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Room Acoustics

Room Acoustics. Bouncing Around October 27, 2006. 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.

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Room Acoustics

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  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 WALL Sound Changes Different Travel Distances

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

  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. Reflection SPECULAR DIFFUSE SMOOTH ROUGH

  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

  16. Closer Audience“Band Shell”

  17. 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!

  18. 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!

  19. EXAMPLE: The Ellipse A & B = foci

  20. 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.

  21. APPROXIMATION ??

  22. Parabolic Reflector

  23. ParabolicReceiver

  24. What about REAL Rooms???

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

  26. Another Factor RESONANCE

  27. Resonance Examples

  28. Speakers?

  29. 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.

  30. A different phenomonon DIFFRACTION

  31. 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.

  32. A Bad Photo .. sorry ploop

  33. Huygen's Principle 1678 Polaroid Photo

  34. 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

  35. Another View

  36. A Slit (Window)

  37. Diffraction Through a SMALL Opening (comparable to l) DIFFRACTION

  38. An Edge

  39. 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.

  40. 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.

  41. We discussed Reflections

  42. What Do You Think?

  43. Or a school performance hall

  44. Professional Concert Hall(mucho Dolleros )

  45. Surfaces

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