Exercise Class 9 Science Chapter 12 Study of Sound

 

Exercise Class 9 Science Chapter 12 Study of Sound

Q.1 Fill in the blanks and explain.

A. Sound does not travel through Vacuum. 

B. The velocity of sound in still is Greater than the velocity of sound in water.

C. The incident of lighting in daily life shows that the velocity of sound is less than the velocity of light.

D. To discover a sunken sheep or objects dip inside the sea SONAR technology is used.

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Q.2 Explain giving scientific reason.

A. The roof of a movie theatre and a conference hall is curved.

ANSWER :  

1. The reflection of sound curved surface occurs in the same ways as the light.
2. When a source of sound is kept at the focus of concave reflector, the sound wave on reflection travel parallel to the principal axis.
3. To avoid undesirable reverberation,  a conference hall and roof of a movie theatre is curved.

B. The intensity of reverberation is higher in closed and empty house.

ANSWER : 

1. In a closed and empty house, there is a absorption of sound only by the floor , walls and ceilings
2. There is no other sound absorbing materials.
3. Hence, The intensity of reverberation is higher in closed and empty house.

C. We cannot hear the echo produced in a classroom.

ANSWER : 

The classroom designed to be less than 17.2 m from the wall. Therefore, the sound appears on the wall or reaches our ears by echoing before 0.1 second. Therefore, we cannot distinguish between the original sound and the noise in the classroom.

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 Q.3 Answer the following questions in your own words.   

1. What is an echo? What factors important to get a distinct echo?
2. Study the construction of the Golghumat at Vijapur and discuss the reasons for the multiple echoes produced there.
3. What should be the dimensions and the shape of classrooms so that no echo can be produced there?

ANSWER :

1. The repetition of sound caused by its reflection off a hard surface is known as echo. Factors important to get a distinct echo are:

• The minimum distance between the source and reflector of sound should be 17 m.
• The size of reflector must be large enough as compared to the wavelength of the sound wave.

2.  Golghumbat has a central dome which stands without any support. The sound produced here gets reflected throughout the dome producing the echo at least three to four times in a second. If the surroundings inside the architecture are very quiet, then we would be able to hear echo at least seven to ten times. So, this central dome is the main reason for the production of multiple echoes in Golghumat.

3. For no echo to be produced in a classroom, the classroom should be square shaped with distance between the walls to be less than 17.2 m. Also, the ceiling should be curved shaped so that the reflection of sound wave is uniform throughout the room.

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Q.4 Where and why are sound absorbing materials used?     

ANSWER :

Field, concert, theater etc. The roofs and walls of the spaces are made of acoustic materials, and the interiors of these rooms are also made of acoustic materials. More reverberation in these places is highly undesirable. Thanks to the acoustic material, the sound reflected from the narrow area is absorbed by this material and thus the echoing is suppressed