


7. Sound 
? SOUND: “Is energy which produces in us the sensation of hearing.” It is produced 
by vibration of body. 
? Sound needs a medium for its propagation. Sound cannot travel in vacuum. 
? Speed of sound is maximum in solids. 5000 ms
-1
 in steel, in water 1500 ms
-1
 and in 
air it is least 330 ms
-1
 nearly. 
? When a body vibrates, the particles of medium also start vibrating and K.E. of 
particles changes into potential energy and P.E. into 
K.E. This is why sound in energy. 
? Sound travels in a medium in the form of wave. 
 
? Longitudinal wave : When the particles of medium move in the direction of motion 
of wave by forming compression and rarefaction. 
 
? AMPLITUDE : “The maximum displacement of the particle of medium on either 
side of mean position.” 
 
? TIME PERIOD: “The time taken by a particle of medium to complete its one 
vibration” “t” 
?  FREQUENCY: “The number of vibrations made by a particle of 
the medium in one second. ƒ measured in Hertz (Hz) 
?  FREQUENCY  ƒ = 1/ t or t = 1 / ƒ 
?  WAVE LENGTH: “The distance travelled by the wave in one one time period of 
vibration of particle of medium.” 
Or 
“The distance between two consecutive compressions or between two 
consecutive rarefactions.” It is denoted by ‘ ? ’ and S.I. unit of wave length is metre 
(m). 
 
?  CHARACTERISTICS OF SOUND : 
(i) Loudness. 
(ii) Pitch (or shrillness). 
(iii) Quality (or timbre or wave form). 
LOUDNESS : is the characteristic of sound by virtue of which a loud sound can be 
distinguished from a faint sound, both having same frequency and same wave 
form. 
?  It depend on: (i) Amplitude of wave (ii) Surface area of vibrating body (ii) Distance 
from the source of sound (iv) Sensitivity of listener: Unit of loudness is (dB) 
decibel. 
?  PITCH: It depends on number of vibrations per second or frequency : more 
frequency is high pitch shrilled sound and low frequency is flat sound. 
?  QUALITY: is the characteristic which distinguishes two sounds’of the same pitch 
and same loudness. It depends on wave form. 
Test yourself 
A. Objective Questions 
1. Write true or false for each statement 
(a) When sound propagates in air, it does not carry energy with it. 
Answer. False. 
(b) In a longitudinal wave, compression and rarefaction are formed. 
Answer. True. 
(c) The distance from one compression to nearest rarefaction is called wavelength. 
Answer. False. 
(d) The frequency is measured in second. 
Answer. False. 
(e) The quality of a sound depends on the amplitude of wave. 
Answer. False. 
(f) The pitch of sound depends on frequency. 
Answer. True. 
(g) Decibel is the unit of pitch of a sound. 
Answer. False. 
2. Fill in the blanks 
(a) The time period of a wave is 2 s. Its frequency is 0.5 S
-1
. 
(b) The pitch of a stringed instrument is increased by increasing tension in string. 
(c) The pitch of a flute is decreased by increasing length of air column. 
(d) Smaller the membrane, higher is the pitch. 
(e) If a drum is beaten hard, its loudness increases. 
(f) A tuning fork produces sound of single frequency. 
 
3. Match the following 
 
4. Select the correct alternative 
(a) Sound can not travel in 
1.  solid 
2.  liquid 
3.  gas 
4.  vacuum 
(b) When sound travels in form of a wave 
1. the particles of medium move from the source to the listener 
2. the particles of medium remains stationary 
3. the particles of medium start vibrating up and down 
4. the particles of medium transfer energy without leaving their mean 
positions. 
(c) The safe limit of loudness of audible sound is 
1.  0 to 80 dB 
2.  above 80 dB 
3.  120 dB 
4.  above 120 dB 
(d) The unit of loudness is 
1.  cm 
2.  second 
3.  hertz 
4.  decibel 
(e) In a piano, pitch is decreased by 
1.  using thicker string 
2.  increasing tension 
3.  reducing length of string 
4.  striking it hard Ans. 
B. Short/Long Answer Questions 
Question 1. 
How does sound travel in air ? 
Answer: 
A periodic disturbance in the medium (Air) is created by the vibration of sound and the 
particles of the medium vibrate about their mean position and transfer of energy in the 
form of sound waves takes place, i.e. in the LONGITUDINAL WAVES. 
Question 2. 
What is longitudinal wave ? 
Answer: 
Longitudinal wave: 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. Such 
a wave can be produced in solids, liquids as well as gases. 
Question 3. 
Explain the mechanism of formation of a longitudinal wave 
when source vibrates in air. 
Answer: 
PROPAGATION OF SOUND IN AIR : When a source of sound vibrates, it creates a 
periodic disturbance in the medium near it (i.e., the condition of medium changes). The 
disturbance then travels in the medium in form of waves. This can be understood by the 
following example. 
Example: Take a vertical metal strip with its lower end fixed. Push its upper end to one 
side and then release it. As it vibrates, i.e. moves alternately to the right and left, sound 
is produced. Figure shows the steady (or mean position) of the metal strip and normal 
condition of air layers near the strip. 
 
As the strip moves to the right from a to b in Figure it pushes the particles of air layer in 
front of it. So the particles of air in this layer come closer to each other i. e., air in that 
layer gets compressed (or compression is formed at C). The particles of this layer while 
moving towards right, pushes and compresses the layer next to it, which then 
compresses the next layer and so on. Thus the disturbance moves forward in form of