1. Fill in the blanks and explain.
a. Sound does not travel through Vacuum
Explanation: Sound requires a material medium to travel. As vacuum is not a material medium, sound cannot travel in vacuum.
b. The velocity of sound in steel is greater than the velocity of sound in water.
Explanation: Velocity of sound in solids is greater than liquid and which in turn is greater than gases. Therefore, velocity of sound in steel is greater than velocity of sound in water.
c. The incidence of lightning in daily life shows that the velocity of sound is less than the velocity of light.
Explanation: During thundering, we observe lighting first and the sound is heard later. This proves that velocity of light is greater than velocity of sound.
d. To discover a sunken ship or objects deep inside the sea, SONAR technology is used.
Explanation: SONAR is used to determine the direction, distance of an underwater object with the help of ultra sonic sound waves.
2. Explain giving scientific reasons.
a. The roof of a movie theatre and a conference hall is curved.
Ans. (1) 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. (2) The time between successive reflections of a particular soundwave reaching us becomes smaller and the reflected sounds get mixed up and produce a continuous sound of increased loudness which cannot be deciphered clearly.
(3) This is the reason why some auditoriums or some particular seats in an auditorium have inferior sound reception. (4) To avoid this, the roof of a movie theatre and conference hall is curved.
b. The intensity of reverberation is higher in a closed and empty house.
Ans. In a closed and empty house, apart from the floor, walls and the ceiling there is no other objects to absorb sound. Hence, the intensity of reverberation is higher.
c. We cannot hear the echo produced in a classroom.
Ans. (1) The dimensions of a classroom usually do not exceed 17.2m. (2) If the dimensions are greater than 17.2m , echo is produced. (3) Also many other objects in classroom that absorbs sounds are responsible to prevent echo of sound. So echo is not produced 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?
Ans. An echo is the repetition of the original sound because of reflection by some surface.
(1) At 22°C, the velocity of sound in air is 344 m/s.
(2) Our brain retains a sound for 0.1 s. Thus, for us to be able to hear a distinct echo, the sound should take more than 0.1 s after starting from the source to get reflected and come back to us.
(3) 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.
(4) 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.
Ans.
(1) In Golghumat at Vijaypur the reason for multiple echoes produced is due to multiple reflections of sound by the tomb.
(2) The sound produced gets reflected several times throughout the tomb which is responsible for echoes.
c. What should be the dimensions and the shape of classrooms so that no echo can be produced there?
Ans.
(1) A classroom should be rectangular in shape and 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) Sound absorbing material should be also used for soundproofing the room.
4. Where and why are sound absorbing materials used?
Ans.
(1) The sound absorbing materials are used on the roofs and walls of auditoriums, concert halls, theatres, school, cinema hall, etc. (2) They prevent reverberation of sound and also prevent echoes.
5. Solve the following examples.
a. The speed of sound in air at 0 °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?
b. Nita heard the sound of lightning after 4 seconds of seeing it. What was the distance of the lightning from her?
c. Sunil is standing between two walls. The wall closest to him is at a distance of 660 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?
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 many times as fast as the other?
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?