1. Sound is a mechanical energy which produces sensation of hearing.
2. For hearing sound there must be (i) a vibrating body, (ii) a material medium for its propagation and (iii) a receiver, such as human ear.
3. Sound energy does not propagate through vacuum.
4. When the particles of a medium oscillate in the same direction, in which wave is being propagated, such a wave is called longitudinal wave.
5. When the particles of a medium oscillate at right angles to the direction of wave propagation, a transverse wave is produced.
6. Transverse waves can be produced in solids and liquids, but not in gases.
7. The highest point on the elevation or hump of a transverse wave is called crest.
8. The lowest point on the depression or hollow of a transverse wave is called trough.
9. Compression is a region in a longitudinal wave, where the particles of medium are crowded together. It is a region of high pressure and high density.
10. Rarefaction is a region in a longitudinal wave, where the particles of medium are spread wide apart. It is a region of low pressure and low density.
11. The change in density of a medium from maximum value to minimum value and again to maximum value in case of longitudinal wave is called one oscillation.
12. The number of compressions and rarefactions (taken together) passing through a point in one second is called frequency. Its unit is hertz.
13. The time taken by two consecutive compressions or rarefactions to cross a point is called time period.
14. The magnitude of maximum displacement of a vibrating particle about its mean position is called amplitude.
15. The pitch of sound is determined by its frequency, i.e., higher the frequency, more is the pitch, and hence, shriller is the sound.
16. The loudness of sound is determined by the amplitude, i.e., more the amplitude, louder is the sound.
17. The property by virtue of which the note of same pitch and same frequency can be distinguished is called timbre or quality of sound.
18. Sound travels fastest in solids, slower in liquids and slowest in gases.
19. A conical tube commonly used for addressing a small group of people is called megaphone.
20. The phenomenon due to which repetition of sound is heard after reflection from a distant object, after the original sound from a given source dies is called an echo.
21. For hearing an echo, the minimum distance between the source of sound and reflecting body should be 17 m.
22. Vibrations within the frequency range of 0 Hz to 20 Hz are called infrasonic vibrations. Humans cannot hear them.
23. Vibrations within the frequency range of 20 Hz to 20000 Hz are called sonic vibrations. They can be heard by humans.
24. Vibrations above the frequency range of 20,000 Hz known as ultrasonic vibrations. Humans cannot hear them.
25. A device used to locate depth of sea or submarines, etc, is called sonar.
1. Distinguish between loudness and intensity of sound
2. An echo returned in 3 s. What is the distance of the reflecting surface from the source, given that the speed of sound is 342 m/s?
3. Why are ceilings of concert halls curved?
4. A submarine emits a sonar pulse, which returns from an underwater cliff in 1.02 s. If the speed of sound in salt water is 1531 m/s, how far away is the cliff?
5. Why sound wave is called a longitudinal wave?
6. Flash and thunder are produced simultaneously, but thunder is heard a few seconds after the flash is seen. Why?
7. Two children are at the 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 aluminium and in air are 6420 m/s and 346 m/S respectively).
8. When a sound is reflected from a distant object, an echo is produced. If the distance between the reflecting surface and the source of sound production remains the same. Do you hear echo sound on a hotter day?
9. 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 splash heard at the top? Given g = 10 ms–2 and speed of sound is 340 m/s.
10. A sound wave travels at a speed of 339 m/s. If its wavelength is 1.5 cm, what is the frequency of the wave? Will it be audible?
11. What is reverberation? How can it be reduced?
12. Explain how bats use ultrasound to catch prey.
13. How is ultrasound used for cleaning?
14. A sonar device on a submarine sends out a signal and receives an echo 5 seconds later. Calculate the speed of sound in water, if the distance of object from the submarine is 3625 m.
15. A girl hears an echo of her own voice from a distant tall building after 2 s. What is the distance of the girl from the building? (Given speed of sound in air = 332 m/s)
16. A hospital uses an ultrasonic scanner to locate tumours in a tissue. What is the wavelength of sound in a tissue is which the speed of sound is 1.7 km/s. The operating frequency of the scanner is 4.2 MHz. (1MHz = 106 Hz).
17. The sound of an explosion on the surface of lake is heard by a boatman 100 m away and a driver 100 m below the point of explosion. Of the two persons mentioned (boatman or driver) who would hear the sound first? And why?
18. Calculate the wavelength of a sound wave whose frequency is 220 Hz and speed is 440 m/s in a given medium.