9th Students Chapter 11 Sound

Sound Exercises Answers

1. What is sound and how is it produced?

Sound is a form of energy that produces the sensation of hearing. It is produced by vibrating objects. When an object vibrates, it makes the particles of the medium around it vibrate, and these vibrations reach our ear, producing sound.

2. Describe with the help of a diagram, how compressions and rarefactions are produced in air near a source of sound.

When a vibrating object moves forward, it pushes the air in front, creating a compression (region of high pressure). When it moves backward, it creates a rarefaction (region of low pressure).

Source of Sound --> Pushes Air --> Compression
Moves Back --> Rarefaction

These compressions and rarefactions travel in the air as sound waves.

3. Why is sound wave called a longitudinal wave?

Sound waves are called longitudinal waves because the particles of the medium move back and forth in the same direction as the wave moves.

4. Which characteristic of the sound helps you to identify your friend by his voice while sitting with others in a dark room?

The quality or timbre of the sound helps us to identify our friend’s voice while sitting with others in a dark room.

5. Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen. Why?

Light travels faster (3 × 10⁸ m/s) than sound (340 m/s), so we see the flash immediately, but we hear the thunder after some seconds.

6. A person has a hearing range from 20 Hz to 20 kHz. What are the typical wavelengths of sound waves in air corresponding to these two frequencies? Take the speed of sound in air as 344 m s⁻¹.

Given:
Speed of sound, v = 344 m/s
Frequencies: f₁ = 20 Hz, f₂ = 20,000 Hz

Using λ = v / f

For 20 Hz:
λ = 344 / 20 = 17.2 m

For 20,000 Hz:
λ = 344 / 20000 = 0.0172 m

So, the typical wavelengths are 17.2 m and 0.0172 m.

7. Two children are at opposite ends of an aluminium rod. One strikes the end of the rod with a stone. Find the ratio of times taken by the sound wave in air and in aluminium to reach the second child.

Speed of sound in air = 344 m/s
Speed of sound in aluminium = 6420 m/s

Time = Distance / Speed

Ratio = v(Al) / v(Air) = 6420 / 344 = 18.66

Time in air is about 18.66 times the time in aluminium.

8. The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute?

Frequency = 100 Hz
Time = 60 seconds
Vibrations = 100 × 60 = 6000

It vibrates 6000 times in a minute.

9. Does sound follow the same laws of reflection as light does? Explain.

Yes, sound follows the same laws of reflection as light. The angle of incidence is equal to the angle of reflection, and the incident wave, reflected wave, and normal lie in the same plane.

10. When a sound is reflected from a distant object, an echo is produced. Let the distance between the reflecting surface and the source of sound production remains the same. Do you hear echo sound on a hotter day?

No, because the speed of sound increases on a hotter day, reducing the time for the echo to return, and if it is less than 0.1 seconds, we cannot hear a separate echo.

11. Give two practical applications of reflection of sound waves.

1. Megaphones and loudspeakers use reflection to direct sound.
2. SONAR uses reflection of sound to measure the depth of water.

12. A stone is dropped from the top of a tower 500 m high into a pond of water at the base of the tower. When is the splash heard at the top? Given, g = 10 m s⁻² and speed of sound = 340 m s⁻¹.

Step 1: Time taken by the stone to fall:
h = 500 m, g = 10 m/s²
h = (1/2) g t² ⇒ 500 = 5 t² ⇒ t² = 100 ⇒ t = 10 s

Step 2: Time for sound to travel up:
t = 500 / 340 ≈ 1.47 s

Total time = 10 + 1.47 = 11.47 s

13. A sound wave travels at a speed of 339 m s⁻¹. If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible?

Speed = 339 m/s, Wavelength = 0.015 m
f = v / λ = 339 / 0.015 = 22600 Hz (22.6 kHz)

This is above 20 kHz and will not be audible to humans.

14. What is reverberation? How can it be reduced?

Reverberation is the persistence of sound due to repeated reflection. It can be reduced using sound-absorbing materials like curtains, carpets, and false ceilings.

15. What is loudness of sound? What factors does it depend on?

Loudness is the perceived intensity of sound by our ears. It depends on the amplitude of the sound wave and the sensitivity of the ear.

16. How is ultrasound used for cleaning?

Ultrasound is used in cleaning by placing objects in an ultrasonic bath where high-frequency vibrations remove dirt and particles from the objects.

17. Explain how defects in a metal block can be detected using ultrasound.

Ultrasound waves are passed through the metal block. If there is a defect, the waves get reflected back and can be detected, helping to locate the defect.