Physics 115 Lecture 20. Reflection and Reverberation March 9, 2018

Transcription

1 Physics 115 Lecture 20 Reflection and Reverberation March 9,

2 Attenuation by absorption The wave intensity decreases due to internal friction among the molecules of the medium Attenuation is described by a bulk absorption coefficient called m: I I e 2mx 0 I = final intensity in W/m 2 I 0 = initial intensity in W/m 2 e = base of the natural logarithm m = bulk absorption coefficient x = distance the wave has traveled

3 Atmospheric absorption The amount by which air absorbs sound depends on frequency, relative humidity, and other factors. Maximum absorption around 15% RH High frequencies are attenuated more

4 Another (easier?) way to calculate The coefficients m for absorption in air are small. For instance, at 4000 Hz (peak sensitivity for human ear) at 20 C and 50% RH, m = m 1. The coefficient can also be expressed as decibels per kilometer L L a x L 0 = initial sound level in decibels L = final sound level in decibels a = absorption coefficient in db/km x = distance in kilometers 0

5 10log The two are related (FYI only) 0 2mx I I I I e 2mx log log log r 0 I I I I m x e 10log I I 20m xlog e has units of 1/m log log 2 log r I I r I I e m L L ax where a 20mlog e r r 1000 m km a m db/km r

6 Example from Figure 6.1 Interactive applet for finding absorption coefficient a. Figure 6.1 shows for 20 C, 20% relative humidity, f = 6000 Hz, that m = m 1. a 8686m m db/km Wolfram value: db/km (m = m 1 ) Handbook of Chemistry and Physics 1995: db/km at 6300 Hz (m = m 1 ) I conclude that figure 6.1 y-axis labels should all be multiplied by ten. [publisher confirmed 3/12/2013]

7 Attenuation by reflection The wave intensity decreases due to partial absorption and transmission of the sound wave at a boundary (external absorption) Attenuation is described by a boundary absorption coefficient called α: absorbed or transmitted intensity incident intensity reflected intensity incident intensity reflected 1 (total energy is conserved) I I 0 I absorbed I 0

8 A cap gun emits a sound wave of intensity I 0 = W/m 2 after it has traveled 101 m. What will I be if absorption by a painted concrete wall is taken into account? Let α = A W/m 2 B W/m 2 C W/m 2 D W/m 2

9 A cap gun emits a sound wave of intensity I 0 = W/m 2 after it has traveled 101 m. What will I be if absorption by a painted concrete wall is taken into account? Let α = A W/m 2 B W/m 2 C W/m 2 D W/m 2 I reflected 0 I W/m W/m 7 2 The reflection reduced the level from 52.9 db to 52.5 db

10 A cap gun emits a sound level L 0 = 52.5 db after it has traveled 101 m and reflected from a concrete wall. What will L be if atmospheric absorption is taken into account? Let a = db/km. A db B db C db D db

11 A cap gun emits a sound level L 0 = 52.5 db after it has traveled 101 m and reflected from a concrete wall. What will L be if atmospheric absorption is taken into account? Let a = db/km. A db B db C db D db L L a x db db/km km L 52.5 db 2.99 db 49.5 db We would get 52.9 db if reflection and absorption not taken into account. The values given in these examples are intended to be realistic for the cap gun experiment in the basement hallway last week. The temperature was assumed to be 20 C, RH 50%, f = 4000 Hz, a value from Wolfram web site, α from Table 6.1 p. 109 for painted concrete.

12 What happens to essentially all the sound energy in a dorm room after it is generated by a pair of stereo speakers? A. It bounces around until it escapes out the window or door. B. It is absorbed by your ears. C. It is converted back to electrical energy by the speakers. D. It heats up the room.

13 What happens to essentially all the sound energy in a dorm room after it is generated by a pair of stereo speakers? A. It bounces around until it escapes out the window or door. B. It is absorbed by your ears. C. It is converted back to electrical energy by the speakers. D. It heats up the room.

14 LAW OF REFLECTION SURFACE

15 LAW OF REFLECTION SURFACE

16 LAW OF REFLECTION SURFACE

17 LAW OF REFLECTION SURFACE

18 LAW OF REFLECTION NORMAL SURFACE

19 LAW OF REFLECTION NORMAL A B SURFACE

20 LAW OF REFLECTION ANGLE OF INCIDENCE = ANGLE OF REFLECTION A=B NORMAL A B Interactive applet for law of reflection

21 A beam of light reflects from a mirror as shown in the figure. If the angle of incidence of the beam is increased, the reflected beam A. stays in the same direction B. shifts in direction 1 C. shifts in direction 2

22 A beam of light reflects from a mirror as shown in the figure. If the angle of incidence of the beam is increased, the reflected beam A. stays in the same direction B. shifts in direction 1 C. shifts in direction 2 θ

23 Reflection of sound waves: ultrasound imaging

24 Reflection of sound waves: echo location

25 Reflection from Curved Surfaces PARABOLIC REFLECTOR

26 Reflection from Curved Surfaces ELLIPSOIDAL REFLECTOR

27 Reflection from Curved Surfaces CONVEX REFLECTOR

28 Diffuse Reflection REFLECTION FROM A ROUGH SURFACE

29 Reflection of light waves Plane mirror Convex mirror

30 Reflection of light waves Concave mirror Interactive applet for ray tracing (FYI only)

31 Parabolic reflector

32 Reflection of light waves Fun house mirrors

33 Reflection: Focusing light waves in a reflecting telescope

34 Reflection: Focusing light waves solar furnace

35 Reflection: Focusing light waves solar furnace

36 ARCHIMEDES Mirror to focus light waves...

37 ... for military purposes

38 Reflection: Focusing microwaves

39 Reflection: Focusing radio waves

40 Reflection of radio waves

41 Reflection of sound waves: Sound focusing at the Cathedral of Agrigento in Sicily

42 Reflection of sound waves: Sound focusing in domes (Whispering galleries) St. Paul s Cathedral (London) U.S. Capitol Building (Washington D.C.)

43 Reflection of sound waves: Focusing

44 Reflection and Room Acoustics DIRECT SOUND

45 Reflection and Room Acoustics FIRST REFLECTION

46 Reflection and Room Acoustics SECOND REFLECTION

47 Reflection and Room Acoustics OTHER REFLECTIONS

48 Reflection and Room Acoustics Impulse (not detected at microphone) Air pressure Direct sound 1st Reflection 2nd Reflection Time

49 Reflection and Room Acoustics Impulse Air pressure Direct sound 50 ms EARLY SOUND (First 50 ms) Time

50 Reflection and Room Acoustics Impulse Air pressure Direct sound 50 ms EARLY SOUND (First 50 ms) Time REVERBERANT SOUND (t > 50 ms)

51 Reverberation Time REVERBERATION TIME is the time required for the sound intensity in the auditorium to decrease to 1 millionth of its original magnitude. [This is equivalent to the sound intensity level decreasing by 60 db.]

52 Reverberation Time Analogy... $ 1200 v

53 Reverberation Time Analogy... $ %, please

54 Reverberation Time Analogy... $ 960

55 Reverberation Time Analogy... $ 960 v

56 Reverberation Time Analogy... $ %, please

57 Reverberation Time Analogy... $ 768

58 Reverberation Time Analogy... $ 768 v

59 Reverberation Time Analogy... $ %, please

60 Reverberation Time Analogy... $

61 Reverberation Time Analogy... $ v

62 Reverberation Time Analogy... $ %, please

63 Reverberation Time Analogy... $

64 Reverberation Time Analogy... $ v

65 Reverberation Time Analogy... $ %, please

66 Reverberation Time Analogy... $

67 Reverberation Time Analogy... $ v ETC.

68 Reverberation Time WEALTH VS. TIME $ etc. etc.

69 1500 WEALTH VS. TIME DOLLARS TIME

70 SOUND INTENSITY INTENSITY VS. TIME TIME

71 INTENSITY VS. TIME SOUND INTENSITY LONG RT SHORT RT TIME

72 Reverberation time The Sabine equation gives the reverberation time RT of a room in seconds: RT constant volume of room total surface absorption V k A

73 In order to decrease the reverberation time you should the surface absorption coefficient α, or make the room. A. increase larger B. increase smaller C. decrease larger D. decrease smaller

74 In order to decrease the reverberation time you should the surface absorption coefficient α, or make the room. A. increase larger B. increase smaller C. decrease larger D. decrease smaller More absorption means a faster decay of sound intensity, shorter RT. A smaller room means less time delay between reflections, shorter reverberation time.

75 A room has a reverberation time of s when its total absorption A = Changing the carpet decreases the total absorption to A = What is the new reverberation time? A s B s C s D s

76 A room has a reverberation time of s when its total absorption A = Changing the carpet decreases the total absorption to A = What is the new reverberation time? A s B s C s D s Use a ratio: RT RT RT RT new old new kv A kv A new old A A kv A old old new RT s s (Optional at this time) You can learn how to calculate the A values on pp This question is an adaptation of Practice Problem 2 on p. 114, which assumes k = for 22 C air.