Science and Technology Class 9 Question Answers Chapter 12 Maharashtra Board

Study of Sound

1. Fill in the blanks and explain.

2. Explain giving scientific reasons.

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

Answer:

• Sound waves get reflected from the walls and roof of a room multiple times. This causes a single sound to be heard not once but continuously. This is called reverberation.
• Due to reverberation, some auditoriums or some particular seats in an auditorium have inferior sound reception. This can be compensated with curtains.
• Ceilings of these halls are made curved so that sound after reflecting from the ceiling, reaches all parts of the hall and the quality of sound improves.

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

Answer:

• Reverberation occurs due to multiple reflections of sound.
• The furniture in the house acts as a sound-absorbing material.
• So if the house is closed and empty, a reflection of sound will be maximum and hence, intensity of reverberation is higher.

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

Answer:

• For distinct echoes, the minimum distance of the reflecting surface from the source of sound must be 17.2 m.
• Benches in the classroom are sound absorbing materials which prevent echo of sound.
• Because of these two reasons echo is not heard in a classroom.

3. Answer the following questions in your own words.

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

Answer:

• An echo is the repetition of the original sound because of reflection by some surface.
• At 22°C, the velocity of sound in air is 344 m/s.
• Our brain retains a sound for 0.1 seconds Thus, for us to be able to hear a distinct echo, the sound should take more than 0.1 seconds after starting from the source to get reflected and. come back to us.
• We know that,
  Distance = speed x time
  = 344 m/s x 0.1 s
  = 34.4 m
• Thus, to be able to hear a distinct echo, the reflecting surface should be at a minimum distance of half of the above, i.e. 17.2 m.
• As the velocity of sound depends on the temperature of air, this distance depends on the temperature.

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

Answer:

• Goighumat with a height of 51 metres and diameter of 37 metres with 3 metres thick walls is spread over approximately 1700 square metres.
• This meets the conditions for echo i.e. : 17.2 metres minimum.
• The dome of the golghumat is curved and hence, sound reflects multiple times before reaching the observer.
• This is the reason for multiple echoes being produced.

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

Answer:

1. Dimensions: The distance between opposite walls in a classroom must be less than 17.2 m so that the reflected sound returns to the observer within 0.1 s.
2. Shape: The classrooms should have curved ceilings and walls so that the reflected sound is directed towards the observer instantly within 0.1 s

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

Answer:

The sound absorbing materials are used in :

• School, cinema hall, concert hall, houses or places where quality of sound is important.
• In the absence of sound absorbing material the sound will undergo multiple reflection causing reverberation of sound.

5. Solve the following examples.

a. The speed of sound in air at O °C 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?

Answer:
Given:
Initial speed of sound at 0°C 332 m/s.
Final speed of sound -344 m/s.
Rate of increase per degree rise in temp. = 0.6m/s
To find:
Temperature when speed is 344m/s
Formulae:
Increase in temperature

Temperature when the speed of sound is 344 m/sis 20°C

b. Nita heard the sound of lightning after 4 seconds of seeing it. What was the distance of the lightning from her? (The velocity of sound in air is 340 m/s?)

Answer:

Given : Speed of sound (v) = 340 m/s
Time taken (f) = 4 sec
To find : Distance (s) = ?

The lightning has struck at a distance of 1360 m from the observer.

c. Sunil is standing between two walls. The wall closest to him is at a distance of 360 m. If he shouts, he hears the first echo after 4 s and another after another 2 seconds.

1. What is the velocity of sound in air?

2. What is the distance between the two walls? (Ans: 330 m/s; 1650 m)

Answer:
Given:
Distance of the closer wall (S₁) = 660 m
Time of echo from closer wall = 4 sec
∴ Time taken (t₁) = 4/2 sec = 2 sec
Time of echo from distant wall = 6 sec
∴ Time taken (t₂) = 6/2 sec = 3 sec
To find :
Velocity of sound in air (y) =?
Distance between two walls (S₁ + S₂) = ?

The velocity of sound in air is 330 mIs and the distance between two walls is 1650 m.

d. Hydrogen gas is filled in two identical bottles, A and B, at the same temperature. The mass of hydrogen in the two bottles is 12 gm and 48 gm respectively. In which bottle will sound travel faster? How may times as fast as the other? (Ans: In A; Twice)

Answer:

In A; Thrice

e. Helium gas is filled in two identical bottles A and B. The mass of the gas in the two bottles is 10 gm and 40 gm respectively. If the speed of sound is the same in both bottles, what conclusions will you draw? (Ans: Temperature of B is 4 times the temperature of A.)

Answer :

Given:
Mass of Helium in bottle A = (mA) = 10gm
Mass of Helium in bottle B = (mB) = 40gm

The temperature of B is 4 times the temperature of A