1. Bouncing Around October 26, 2007
Room Acoustics
Bouncing Around
October 26, 2007

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.
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Room Acoustics

3. Consider a Pulse of Sound
WALL
Reflection
Sound Changes
Different Travel Distances
Many reflections occur at the same “time”
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4. Consider the following:
270 ft
100 ft
ear
source
200 ft
250 ft
500 ft.
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5. Note
Our ear will hear two sounds 40 ms apart as a but “rough” and, perhaps, as a fast echo.
A room should be carefully designed to maintain a “pleasant” aural experience.
This will be our concern today.
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Room Acoustics

6. A 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.
There may be an echo.
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7. Example - Interference
“Wall
Wall
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8. Also consider Wavelengths in music
Note different wavelengths and
compare with the size of a room.
Wavelength will be an important
variable in a room.
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9. Surfaces (Walls, floors, etc.)
Rough or Smooth
Hard or soft
Location with respect to listener
Characteristics depend on the sound being detected.
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10. Two surfaces
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11. IS THIS A ROUGH SURFACE??? 1 nm = 10-12 meters =0.000000000001 m
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12. 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.
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13. Reflection
SMOOTH
ROUGH
SPECULAR
DIFFUSE
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14. 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.
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15. 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
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16. Useful aspects of reflection
Think about the reverse!
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17. The old Greek Amphitheater
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19. Closer Audience “Band Shell”
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22. 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!
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23. 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!
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24. EXAMPLE: The Ellipse
A & B = foci
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25. 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.
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26. APPROXIMATION ??
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28. Parabolic Reflector
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29. Parabolic Receiver
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30. What about REAL Rooms???

31. In a Real Room What about the walls? Smooth Rough
How Smooth?
Rough
How Rough?
Transmission properties?
WALL
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32. Another Factor RESONANCE
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33. Resonance Examples
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34. Speakers?
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35. 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.
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36. A different phenomonon
DIFFRACTION
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37. 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.
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38. A Bad Photo .. sorry
ploop
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39. Huygen's Principle 1678
Polaroid Photo
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40. 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
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41. Another View
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42. A Slit (Window)
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43. Diffraction Through a SMALL Opening (comparable to l)
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45. An Edge
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46. 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.
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47. 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.
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48. We discussed Reflections
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49. What Do You Think?
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50. Or a school performance hall
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51. Professional Concert Hall (mucho Dolleros )
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52. Surfaces
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53. $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$
Baffles
$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$
Soft Walls
People???
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54. Note Modern halls are adjustable for The piece being played
The size of the audience
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55. Create a SUDDEN Sound Listen & Record with a microphone loudness time
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56. Real Example: Royal Festival Hall
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58. Room Reflections
Room full of sound!
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59. 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.
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60. Listen to the Room!
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61. Lets start a musical tone and listen to the auditorium with a sound recorder.
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62. 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 1/1,000,000th of the steady level is called the REVERBERATION TIME.
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63. The Return to Peace Reverberation Time
Reverberation time is the time the sound takes to be reduced to one
millionth of its initial level.
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64. Absorbing Materials
More Absorbing
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65. A Formula NOT to be Remembered
Wallace Sabine
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66. Let’s try a calculation – Living Room @ 500 Hz (Book states this wrong)
Ceiling Area = 4 x 5 = 20 m2
Effective = 0.1 x 20 = 2m2 3m 4m 5m
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67. Another Example
300 x 0.1
same
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68. Reverberation Times Desired
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69. 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.
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70. http://www. crutchfieldadvisor
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