1. Room Acoustics Bouncing Around October 27, 2006

2. Music and Other Sounds Come from a source. The source is not isolated, it is in an environment. The environment can affect what the listener will hear: –Ambient noise level –Properties of the wall, ceiling, etc. –Other sources producing sound at the same time.

3. Consider a Pulse of Sound Different Travel Distances WALL Sound Changes

4. The wall Sends a “delayed” reflection of the sound to the ear. –A matter of distance. The reflection may be synchronized with the source so that they may “interfere” The reflection may, be hindered by the absorption of the sound energy by the wall.

5. Example - Interference “Wall Wall

6. Wavelengths in music Note different wavelengths and compare with the size of a room. Wavelength will be an important variable in a room.

7. Surfaces (Walls, floors, etc.) Rough or Smooth Hard or soft Location with respect to listener Characteristics depend on the sound being detected.

8. Two surfaces

9. 1 nm = 10 -12 meters =0.000000000001 m IS THIS A ROUGH SURFACE???

10. Again, Consider a Wall How smooth is it? Smooth is in the feel of the feeler! Smooth or Rough are Relative terms. We define: –SMOOTH – Variations occur on a scale much smaller than a wavelength of the sound we are considering. –ROUGH – The variations in the surface are comparable to the size of the wavelength.

11. SMOOTH ROUGH SPECULAR DIFFUSE

12. SOFT Walls A soft wall (like rubber or cork) will yield when you push on it. Sound (music) pressure pushes on the wall. IF the wall deforms, than a force (pA) times a distance (the deformation), means that the wave does WORK. The sound therefore loses some energy when it hits such a wall. The reflection isn’t as strong as one from an “un- yielding” wall.

13. Consider an outdoor concert Musicians on stage People in the audience No Walls or Ceilings Only reflections possible are from structures in back of the musicians. –And possibly the ground

14. Useful aspects of reflection Think about the reverse!

15. The old Greek Amphitheater

17. Closer Audience “Band Shell”

20. Care in a band-shell The focus can’t be too good because then all of the performers need to be at the same place. Since they can’t be, a vertical wall might be better. Real Band shells look right but really do NOT properly focus. ON PURPOSE!

21. What does “focus” mean Sound waves hit a surface which can be called a mirror. The mirror surface can be curved so that rays of sound from different directions can be made to come together at the same place. –Like a lens In a concert hall, too much focusing can also mean that there is only ONE good seat in the house!

22. EXAMPLE: The Ellipse A & B = foci

23. Whispering Gallery Note – This Wren design was actually a spherical surface that doesn’t really focus that well. It probably comes close to a portion of an ellipse.

24. APPROXIMATION ??

26. Parabolic Reflector

27. Parabolic Receiver

28. What about REAL Rooms???

29. In a Real Room What about the walls? Smooth –How Smooth? Rough –How Rough? Transmission properties? WALL

30. Another Factor RESONANCE

31. Resonance Examples

32. Speakers?

33. At home with Shostakovich If you can see it, you can hear it! Wherever you see your speaker reflected in the mirror, that's a point of reflection that should receive absorptive, or in some cases, diffusive acoustic treatment.

35. Diffraction Sound can “bend” around objects. Sound can change its properties depending upon the size of the wavelength compared to objects. The Diffraction effect can be understood via one of the early theories of waves.

36. A Bad Photo.. sorry ploop

37. Huygen's Principle 1678 Polaroid Photo

38. Huygen's Principle Every point on the front of a wave (wave front) acts as a source of spherical waves. The next position of the wave front will be the surface that is tangent to all of the other parts of the surface created in the same way. The spherical wave travels at the speed of sound. vt

39. Another View

40. A Slit (Window)

41. Diffraction Through a SMALL Opening (comparable to ) DIFFRACTION

43. An Edge

44. Sound Travels in straight Lines. Travels in crooked lines. Can be focused. Can be absorbed by a surface Can be diffracted Can interfere “with itself” Is dependent on the properties of the room.

45. What else? Small objects will scatter or diffract sound so it can be heard in non-straight lines. –Around edges, etc. Small objects do very little to long wavelength sounds (low tones). They are like the Eveready Battery … they keep going and going and going ….. Higher frequency sounds will be deflected or absorbed more than low frequency sounds.

46. We discussed Reflections

47. What Do You Think?

48. Or a school performance hall

49. Professional Concert Hall (mucho ) Professional Concert Hall (mucho Dolleros )

50. Surfaces

51. Baffles Soft Walls People??? $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$

52. Note Modern halls are adjustable for –The piece being played –The size of the audience

53. Create a SUDDEN Sound time loudness Listen & Record with a microphone

54. Real Example: Royal Festival Hall

56. Room Reflections Room full of sound!

57. Room Full Of Sound Cut a small Window into the wall EACH SECOND THE SAME FRACTION OF SOUND WILL LEAK FROM THE ROOM LEADING TO WHAT IS CALLED EXPONENTIAL DECAY.

58. Listen to the Room!

59. Lets start a musical tone and listen to the auditorium with a sound recorder.

60. How about the return to silence? There is a steady musical sound in the auditorium. The symphony is over. The music suddenly stops. It takes a certain time for the sound level to get to a very small level. The time it takes for the auditorium sound to drop to 1/1,000,000 th of the steady level is called the REVERBERATION TIME.

61. The Return to Peace Reverberation Time Reverberation time is the time the sound takes to be reduced to one millionth of its initial level.

62. Absorbing Materials More Absorbing

63. A Formula NOT to be Remembered

64. Let’s try a calculation – Living Room @ 500 Hz (Book states this wrong) 3m 5m4m Ceiling Area = 4 x 5 = 20 m 2 Effective = 0.1 x 20 = 2m 2

65. Another Example 300 x 0.1 same

66. Reverberation Times Desired

67. For Music Rooms must be carefully designed. The “engineering” contains a lot of “Kentucky Windage”. Different kinds of music require different acoustical designs. In the right room, you hear what the composer intended you to hear.

68. http://www.crutchfieldadvisor.com/ISEO- rgbtcspd/learningcenter/home/speakers_roomacoustics.html