# Class-10 Goyal Brothers Sound ICSE Physics Solutions Ch-7

Class-10 Goyal Brothers Sound ICSE Physics Solutions Ch-7. We Provide Step by Step Answer of Exercise, MCQs, Numerical Problem Questions of Exercise-1 Goyal Brothers Sound ICSE Class-10 . Visit official Website CISCE  for detail information about ICSE Board Class-10 Physics.

## Sound Class-10 Goyal Brothers ICSE Physics Solutions Ch-7

-: Select Topics :-

Exercise-1

MCQ -1

Numerical Problem-1

Exercise-2

Exercise-3

### Exe-1 Class-10

Sound Goyal Brothers ICSE Physics Solutions Ch-7

Question 1.
(a) State the laws of reflection of sound.
(b) How will you verify laws of reflection of sound experimentally ?
(a) Laws of reflection of sound :

1. Angle of incidence is equal to the angle of reflection. ∠i = ∠l
2. Incident wave, reflected wave and the normal lie in the same plane.

(b) Verification of laws of reflection : Take a smooth polished large wooden board and mount it vertically on a table. At right angle to the board fix a wooden screen on each side of the screen place a long, narrow and highly polished tube from inside. Place a watch at the end A. Move the tube B slightly left or right till distinct tick of water is heard. Measure ∠PKN and
∠BKN.

It is found that ∠PKN = ∠BKN. This verifies the laws of reflection.

Question 2.
Describe any two applications of reflection of sound.
(i) Megaphone : People use horn shaped metal tubes commonly called megaphones while addressing a group of people in fairs or tourist spots. Sound energy is prevented from spreading out by successive reflections from the horn shaped tubes.
(ii) Hearing aid : Its shape is like a trumpet the narrow end is kept in the ear tube of the person who is hard of hearing. Where as the wider end towards the speaker collects the waves and reflects into the narrow end. This increases the intensity of sound energy hence the person who is hard of hearing can hear clearly.

Question 3.
(a) What is an echo ?
ECHO : “The repeated sound heard after reflection from a distant rigid obstacle (such as cliff, a hill side, wall of a building, edge of forest etc.) after the original sound has ceased is called an ECHO.”

(b)
State two conditions necessary for the formation of an echo.
Two conditions for forming an echo :

1. The minimum distance between the source of sound and the reflecting body should be 17 metres.
2. The intensity of sound should be sufficient so that it can be heard after reflection.

Question 4.
What are reverberation ? Give two examples.
Reverberation : “Due to repeated reflections at the reflecting surface (reflector is less than 17 metres from original sound) the sound gets prolonged, This effect is known as reverberation.”
Example :

1. Speaking in a large empty room.
2. Clapping in tombs like TajMahal.

Question 5.
How will you determine speed of sound by the method of echos ?
In order to determine the speed of sound in air, sound produced from a place at known distance d at least 50 m from the reflecting surface. The time interval t in which the echo reaches the place from where sound was produced is noted by a stop watch. Then speed of sound is calculated
as v = total distance travelled /time interval = 2d/ t ms-1

Question 6.
What is sonar ? State its principle. How is it used to find the depth of sea ?
SONAR : “Sound Navigation and Ranging.” It is based on the principle of ECHO.

ultra- sonic waves are sent in all directions from the ship and then are received on their return after reflection. Using the formula an obstacle such as enemy submarine, ice berg, a sunken ship etc. The time interval between sending and receiving of wave is noted. The distance (depth) can be calculated by vis speed of ultrasonic waves in water. v = 2d/t or d = Vt/2

Question 7.
How do bats locate their prey ? Explain in detail.
Location of prey by bats :
Bats can produce and detect the sound of very high frequency up to 10 kHz. The sound produced by bats get reflected back from an obstacle in front of it. These echoes tell the bat how they must turn in the air to avoid collision with obstacles. By using their ears, the bats, can fly skilfully at night in the utter darkness of caves. The highly sensitive nose of a bat acts as a recorder and picks up air vibrations set in motion by the movements of other animals. It appears that the nose and ears of the bats are important factors in the radar like operation.

Question 8.
How do the following use echoes ?

1. army,
2. geologists,
3. fishermen.

(i) army : Echoes are used by army to locate the gun position of enemy. Radar an instrument is used to locate an enemy air-craft ship.
(ii) geologists : Echoes are used by geologists for mineral prospecting.
(iii) fishermen : for locating fishes ultrasonic waves are sent into water. If these vibrations strike a fish, they are reflected back to the receivers. The time for hearing the echoes recorded. The position of fish is calculated by d = v × t/2 using vel. of sound in water as 1450 ms-1.

### Multiple Choice Questions-1

Sound Class-10 Goyal Brothers  ICSE Physics Solutions

Question 1.
The practical application based on the reflection of sound is:
(a) megaphone
(b) sounding board
(c) sonometer
(d) both (a) and (b)
(d) both (a) and (b)

Question 2.
Which is not the condition for the formation of echoes ?
(a) Minimum distance between the source of sound and reflecting body should be 17 m.
(b) The temperature of air should be above 20°C.
(c) The wavelength of sound should be less than the height of the reflecting body.
(d) The intensity of sound should be sufficient so that it could be heard after reflection.
(a) Minimum distance between the source of sound and reflecting body should be 17 m.

Question 3.
For hearing an echo, the minimum distance between the source of sound and reflecting body should be
(a) 12 m
(b) 24 m
(c) 17 m
(d) 51 m
(c) 17m

Question 4.
To locate its prey in the darkness the owl or the bat emits:
(a) infrasonic waves
(b) ultrasonic waves
(c) sonic waves
(d) infrared waves
(b) ultrasonic waves

### Numerical Problem on Echoes

Sound Class-10 Goyal Brothers  ICSE Physics Solutions

Practice Problems 1

Question 1.
A person fires a gun in front of a building 167 m away. If the speed of sound is 334 ms-1, calculate time in which he hears an echo.
d = 167 ∴ 2d = 167 × 2 m
Speed of sound = 334 ms-1
t for echo to be heard = ?

Question 2.
An echo is heard after 0.8 s, when a person fires a cracker, 132.8 m from a high building. Calculate the speed of sound.
t = 0.8 s d = 132.8 m

Question 3.
The speed of sound is 310 ms-1. A person fires a gun. An echo is heard after 1.5 s. Calculate the distance of person from the cliff from which echo is heard.
Speed of sound = 310 ms-1
time after which echo is heard t = 1.5 s distance from cliff d = ?
2d = Speed × t

Practice Problems 2

Question 1.
An echo is heard by a radar in 0.08 s. If velocity of radio waves is 3 × 108 ms-1, how far is the enemy plane ?
t = 0.08 s v = 3 × 108 ms-1

Question 2.
An enemy plane is at a distance of 300 km from a radar. In how much lime the radar will be able to detect the plane ? Take velocity of radiowaves as 3 × 108 ms-1.
d=300 t = ? v = 3 × 108 ms-1
t=2d/v = 2 × 300 × 1000m/3 ×108 ms-1
=0.002 s = 2 × 10-3 s

Practice Problems 3

Question 1.
A man stands in between two parallel cliffs and explodes a cracker. He hears the first echo after 0.6 s and second echo after 2.4 s. Calculate the distance between the cliffs. [Speed of sound is 336 ms-1]
Let the 2 buildings A and B situated at a distance d1 and d2 from a man’s point.

Question 2.
A man stands in between two parallel cliffs and explodes a cracker. He hears the first echo after 0.6 s and second echo after 2.4 s. Calculate the distance between the cliffs. [Speed of sound is 336 ms-1]
Let A and B be two cliffs at distance d1 and d2 metre from a man.

Practice Problems : 4

Question 1.
A man stands in between two cliffs, such that he is at a distance of 133.6 m from nearer cliff. He fires a gun and hears first echo after 0.8 s and second echo after 1.8 s. Calculate :

1. speed of sound
2. distance between two cliffs.

Question 2.
A person stands in between two parallel cliffs which are 99 m apart. He fires a gun and hears two successive echoes after 0.2 s and 0.4 s. Calculate :

1. the distance of the person from the nearer cliff
2. speed of sound.

Practice Problems : 5

Question 1.
A man stands in front of a vertical cliff and fires a gun. He hears an echo after 2.5 s. On moving 80 m closer to the cliff he again fires the gun and he hears an echo after 2 s. Calculate:
(a) distance of man from cliff to his initial position
(b) speed of sound.

Question 2.
A boy stands in front of a cliff, on the other side of a river. He fires a gun and hears an echo after 6 seconds. The boy then moves 170 m backwards and again fires the gun. He hears an echo after 7 seconds. Calculate :
(a) width of river
(b) speed of sound.

### Exe-2 Class-10

Sound Goyal Brothers ICSE Physics Solutions Ch-7

Question 1.
Define the following :

1. natural vibrations
2. forced vibrations
3. damped vibrations
4. natural frequency

(i) Natural vibrations : “The vibrations produced in a body, on being slightly disturbed from its mean position are caiied natural vibrations.”
(ii) Forced vibrations : “The vibrations which lake plat e under the influence of an external periodic force arc called forced vibrations.”
(iii) Damped vibrations : “The periodic vibrations of continuously decreasing amplitude in the presence of resistive force are called damped vibrations.”
(iv) Natural frequency : “The number of vibrations executed per second by a freely vibrating body is called natural frequency.”

Question 2.
State four characteristics of forced vibrations.
Characteristics of forced vibrations :

1. The body acquires the frequency of external periodic force.
2. The amplitude of forced vibration is very small, if the frequency of external force is much different from natural frequency of the body.
3. The amplitude of vibration remains constant with time, but magnitude depends upon the frequency of the driving force.
4. If the frequency of external force is exactly equal or is an integral multiple vibrating body, the amplitude of oscillation is very large.

Question 3.
Give two examples of forced vibrations.
Examples:

1. When a guitar is played, the artist forces the strings of the guitar to execute forced vibrations.
2. The vibrations produced in hollow sound box containing air are forced vibrations.

Question 4.
Why are the stringed instruments provided with large wind box?
So that on plucking the strings, forced vibrations of the air send greater energy and cause a loud sound.

Question 5.
What do you understand by the term resonance ? Give two conditions for producing resonance.
Resonance : “is a special case of forced vibrations. When the frequency of an externally applied periodic force on a body is equal to its natural frequency, the body readily begins to vibrate with an increased amplitude. This phenomenon is called resonance.”
Two conditions of resonance :

1. The natural frequency of the given body (non-vibrating body) must be equal to (or its integral multiple) the frequency of the vibrating body.
2. The vibrating body must have sufficient force, so as to. set the other body into vibrations.

Question 6.
Explain the following :
(a) Why does the frame of a motorbike vibrate violently at some particular speed ?
The vibrations caused by the movement of the piston, the frame of motorbike may have frequency (natural) be equal to the frequency of piston and due to resonance, it vibrates violently at that particular speed.

(b) Why does an odd piece of cutlery start vibrating violently when a note of some particular frequency is played ?
When note of some particular frequency is played its frequency may have matched the natural frequency of odd piece of cutlery and due to resonance the piece starts vibrating.

(c) Why are the soldiers instructed to march out of step while crossing a bridge ?
While crossing the bridge when soldiers march in steps, each soldier exerts a periodic force in same phase and therefore the bridge executes the forced vibrations of frequency equal to the frequency of their steps. Now the natural frequency of the bridge happens to be equal to the frequency of the steps, the bridge will vibrate with a large amplitude due to resonance and bridge may collapse. So, the soldiers are advised to break their steps while crossing the bridge.

### Multiple Choice Questions-2

Sound Class-10 Goyal Brothers  ICSE Physics Solutions

Question 1.
A string is stretched between two nails fixed in the opposite
walls and plucked from middle. The vibrations produced by the string are : –
(a) forced vibrations
(b) free vibrations
(c) damped vibrations
(d) resonant vibrations
(b) free vibrations

Question 2.
Water from a tap is allowed to fall in a vessel with a thin neck. The pitch of sound produced by falling water with the volume of water in the vessel.
(a) decreases
(b) increases
(c) remains same
(d) none of these
(b) increases

Question 3.
The amplitude of forced vibrations is generally than the amplitude of applied external force.
(a) more
(b) less
(c) equal to
(d) none of these
(b) less

Question 4.
A tuning fork has a frequency of 212 Hz. It will produce resonance in a wooden board of frequency
(a) 106 Hz
(b) 318 Hz
(c) 212 Hz
(d) 448 Hz
(c) 212 Hz

### Exe-3  Class-10

Sound Goyal Brothers ICSE Physics Solutions Ch-7

Question 1.
Define the following

1. musical sound,
2. noise

(i) Musical sounds : “Sound waves which produce pleasant sensation in our ears and are acceptable, are called musical sounds.”
(ii) Noise : “Sound waves which produce trouble-some sensation and are unacceptable are known as noise.

Question 2.
Give three differences between musical sound and noise.
Difference between musical sound and noise
Musical sound:

1. Produce pleasant effect on the ear
2. Proceeds at regular intervals in quick succession.

Noise:

1. Produce displeasing effect on ear. .
2. Proceed at irregular intervals.

Question 3.
State three characteristics of musical sound.
Three characteristics of musical sound :

1. Pitch
2. Loudness or intensity of sound
3. Quality or timbre.

Question 4.
(a) What do you understand by the term pitch of sound ?
(b) How is the pitch of sound related to the frequency of a vibrating body ?
(a) Pitch : “This characteristic enables us to differentiate between two sounds with equal loudness, coming from different sources and having different frequencies.” Sounds with equal loudness can be produced by pressing different keys of the harmonium, but they are easily distinguished due to different pitches.
(b) The higher the frequency of a note the higher is its pitch.

Question 5.
(a) What do you understand by the term loudness of sound ?
(b) How is the loudness of sound related to :

1. amplitude of the vibrating body,
2. distance of the observer from the vibrating body,
3. density of the medium producing sound,
4. frequency of sound.

(a) Loudness : “Is the time rate at which the sound energy flows through a unit area.” Different bodies of same frequency have different loudness due to different amplitude. More the amplitude louder it is.
(b) :

1. More the amplitude, louder it is.
2. Loudness or intensity of sound I ∝ 1/(Distance) Loudness decreases with increase in distance.
3. Loudness is directly proportional to density of medium.
4. Sound waves of the same Intensity but of different frequencies usually have different loudness.

Question 6.
(a) What do you understand by the term intensity of sound?
(b) Name the unit in which intensity of sound (loudness) is measured.
(c) What is the normal range of loudness ?
(d) What is the range of loudness when sound becomes painful?
(a) Intensity of sound or Loudness : “Is the time rate at which the sound energy flows through a unit area.” Different bodies of same frequency have different loudness due to different amplitude. More the amplitude louder it is.
(b) Unit of Intensity of loudness is (dB) decibels.
(c) Normal range of loudness is 50 (dB) to 80 (dB).
(d) above 80 dB it becomes painful.

Question 7.
List one source of noise in :
(a) transportation,
(b) homes,
(c) factories,
(d) surroundings.
Source of noise in
(a) Transportation : Petrol and diesel vehicles.
(b) Homes : Power music system, desert cooler.
(c) Factories : Running of machines, grinding.
(d) Surroundings : Loud speakers used in marriages and religious places.

Question 8.
List four harmful effects of sound pollution.
Harmful effects of sound pollution :

1. Noise produces headaches, irritatibility and nervous tension.
2. A long exposure to noise pollution may result in the loss of hearing to deafness.
3. Noise in the surroundings interferes with the conversation with another.
4. It causes anger, tension and interferer with the sleep pattern of individuals.

Question 9.
List four ways of reducing noise pollution.
Ways of reducing noise pollution :

1. Factories should be located far away from residential areas.
2. Heavy vehicles should not be allowed in residential areas.
3. At homes T.V. radio, power music system, should played at a low volume.
4. Machines should be designed in such a way, so that they produce minimum noise.

Question 10.
What do you understand by the term quality of sound ?
Quality of sound : “The notes of different instruments having
the same frequencies and same loudness are distinguished by this characteristics.” It is because different waveforms are produced by different musical instruments.

### Multiple Choice Questions-3

Sound Class-10 Goyal Brothers  ICSE Physics Solutions

Question 1.
The amplitude of a sound wave is increased from 1 mm to 2 mm. The loudness of the sound will:
(a) increase two time
(b) increase four times
(c) same
(d) decrease
(b) Increase four times ∵ IOC (amp)2

Question 2.
By decreasing the amplitude of a pure note its :
(a) speed decreases
(b) wavelength decreases
(c) quality changes
(d) loudness decreases
(d) loudness decreases

Question 3.
Two notes are produced from a flute and piano, such that they have same loudness and same pitch. The notes so produced differ in their :
(a) waveform
(b) wavelength
(c) frequency
(d) speed
(a) waveform

Question 4.    Class-10 Goyal Brothers Sound
The voice of women is shrill as compared to men because of the difference in their :
(a) speed
(b) loudness
(c) frequency
(d) all these
(c) frequency

Question 5.
The sound produced by two tuning forks A and B have same amplitude and same waveform, but the frequency of A is three times more than B. In such a case :
(a) quality of sound of A differs from B
(b) the note produced by A is shriller than B
(c) the note produced by B is shriller than A
(d) the note produced by A has more speed than B.