# Frank Solutions for Chapter 7 Sound- Production and Propagation Class 9 Physics ICSE

1. Is sound a form of energy?

Yes, Sound is form of energy.

2. How is sound produced?

Sound is produced by the vibrating objects.

3. Can sound travel through vacuum ?

No, sound cannot travel through vacuum.

4. Can sound propagate in solids and liquids?

Yes, sound can propagate through solids and liquids.

5. Can we produce sound in vacuum?

No, sound cannot be produced in vacuum. It needs a material medium.

6. State the main factors on which the speed of sound in a medium depends.

Speed of sound depends on the density, pressure, temperature, humidity and Wind.

7. What is the approximate value of the speed of sound in air at N.T.P.?

330 m/s is the value of speed in air at N.T.P

8. Compare approximately the speed of sound in air, water and iron.

In water, speed of sound is 4.5 times more than that in air. And, in iron, the speed of sound is 15 times as fast in air.

9. In which medium is the speed of sound more: humid air or dry air? Give a reason to your answer.

The speed of sound is more in the humid air as compared to dry air. Because, the air moisture decreases the density of air and speed of sound is inversely proportional to density of air. So, sound travels faster in humid air.

10. How does the speed of sound depend on (i) Amplitude, and (ii) Wavelength?

Speed of sound does not depend on the amplitude and wavelength.

11. On placing your ear close to an iron railing which is tapped some distance away, sound is heard twice. Explain, why?

The sound produced by the moving wheels of the train travels faster on the iron track than in the air. So, firstly Sound produced because of railway track is heard and then heard in the air.

12. What causes sound in air?

The waves in the air causes sound. Through waves, energy get transferred from one place to another and hence the sound.

13. What is the ratio of speed of light to speed of sound?

The ratio of speed of light to the speed of sound is 9.1 x 105.

14. State the effect of pressure on the speed of sound in air.

Speed of sound in air is independent of variation of pressure.

15. How does the speed of sound in air vary with the temperature?

The speed of sound in air increases with increase in temperature because with increase in temperature, the density of air gases decreases and speed of air is inversely proportional to density of air.

16. The sound of an explosion on the surface of a lake is heard by a boatman 120 m away and by a diver 120 m below the point of explosion. Out of the two persons, who would hear the sound first? Why?

The diver will hear the sound first because the sound travels faster in water than in air.

17. Describe a simple experiment to determine the speed of sound in air.

The simple experiment that a person can do to calculate the speed of sound in air is that a person stands at a known distance (d meter) from the cliff and fires a pistol and simultaneously start the stop watch. He stops the stop-watch as soon as he hears a echo. The distance travelled by the sound during the time (t) seconds is 2d.

So, speed of sound = distance travelled/time taken = 2d/t

18. How do the following factors affect the speed of sound in air?

(i) Moisture in air,

(ii) Pressure in air,

(iii) Temperature of air?

(i) Moisture in Air : The speed of sound increase with increase in humidity because the moisture in air decrease the density of air and speed of air is inversely proportional to density of air. Hence, the sound travels faster in moist air.

(ii) Pressure in air: Speed of sound is independent of the air pressure.

(iii) The speed of sound increases with the increase in temperature of the gas.

19. The time interval between a lighting flash and the first sound of thunder was found to be 6 s. If the speed of sound in air is 330 ms-1, find the distance of the flash from the observer.

The distance of flash from the observer = speed in air x time taken for the first sound to reach

330×6 = 1980 m

20. A person fires a gun and other boy at a distance hears the sound of the fire 2.5 s after seeing the flash. If the speed of sound in air is 330 m-1, find the distance between the boys.