1. 1 13.2 Properties of sound Sound in a concert Some facts about sound Sound waves are longitudinal waves Speed of sound Sound and light waves compared Audible sound and ultrasound Check-point 2 E ? ?

2. 2 13.2 Properties of sound Sound in a concert You can hear the sound of different musical instruments in a concert.

3. 3 13.2 Properties of sound Sound in a concert Discuss: What are common to all of them? All of them have vibrating parts causing nearby air particles to vibrate and produce sound waves.

4. 4 13.2 Properties of sound guitar loudspeaker tuning fork aSound is produced by vibrations 1Some facts about sound

5. 5 13.2 Properties of sound Sound source vibrates. Nearby air particles vibrate. Sound waves are produced. aSound is produced by vibrations

6. 6 13.2 Properties of sound bSound cannot travel through a vacuum Less and less sound is heard in pumping the air out. electric bell to vacuum pump

7. 7 13.2 Properties of sound bSound cannot travel through a vacuum Astronauts can only talk to each other via a radio system.

8. 8 13.2 Properties of sound cSound can travel through solids, liquids and gases You can hear sound when swimming underwater. Floors, ceilings and brick walls also transmit sound.

9. 9 13.2 Properties of sound 2Sound waves are longitudinal waves Put a buring candle in front of a loudspeaker... move forwards move backwards

10. 10 13.2 Properties of sound 2Sound waves are longitudinal waves Video

11. 11 13.2 Properties of sound 2Sound waves are longitudinal waves loudspeaker cone Sound is a longitudinal wave. wave vibrations of air (movement of flame ) CRCRC C = compression R = rarefaction

12. 12 13.2 Properties of sound 3Speed of sound It increases with the temperature. It varies with the medium: v solid > v liquid > v gas MediumSpeed of sound / m s –1 Air at 0°C330 Air at 20°C340 Water1400 Concrete5000 Steel6000

13. 13 13.2 Properties of sound 4Sound and light waves compared Light travels faster than sound. E

14. 14 13.2 Properties of sound 4Sound and light waves compared Difference between Sound and Light waves. E Sound wavesLight waves Production Mechanical vibrations Electrical and magnetic vibrations Type of waves Longitudinal waves Transverse waves

15. 15 13.2 Properties of sound 4Sound and light waves compared E Difference between Sound and Light waves. Sound wavesLight waves Travelling medium Solids, liquids and gases (but not vacuum) Transparent materials (and a vacuum) Travelling speed in the air 330 m s –1 (at 0°C) 3 × 10 8 m s –1

16. 16 13.2 Properties of sound 4Sound and light waves compared E

17. 17 13.2 Properties of sound 5Audible sound and ultrasound Human beings can hear sound waves only within a range of frequencies. Many animals can hear sound waves within a wider range of frequencies.

18. 18 13.2 Properties of sound Audible range of frequencies Connect a signal generator to a loudspeaker and a CRO. Start with a frequency of 1 Hz and gradually increase it to beyond 20 kHz. Find out the range of frequency that you can hear. Experiment 13b

19. 19 13.2 Properties of sound Experiment 13b Video Audible range of frequencies

20. 20 13.2 Properties of sound 5Audible sound and ultrasound aAudible sound Human beings can hear sound of frequency from about 20 Hz to 20 kHz. It is called audio frequency range.

21. 21 13.2 Properties of sound aAudible sound Sounds within audible frequency range: Frequency 20 000 HzHighest frequency heard 10 000 HzWhistle 1000 HzHigh note from singer (soprano) 100 HzLow note (bass) 20 HzLowest frequency heard

22. 22 13.2 Properties of sound aAudible sound

23. 23 13.2 Properties of sound bUltrasound Dolphins and bats can emit ultrasound. E Ultrasound (or ultrasonic waves): Sound waves with frequency above 20 kHz

24. 24 13.2 Properties of sound Applications of ultrasound Sonar for detecting shoals of fish. ultrasound from sonar echo shoal of fish E

25. 25 13.2 Properties of sound Applications of ultrasound Ultrasound scanner for scanning unborn babies. E Ultrasound flaw detector for detecting flaws at railway tracks. detector flaw Ultrasound cleaner for cleaning small and delicate objects.

26. 26 13.2 Properties of sound Applications of ultrasound Video E

27. 27 13.2 Properties of sound bUltrasound

28. 28 13.2 Properties of sound Check-point 2 1Which of the following is...Which of the following is... 2A boy drops a stone into a...A boy drops a stone into a... 3In a 100-m run…In a 100-m run… E E

29. 29 13.2 Properties of sound Check-point 2 - Q1 Which of the following is INCORRECT? ASound is a wave. BThe air vibrates along the travelling direction of sound. CSound can travel in all media, including a vacuum. DThe speed of sound is highest in solids.

30. 30 13.2 Properties of sound Check-point 2 - Q2 A boy drops a stone into a well. Which of the following is correct? AHe hears the splash first. BHe sees the splash first. CHe sees and hears the splash at the same time. E

31. 31 13.2 Properties of sound Check-point 2 - Q3 In a 100-m run, the timekeeper starts the stop-watch when he sees the flag wave. What is the error in timing if the timekeeper relies on listening to the starter pistol? timekeeper 100 m E

32. 32 13.2 Properties of sound Check-point 2 - Q3 Since light travels much ________ than sound, the time taken for light to travel from starter to timekeeper is __________. Speed of sound in air = 340 m s  1 faster neglected timekeeper 100 m E

33. 33 13.2 Properties of sound Check-point 2 - Q3 The time interval for the sound of pistol reaches the timekeeper timekeeper 100 m = ( ) speed of sound = = _______ s ( ) 340 distance between timekeeper & starter 100 0.294 E

34. 34 13.2 Properties of sound Check-point 2 - Q3 E timekeeper 100 m The time measured by the timekeeper would be __________ (shorter/longer). shorter

35. 35 13.2 Properties of sound The End

36. 36 13.2 Properties of sound Example 2 A girl sees a flash of lightning in the sky. Five seconds later she hears the bang. How far away is she from the thunderstorm? Take speed of sound as 340 m s –1. Lightning: the flash and the bang

37. 37 13.2 Properties of sound Example 2 Light travels so fast, we can neglect the time lapse from the occurrence of lightning to the flash arrives at the girl’s eyes. The girl’s distance from the thunderstorm = speed of sound × time = 340 × 5 = 1700 m Lightning: the flash and the bang

38. 38 13.2 Properties of sound Return

39. 39 13.2 Properties of sound Example 3 Find the wavelength of the following sounds. Given that the speed of sound is 340 m s –1. (a)The lowest frequency heard (20 Hz). Apply v = f λ Wavelengths of different sounds λ=λ= v f = 340 20 = 17 m

40. 40 13.2 Properties of sound Example 3 Find the wavelength of the following sounds. Given that the speed of sound is 340 m s –1. (b)The highest frequency heard (20 000 Hz). Apply v = f λ Wavelengths of different sounds λ=λ= v f = 340 20 000 = 0.017 m

41. 41 13.2 Properties of sound Return

42. 42 13.2 Properties of sound Example 4 Echolocation

43. 43 13.2 Properties of sound Example 4 (a) What is the wavelength of the whale’s echolocation wave? Take the speed of sound in water as 1400 m s –1. (b)If an obstacle is 100 m from the whale, how long after the whale emits a wave will the reflected wave return to him? Echolocation

44. 44 13.2 Properties of sound Example 4 (a)Apply v = f λ (b)Distance between the whale and the obstacle = 100 m ⇒ Distance travelled by sound waves = 2 × 100 m = 200 m Echolocation λ=λ= v f = 1400 20 000 = 0.007 m Time = distance speed = 0.143 s

45. 45 13.2 Properties of sound Return