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Room Acoustics

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  1. 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. Room Acoustics

  3. Consider a Pulse of Sound WALL Reflection Sound Changes Different Travel Distances Many reflections occur at the same “time” Room Acoustics

  4. Consider the following: 270 ft 100 ft ear 200 ft source 250 ft 500 ft. Room Acoustics

  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. 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. Room Acoustics

  7. Example - Interference “Wall Wall Room Acoustics

  8. Also considerWavelengths in music Note different wavelengths and compare with the size of a room. Wavelength will be an important variable in a room. Room Acoustics

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

  10. Two surfaces Room Acoustics

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

  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. Room Acoustics

  13. Reflection SPECULAR DIFFUSE SMOOTH ROUGH Room Acoustics

  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. Room Acoustics

  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 Room Acoustics

  16. Useful aspects of reflection Think about the reverse! Room Acoustics

  17. The old Greek Amphitheater Room Acoustics

  18. Room Acoustics

  19. Closer Audience“Band Shell” Room Acoustics

  20. Room Acoustics

  21. Room Acoustics

  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! Room Acoustics

  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! Room Acoustics

  24. EXAMPLE: The Ellipse A & B = foci Room Acoustics

  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. Room Acoustics

  26. APPROXIMATION ?? Room Acoustics

  27. Room Acoustics

  28. Parabolic Reflector Room Acoustics

  29. ParabolicReceiver Room Acoustics

  30. What about REAL Rooms???

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

  32. Another Factor RESONANCE Room Acoustics

  33. Resonance Examples Room Acoustics

  34. Speakers? Room Acoustics

  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. Room Acoustics

  36. A different phenomonon DIFFRACTION Room Acoustics

  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. Room Acoustics

  38. A Bad Photo .. sorry ploop Room Acoustics

  39. Huygen's Principle 1678 Polaroid Photo Room Acoustics

  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 Room Acoustics

  41. Another View Room Acoustics

  42. A Slit (Window) Room Acoustics

  43. Diffraction Through a SMALL Opening (comparable to l) DIFFRACTION Room Acoustics

  44. Room Acoustics

  45. An Edge Room Acoustics

  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. Room Acoustics

  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. Room Acoustics

  48. We discussed Reflections Room Acoustics

  49. What Do You Think? Room Acoustics

  50. Or a school performance hall Room Acoustics