# Frank Solutions for Chapter 7 Sound Class 9 Physics ICSE

1. Can you hear each other in a dark room ?

Yes, we can hear in the dark room.

2. What causes sound?

Sound is produced by the vibrating objects.

3. Is sound a form of energy?

Yes, sound is a form of energy.

4. Can sound travel through vacuum?

No, sound cannot travel through vacuum. It needs a material medium to travel.

5. Does sound require a material medium for transmission?

Yes, sound requires a material medium for transmission.

6. Name the factors on which the speed of sound depends.

The speed of sound depends on the following factors:

1. Density of air
2. Temperature
3. Humidity in air as well the wind.

7. What is the effect of wavelength and amplitude on the speed of sound?

There is no effect on the wavelength and amplitude on the speed of sound.

8. What is the value of sound of room temperature?

Speed of sound at room temperature is 330 m/s

9. Is the speed of sound different in different media ?

Yes, the speed of sound is different in different medium.

10. Does the speed of sound in a gas depend on the pressure of the gas?

No, the speed of sound in the gas is independent on the change in pressure of the gas.

11. Write the formula for speed of sound in a gas using Laplace correction.

Using Laplace correction, the speed of sound in the gas is given by:

V = square root of (Using Laplace correction, the speed of sound in the gas is given by:

V = square root of (yP/d) where y is ratio of specific heat of gas at constant pressure to that at constant volume.

12. Does the speed of sound depend upon the direction of wind?

Yes, the speed of sound depends on the direction of wind. When wind is blowing in the direction of propagation of sound, the speed of sound increases.

But, when wind blows in the direction opposite to that of sound, the speed of sound decreases.

13. What are ultrasonic ?

Ultrasonic are sounds having frequency higher that 20 KHz and not audible to the human being.

14. What does SONAR stand for?

SONAR stands for the "sound navigation and ranging".

15. Which has the higher frequency: ultrasonic sound or infrasonic sound?

Ultrasonic sound has higher frequency than infrasonic sound.

16. Can you hear the sound produced due to the vibrations of second's pendulum?

No, we cannot hear the sound produced due to the vibration of second's pendulum.

17. Does SONAR make use of infrasonic sound?

No, SONAR does not make use of infrasonic waves. Instead, it make use of ultrasonic waves.

18. Does ultrasonic in a medium travel faster than the audible sound ?

Yes, ultrasound in the medium travels faster than the audible sound.

19. Is it possible to hear your friend's voice in vacuum? Give reason for your answer.

No, it is not possible to hear the friend's voice in vacuum because sound needs a material medium to travel.

20. What are the requisites of the medium for the propagation of sound? Briefly, explain them.

The following are the requisites of medium to travel:

1. The medium should possess elasticity so that particles of medium have tendency to come back to their original position after the displacement.
2. The medium should be frictionless so that there should be loss of energy.
3. The medium must possess inertia so that particles of medium have ability to store energy.

21. What cause sound? Why do we require a material medium for the transmission of sound?

Sound is produced by the vibrating objects. We require a material medium for the propagation of sound because vibrations of object need to get transferred or transmitted from one place to another.

22. What is the effect of temperature and pressure on the speed of sound in a gas?

With increase in temperature of gas, there is increase in speed of gas because increase in temperature decreases the density of gas. And speed of sound is inversely proportional to density of gas. The speed of gas is independent of the pressure of gas.

23. (a) What is the effect of wavelength and amplitude on the speed of sound?

(b) Explain the effect of density on the speed of sound.

(a) No, there is no effect of wavelength and amplitude on the speed of sound.

(b) With increase in density, there is decrease in the speed of sound in gas.

24. What is sound? Explain with example how does sound travel in a medium.

Sound is defined as form of energy that produces the sensation of hearing in our ears. The sound travels in medium through energy transfers by wave motion from one place to another. For example , when the prong of the tuning fork vibrate , the waves start moving from one fork to another , the air start getting compressed . The compression is pushed forward. Now, when the fork vibrate in another direction, the air in the immediate neighbour which was compressed, now get rarefied. As a result, a series of compression and rarefactions are produced and sound energy travels in the direction where compression and rarefactions travel.

25. Compare approximately the speed of sound in air and steel.

The ratio of speed of sound in air to speed of sound in steel is 1/15.

26. The smoke from the gun barrel is seen 2 second before the explosion is heard. If the speed of sound in air is 350 ms-1, calculate the distance of observer from the gun.

The distance of observer from the gun = speed of sound x time taken for the explosion

= 350×2 = 700m

27. Does light travel faster in air than in water ? Why?

Yes, light travels faster in air than water. Because, light does not require any medium for their propagation and they are not mechanical waves. The quantities which oscillate in light are mechanical and electric field. And electric and magnetic field decrease with the medium in between them. So, light travels faster in air than water.

28. Why does the flash of lighting reach earlier than the sound of thunder?

The flash of light reaches earlier than the sound of thunder because the speed of light (its 3 x 108) is much larger than the speed of sound in air (its 330 m/s).

29. The sound of which of the following frequency is audible to us?

2 Hz, 1000 Hz, 200 kHz, 8 MHz

The sound of 1000 Hz frequency will be audible to us. Others 2Hz is infrasonic sound frequency and 200 KHz, 8 MHz is ultrasonic sound frequency.

30. Describe in brief, with the aid of a sketch diagram, an experiment to demonstrate that a material medium is necessary to transmit sound.

The sketch diagram of bell jar is

Take an air tight bell jar having an arrangement of electric bell as shown above in the diagram. The electric bell is suspended inside the glass bell-jar, which is further connected to the vacuum pump. When the circuit of the electric bell is completed, the hammer strikes the gong repeatedly and sound is heard.

Now, start withdrawing the air from the glass bell-jar using the vacuum pump and loudness of the sound will keep decreasing. When the whole of the air is withdrawn, no sound will be heard. So, this experiment shows that a material medium is needed for the propagation of sound.

31. There is no atmosphere on moon. Can you hear each other on moon's surface? Can every kind of sound be heard?

There is no atmosphere on moon. So, we cannot hear each other on moon because sound needs a material medium to travel from one place to another. No kind of sound can be heard.

32. State three characteristics of the medium required for the propagation of sound.

The following are the three characteristics of the medium required for the propagation of sound:

1. The medium must possess elasticity so that the particles of medium have the tendency to return back to their original positions.
2. The medium must be frictionless so that there is no loss of energy during transmission.
3. The medium must possess inertia such that the particles of medium have the ability to store energy.

33. 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

34. What are ultrasonic? State and explain two applications of ultrasounds.

Ultrasonic sound is the sound having frequency more than 20KHz. The application of ultrasonic sound is as follows:

1. It is used to detect the flaws in metal casting of automobile tyres.
2. Used in hospitals to detect defects in certain parts of body.

35. How are sound waves produced? How are sound waves transmitted from one place to the other?

Sound waves are produced by the object motion back and forth in the medium. Due to which it compresses the medium and compression waves are moved forward. The sound travels in medium through energy transfers by wave motion from one place to another. For example, when the prong of the tuning fork vibrate , the waves start moving from one fork to another , the air start getting compressed . The compression is pushed forward. Now, when the fork vibrate in another direction, the air in the immediate neighbour which was compressed, now get rarefied. As a result, a series of compression and rarefactions are produced and sound energy travels in the direction where compression and rarefactions travel.

36. Distinguish between light and sound waves. What are the consequences of the large difference in the speed of sound and that of light? Briefly explain them.

Light waves are electromagnetic waves while the sound waves are mechanical waves. Light waves do not require any material medium for their propagation while sound waves need the material medium for its travel from one place to another.

Due to large magnitude of difference between speed of light (i.e., 3 × 108 m/s) to the speed of sound in air (330 m/s), the light reaches first and then the sound from the object. Like, in thunderstorm, the light reaches first to earth and then person hears the sound of light.

37. How will the following factors affect the speed of sound in air:

(i) Pressure of air,

(ii) Temperature of air,

(iii) Humidity, and

(iv) Wind ?

(i) Pressure of air: The speed of sound in air is independent of pressure of air.

(ii) Temperature of Air: Speed of sound in air is directly proportional to the temperature of air. As the temperature of air increases, the density of air decreases and hence speed of sound increases.

(iii) Humidity: The speed of sound increases with increase in humidity. The increase of moisture decreases the density of the atmosphere and therefore sound travels faster in moist air.

(iv) Wind : The speed of sound depends on the direction of wind. When the wind is blowing in the direction of propagation of sound, the speed of sound increases. But, when the wind blows in the direction opposite to that of the sound, the speed of sound decreases.

38. Explain how the principle of echo is used by

(i) The bat during its flight at night, and

(ii) The dolphin to locate small fish as its prey.