12. Study of Sound | Science | Class 9

1. Fill in the blanks:

a. Sound does not travel through vacuum.

b. The velocity of sound in steel is greater than the velocity of sound in water.

c. The incidence of thunderstorm in daily life shows that the velocity of sound is less than the velocity of light.

d. To discover a sunken ship or objects deep inside the sea, SONAR technology is used.

2. Explain giving scientific reasons.

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

This is because the sound gets reflected uniformly from the ceiling and reaches uniformly throughout the theatre for the audience to have a better sound experience.

b. The intensity of reverberation is higher in a closed and empty house,

This is because, as there are no objects to absorb the sound in an empty house and if closed faces multiple reflections and has no scope to escape. The intensity of sound thereby has no scope to reduce as would be the case in case of a non-empty room. 

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

The classrooms are scientifically designed so that the distance between two walls is always less than 17.2 m. Because of such a design of classroom the reflection of sound reaches our ears before 0.1 s and hence we are not able to distinguish between the original sound and the echo sound.

3. Answer the following questions in your own words.

a. What is an echo? What factors are important to get a distinct echo?

Ans. An echo is the repetition of the original sound because of reflection by some surface.

The factors important to get a distinct echo are:

  • The minimum distance between the source and reflecting surface should be 17.2 m.
  • The size of reflector must be larger enough to the wavelength of sound wave.
  • The intensity of sound at the source should be such that the reflected sound reaching the ear is sufficiently intense to be audible.

b. Study the construction of the Golghumat at Vijapur and discuss the reasons for the multiple echoes produced there.

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Image source: Wikipedia

Ans. The Golghumbat at Vijapur 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 one can hear echo at least 7 to 10 times. Thus, the central dome is the important reason for the production of multiple echoes in Golghumat at Vijapur.

c. What should be the dimensions and the shape of classrooms so that no echo can be produced there?
Ans. (i) 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.

(ii) Also, the ceiling should be curved shaped so that the reflection of sound wave is uniform throughout the room.

4. Where and why are sound absorbing materials used?

Ans. (i) The sound absorbing materials are used on the roofs and walls of auditoriums, concert halls, theatres.

(ii) As the excessive reverberation in such places is highly undesirable. Due to the presence of such sound absorbing materials, the sound reflected from the rigid surfaces is absorbed by these materials and hence reverberation is avoided.

5. Solve the following examples.
a. The speed of sound in air at 0 degree centigrade is 332 m/s. If it increases at the rate of 0.6 m/s per degree, what will be the temperature when the velocity has increased to 344 m/s?

Ans.  Given data: Velocity of sound =  332 m/s at 0 degree centigrade

The increase rate of velocity  = 0.6 m/s per ℃

Solution: 

We know the relation between velocity and temperature: v

Speed at x℃ = (332 + 0.6x)℃

Thus 344 = 332 + 0.6x
344 – 332 = 0.6x
12 = 0.6x
x = 12/ 0.6 = 20℃

Temperature is 20℃

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