Sound                                Important Notes
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 compression and rarefaction (taken together) passing through a point in one second is called frequency. Its unit is hertz.
13. The time taken by two consecutive compression or rarefaction 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.