2. What is sound ?

3. Mystery music of the day You have 20 seconds to shout author and title Worth 1 (one) brownie point

4. Answer: PHILIP GLASS PHILIP GLASS, Rubric (from Glassworks, 1976)

5. Robert Boyle 1660

6. sound needs a medium to propagate

8. sound is air in motion, but it is not wind It’s an oscillation

9. Sound is a compression wave of matter (usually air) Energy propagates. Matter does not.

10. molecules in air are tiny, ~0.0000000003 m= 3 10 -10 m across are fast moving, ~450 m/s ~ 1600 km/h ~ 1000 miles/h ~fast airplane are close to each other, ~ 0.000000003= 3 10 -9 m = 10 molecule sizes fly about 0.0000001 m = 10 -7 m between collisions exert pressure

11. Pressure force on the wall Higher the temperature, density higher the force

12. air denser air (more molecules) hotter air (faster molecules)

13. Pressure (P) is the force (F) per unit of area (A): F = P A The pressure on the air around us is enormous: 10 tons/m 2 or 15 pounds/inch 2 It doesn’t crush us because the pressure is also inside us pushing out

14. We can now understand how sound propagates compressed air

15. Mystery music of the day You have 20 seconds to shout author and title Worth 1 (one) brownie point

16. World destruction World destruction, Afrika Bambaataa & Johnny Lydon (1984)

17. Sound speed in gases is given by : at constant entropy (for scientists only) (for everybody) temperature molecule mass heavier molecules are harder to push around, sounds propagates slowly hotter air, faster molecules, faster sound hotter air, faster molecules, faster sound

18. Our ears can detect tiny variations in air pressure: normal air pressure 100.000 Pa pressure variation for the just audible sound = 0.00002 Pa pressure variation on the pain threshold = 20 Pa

19. Sound exists also on liquids and solids Air (normal temperature) v = 344 m/s = 770 miles/hour or “5 seconds, one mile” Water v =1400 m/s Steel v = 5100 m/s

20. Properties of waves, sound include Reflection hard wall

21. If instead of a hard wall we have something more flexible (but not elastic), much of the sound energy will dissipate acoustic insulator

22. same angle Reflection on an angle

23. Someone at the red point can hear a conversation on the blue point better than anybody else in the room The same is said to occur in some gothic cathedrals

24. Band shell

26. We hear a lot of reflected sound all the time. It usually arrives very close to the direct sound and we don’t notice it as a distinct sound. Still, it contributes to the quality of the sound.

27. Compare how an organ sounds in a cathedral outdoors Compare how your voice sounds in a bathroom living room We will discuss all this with more detail later…

28. Mystery music of the day You have 20 seconds to shout author and title Worth 1 (one) brownie point

29. Answer: DEBUSSY DEBUSSY, Arabesque (1888)

30. Refraction faster medium slower medium Initial and final directions are different. The sound made a curve !

31. As an animation now

32. At night

33. During the day hotter air colder air

34. Acoustic lens

35. Larger wavelength (~ obstacles) : more diffractionLarger wavelength (~ obstacles) : more diffraction Shorter wavelength (<< obstacles) : less diffractionShorter wavelength (<< obstacles) : less diffraction Diffraction

36. “Inverse square law”: how sound intensity decreases with distance Intensity ~ 1/r 2

37. It’s a good time now to read section 2.2 and 2.3 of Berg & Stork (except interference)

38. Mystery music of the day You have 20 seconds to shout author and title Worth 1 (one) brownie point

39. Norwegian wood, Lennon & Mcartney (1966?)

40. How to read graphs figure what’s in the horizontal axis (w/ units) figure what’s in the vertical axis (w/ units) figure what’s in the vertical axis (w/ units) find the value of “savings” at any particular time find the value of “savings” at any particular time

41. visualanalytical

42. WATCH !!! same information

43. Let us watch now the graph of sound pressure variation as a function of time for some real sounds Which sounds are periodic ? What distinguishes noise from “musical” sounds ? What makes a sound louder ? What makes a sound lower or higher in pitch ?

44. Wavetools http://www.physics.mcgill.ca/~grant/225B/Wavetoo ls/Computerstuff.html

45. A very “pure” (but annoying) sound:

46. Mystery music of the day You have 20 seconds to shout author and title Worth 1 (one) brownie point

47. Bachianas Brasileiras #5, Villa-Lobos (1938)

48. It is a good time now to read Berg & Stork, Chapter 1

49. “Musical” sounds are periodic period (T) amplitude (A) frequency (f) = 1/T, T=1/f

50. amplitude = loudness frequency = pitch shape = timbre With a few qualifications …

51. Period (T) = time for one cycle (measured in s, …) Frequency (f) = number of cycles per second (measured in 1/s = Hz) Example

52. “Pure” tones are sine waves Harmonic oscillator

53. amplitudefrequency

54. The mathematics of periodic waves wavelength: distance between two crests period: time between two crests frequency: how many crests per second

55. [Using a wave generator, find the lowest and highest audible frequencies and calculate their period and wavelength]

57. How sounds combine: adding two waves

58. Beats

59. Tartini tones

60. Interference movie http://www.colorado.edu/physics/2000/schroedinger/ big_interference.html

61. It is a good time now to read Berg & Stork, Chapter 2.4, 2.5, 2.6, 2.7, 2.8, 2.9

62. Mystery music of the day You have 20 seconds to shout author (or performer) and title Worth 1 (one) brownie point

63. All along the watchtower, Jimmy Hendrix (1966)

64. http://www.lon- capa.org/~mmp/applist/doppler/d.htm

65. Doppler effect

66. [go to the blackboard and do some real physics] Applications: Doppler radar Doppler radar Doppler sonogram Doppler sonogram Expansion of the Universe Expansion of the Universe

67. Ultrasound and sonograms Typical frequencies: 300 MHz