1. Fill in the blanks and explain.

a. Sound does not travel through ……vacuum ………………….…………

Answer. Sound waves are a type of wave known as longitudinal waves. These waves need a medium, like air, water, or solids, to travel through. They can’t travel through empty space, like a vacuum.

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

Because steel is denser than water, sound travels faster through steel than through water.

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

During thunderstorms, we often notice that we see lightning before we hear the accompanying thunder. This observation suggests that light travels faster than sound waves.

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

With this technology, we can pinpoint the locations of animals or objects deep underwater.

2. Explain giving scientific reasons.

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

• Sound waves undergo multiple reflections off the walls and ceiling within a room, leading to a prolonged auditory perception known as reverberation.
• This phenomenon results in continuous sound rather than a single instance. As a consequence, certain areas within auditoriums may experience subpar sound reception, remedied by the strategic use of curtains.
• The curved design of ceilings in these venues facilitates the dispersion of reflected sound waves, ensuring uniform auditory coverage throughout the hall and enhancing sound quality

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

Answer. Reverberation arises from the numerous sound reflections within an environment.

Household furniture serves as an effective sound-absorbent material, mitigating reverberation effects. Consequently, in an unoccupied and enclosed house, sound reflections reach their peak, leading to heightened reverberation intensity

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

• To produce clear echoes, the reflecting surface must maintain a minimum distance of 17.2 meters from the sound source.
• Benches placed in classrooms act as effective sound-absorbing materials, effectively dampening sound echoes.
• As a result of these dual factors, the occurrence of echoes is nullified within a classroom environment.

3. Answer the following questions in your own words.

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

• An echo results from sound waves reflecting off surfaces, causing the original sound to repeat.
• At a temperature of 22°C, the speed of sound in air measures 344 meters per second.
• Our brain retains sound for approximately 0.1 seconds, necessitating a delay of over 0.1 seconds for a distinct echo to be audible. Utilizing the equation Distance = speed x time, we calculate the minimum distance for echo reception as 34.4 meters.
• Consequently, the reflecting surface should be at least half this distance, or 17.2 meters, for an echo to be discernible. Since sound velocity varies with air temperature, this requisite distance is temperature-dependent.

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

• The Golghumat, towering at 51 meters in height and 37 meters in diameter, features robust 3-meter-thick walls, encompassing an expansive area of approximately 1700 square meters.
• Satisfying the echo criteria of a minimum distance of 17.2 meters, the Golghumat’s vast dimensions allow for distinct echoes.
• With its curved dome structure, sound undergoes numerous reflections before reaching an observer, resulting in the production of multiple echoes.

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

Dimensions: To ensure the reflected sound reaches the observer within 0.1 seconds, the distance between opposing walls in a classroom should be under 17.2 meters.

Shape: Incorporating curved ceilings and walls in classrooms facilitates the instantaneous redirection of reflected sound toward the observer within 0.1 seconds.

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

Answer. Sound-absorbing materials are used in various places such as schools, cinema halls, concert halls, residences, and venues where sound quality is very important. Without these materials, sound experiences multiple reflections, resulting in reverberation.

 Q5 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?

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?)

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

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?

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?