Page 150 - Understanding NCERT Science 09

• Sound properties such as pitch, loudness and quality are
determined by the corresponding wave properties.
• Loudness is a physiological response of the ear to the intensity
of sound.
• The amount of sound energy passing each second through
unit area is called the intensity of sound.
• The audible range of hearing for average human beings is in
the frequency range of 20 Hz – 20 kHz.
• Sound waves with frequencies below the audible range are
termed “infrasonic” and those above the audible range are
termed “ultrasonic”.

• Ultrasound has many medical and industrial applications.

Exercises

1. What is sound and how is it produced?
2. Describe with the help of a diagram, how compressions and
rarefactions are produced in air near a source of sound.
3. Why is sound wave called a longitudinal wave?
4. Which characteristic of the sound helps you to identify your
friend by his 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?
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
–1
sound in air as 344 m s .
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.
8. The frequency of a source of sound is 100 Hz. How many
times does it vibrate in a minute?
9. Does sound follow the same laws of reflection as light does?
Explain.
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?
11. Give two practical applications of reflection of sound waves.
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
–2
heard at the top? Given, g = 10 m s and speed of sound =
–1
340 m s .
138 SCIENCE

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