Answers to Sound-Related Questions for CBSE textbook
1. What is sound and how is it produced?
Sound is a form of energy that travels through a medium (like air, water, or solids) in the form of waves. It is produced when a vibrating object causes the surrounding medium to vibrate, creating pressure waves that can travel through the medium.
2. Describe with the help of a diagram, how compressions and rarefactions are produced in air near a source of sound.
Compressions: These are regions where air particles are close together due to the vibration of the sound source.
Rarefactions: These are regions where air particles are spread apart.
Note: Diagram would show a source like a tuning fork with compressions and rarefactions in air.
3. Why is sound wave called a longitudinal wave?
A sound wave is called a longitudinal wave because the particles of the medium through which the sound travels vibrate in the direction of the wave's propagation. This means that the displacement of the medium is parallel to the direction in which the wave travels.
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, which is the unique tone or 'color' of the voice, allows you to identify your friend. Timbre depends on the complex mixture of frequencies in the sound, making voices distinct.
5. Flash and thunder are produced simultaneously. But thunder is heard a few seconds after the flash is seen, why?
Light travels much faster than sound. The speed of light is approximately 3 × 10^8 m/s, while the speed of sound in air is about 344 m/s. This difference in speed means that light reaches you almost instantly, while sound takes a noticeable amount of time to travel the same distance.
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.
For 20 Hz:λ=fv=20Hz344m/s=17.2m
For 20 kHz:λ=20000Hz344m/s=0.0172m=1.72cm
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.
Assuming the speed of sound in aluminium is about 6420 m/s:
Time in air = distance / speed of sound in air
Time in aluminium = distance / speed of sound in aluminium
Ratio = (time in air) / (time in aluminium) = (344/6420) ≈ 0.0536 or 1:18.6
8. The frequency of a source of sound is 100 Hz. How many times does it vibrate in a minute?
100 Hz means 100 vibrations per second. In one minute (60 seconds):
100 × 60 = 6000 vibrations per 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 equals the angle of reflection.
Both are reflected from a surface, following these laws.
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?
On a hotter day, the speed of sound in air increases, but the time taken for the echo to return (which depends on the distance) remains the same. However, because sound travels faster, the echo might return sooner, but you would still hear it if the conditions for echo formation are met (like sufficient distance for the sound to travel).
11. Give two practical applications of reflection of sound waves.
Stethoscopes: Use reflection of sound to diagnose heart and lung sounds.
Sonar Systems: Utilize reflection of sound waves to measure distances and detect objects under 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.
Time for stone to fall:
t1=g2h=102×500=100=10s
