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Room acoustics

sound absorption,difraction

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Room acoustics

  1. 1. Room AcousticsRoom Acoustics Bouncing ARoundBouncing ARound
  2. 2. soundsound Travels in straight Lines.Travels in straight Lines. Travels in crooked lines.Travels in crooked lines. Can be focused.Can be focused. Can be absorbed by a surfaceCan be absorbed by a surface Can be diffractedCan be diffracted Can interfere “with itself”Can interfere “with itself” Is dependent on the properties of the room.Is dependent on the properties of the room.
  3. 3. sound ABsoRptionsound ABsoRption Acoustic absorption or sound absorption refers to the process by which a material, structure, or object takes in sound energy when sound waves are encountered, as opposed to reflecting the energy. Part of the absorbed energy is transformed into heat and part is transmitted through the absorbing body. The energy transformed into heat is said to have been 'lost'.
  4. 4. diffRActiondiffRAction Sound can “bend” around objects.Sound can “bend” around objects. Sound can change its properties dependingSound can change its properties depending upon the size of the wavelength comparedupon the size of the wavelength compared to objects.to objects. The Diffraction effect can be understoodThe Diffraction effect can be understood via one of the early theories of waves.via one of the early theories of waves.
  5. 5. Listen to a tone and move yourListen to a tone and move your head from side to side.head from side to side. What do you hear?What do you hear? Why do you hear it??Why do you hear it?? ExpERimEntExpERimEnt
  6. 6. movE youR hEAdmovE youR hEAd Different Distances WALLWALL
  7. 7. Two surfacesTwo surfaces
  8. 8. considER A WAllconsidER A WAll How smooth is it?How smooth is it? Smooth is in the feel of the feeler!Smooth is in the feel of the feeler! Smooth or Rough are Relative terms.Smooth or Rough are Relative terms. We define:We define: – SMOOTH – Variations occur on a scale muchSMOOTH – Variations occur on a scale much smaller than a wavelength of the sound we aresmaller than a wavelength of the sound we are considering.considering. – ROUGH – The variations in the surface areROUGH – The variations in the surface are comparable to the size of the wavelength.comparable to the size of the wavelength.
  9. 9. SMOOTH ROUGH spEculAR diffusE
  10. 10. soft WAllssoft WAlls A soft wall (like rubber or cork) will yield whenA soft wall (like rubber or cork) will yield when you push on it.you push on it. Sound (music) pressure pushes on the wall.Sound (music) pressure pushes on the wall. IF the wall deforms, than a force (pA) times aIF the wall deforms, than a force (pA) times a distance (the deformation), means that the wavedistance (the deformation), means that the wave does WORK.does WORK. The sound therefore loses some energy when itThe sound therefore loses some energy when it hits such a wall.hits such a wall. The reflection isn’t as strong as one from an “un-The reflection isn’t as strong as one from an “un- yielding” wall.yielding” wall.
  11. 11. considER An outdooRconsidER An outdooR concERtconcERt Musicians on stageMusicians on stage People in the audiencePeople in the audience No Walls or CeilingsNo Walls or Ceilings Only reflections possible are fromOnly reflections possible are from structures in back of the musicians.structures in back of the musicians.
  12. 12. usEful AspEcts ofusEful AspEcts of REflEctionREflEction Think about the reverse!
  13. 13. The old GreekThe old Greek AmphiTheATerAmphiTheATer
  14. 14. Closer AudienCeCloser AudienCe “BAnd shell”“BAnd shell”
  15. 15. WhAT does “foCus” meAnWhAT does “foCus” meAn Sound waves hit a surface which can be called aSound waves hit a surface which can be called a mirror.mirror. The mirror surface can be curved so that rays ofThe mirror surface can be curved so that rays of sound from different directions can be made tosound from different directions can be made to come together at the same place.come together at the same place. – Like a lensLike a lens In a concert hall, too much focusing can alsoIn a concert hall, too much focusing can also mean that there is only ONE good seat in themean that there is only ONE good seat in the house!house!
  16. 16. eXAmple: The ellipseeXAmple: The ellipse A & B = foci
  17. 17. WhisperinG GAlleryWhisperinG 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.
  18. 18. ApproXimATion ??ApproXimATion ??
  19. 19. pArABoliC refleCTorpArABoliC refleCTor
  20. 20. pArABoliCpArABoliC reCeiverreCeiver
  21. 21. Another OneAnother One
  22. 22. An interesting ApplicAtionAn interesting ApplicAtion With this device, you can magnify faint or distant sounds with a clarity you never thought possible.
  23. 23. WhAt About reAlWhAt About reAl rooms???rooms???
  24. 24. in A reAl roomin A reAl room What about theWhat about the walls?walls? SmoothSmooth – How Smooth?How Smooth? RoughRough – How Rough?How Rough?
  25. 25. WhAt else?WhAt else? Small objects will scatter or diffract sound so itSmall objects will scatter or diffract sound so it can be heard in non-straight lines.can be heard in non-straight lines. – Around edges, etc.Around edges, etc. Small objects do very little to long wavelengthSmall objects do very little to long wavelength sounds (low tones). They are like the eEverreadysounds (low tones). They are like the eEverready Battery … they keep going and going and goingBattery … they keep going and going and going …..….. Higher frequency sounds will be deflected orHigher frequency sounds will be deflected or absorbed more than low frequency sounds.absorbed more than low frequency sounds.
  26. 26. We discussed reflectionsWe discussed reflections
  27. 27. A School Performance HallA School Performance Hall
  28. 28. Professional Concert HallProfessional Concert Hall
  29. 29. Types of SurfacesTypes of Surfaces
  30. 30. Room ReflectionsRoom Reflections Room full of sound!
  31. 31. 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.
  32. 32. Listen to the Room!Listen to the Room!
  33. 33. Lets start a musical tone and listen toLets start a musical tone and listen to the auditorium with a sound recorder.the auditorium with a sound recorder.
  34. 34. How about tHe return toHow about tHe return to silence?silence? There is a steady musical sound in theThere is a steady musical sound in the auditorium.auditorium. The symphony is over.The symphony is over. The music suddenly stops. It takes aThe music suddenly stops. It takes a certain time for the sound level to get to acertain time for the sound level to get to a very small level.very small level. The time it takes for the auditorium soundThe time it takes for the auditorium sound to drop to 1/1,000,000to drop to 1/1,000,000thth of the steady level isof the steady level is called thecalled the REVERREVERBERATION TIMEBERATION TIME..
  35. 35. tHe return to PeacetHe return to Peace Reverberation Time
  36. 36. absorbing Materialsabsorbing MaterialsMoreAbsorbing
  37. 37. Sabine’s FormulaSabine’s Formula       = surfacesabsorbingallofareaeffective"" roomtheofVolumex0.16 ion timereverberat
  38. 38. resonantresonant Panels/absorbersPanels/absorbers A resonant absorber is a vibrational system that “runs” on sound pressure. As vibrational science will tell us a resonant absorber is a mass vibrating against a spring. The mass is the cabinet and front wall or diaphragm. The spring is the air inside the cavity of the resonant absorber.
  39. 39. resonantresonant Panels/absorbersPanels/absorbers If you change the vibrating mass and stiffness of the spring, you can control and tune the resonant absorber to the resonant frequency of choice. The internal mass or cabinet depth determines design frequency. The spring or internal air and cavity are used for achieving rate of absorption above the unit’s designed for resonant frequency. There are two types of resonant absorbers: Helmholtz and Diaphragmatic or membrane.
  40. 40. ResonantResonant Panels/AbsorbersPanels/Absorbers
  41. 41. tHank YoutHank You

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