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L 18 Auditorium and Room Acoustics

L 18 Auditorium and Room Acoustics. Dekelbaum Concert Hall at the U MD Smith Center. Paths of sound in room. Intensity vs. time for pulse.

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L 18 Auditorium and Room Acoustics

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  1. L 18Auditorium and Room Acoustics

  2. Dekelbaum Concert Hallat the U MDSmith Center

  3. Paths of sound in room

  4. Intensity vs. time for pulse

  5. We define the reverberation time T as the time the sound intensity takes to decay to one millionth of its original steady-state intensity.This is the time taken the sound level to decay by 10 log(1,000,000)= 60 db

  6. Average Intensity vs. Time for a constant tone

  7. To calculate T, consider a room with a hole in one wall of area A.Call the reverberation time T.T ˜ volume V, 1/AT= K V/AIt has been worked out that, for V in m3 , A in m2 T= 0.16 V/A

  8. Let us now replace the open window area with an absorbing material of area S and absorption coefficient a.Then A= Sa. If there is more than one type of absorbing material, the A= S1 a1+S2a2 +S3a3+…If you prefer, we can writeT= 0.16 V/(ΣSiai)

  9. We call this new “area” Athe “total room absorption”, measured in sabins (same unit as area, but a somewhat different meaning)

  10. If we wish to calculate T, we need to calculate the total room absorption- from the walls, floor, ceiling, furniture, drapes, people, etc.

  11. An empty room with totally reflecting walls, ceiling, floors, etc. has aa) shorter T than one (empty) of comparable dimensions with perfectly absorbing walls, ceiling, floors, etc.b) shorter T than one(empty) of comparable dimensions with open windowsc) shorter T than one (empty) of comparable dimensions with no windows and with carpets covering the wallsd) a longer T than one of comparable dimensions with perfectly reflecting walls , but with people inside.

  12. Examples of Reverberation Times One way to respond to the question "What is a good range of reverberation times for concert halls?" is to give examples of some of the most famous halls in the world. For the overall average reverberation times: Vienna, Musikvereinsaal : 2.05 seconds Boston, Symphony Hall: 1.8 seconds New York, Carnegie Hall: 1.7 seconds But the overall average reverberation time does not tell the whole story. The variation of reverberation time with frequency is also important.

  13. Ideal Reverberation Times

  14. Search youtube for Notre Dame de Paris organ (try a Bach piece)

  15. Intimacy Room said to be “intimate” when the first reverberation arrives within 20 ms of the direct sound.

  16. Fullness vs. Clarity Refers to the amount of reflected sound relative to the amount of direct sound

  17. Warmth vs. Brilliance Warmth increases with increasing T for low frequencies

  18. Acoustical Design Problems • Focusing of sound • Echoes • Shadows • Resonances • External noise • Double-valued T

  19. Focusing of Sound Occurs with use of parabolic surfaces either behind performers or at rear of auditorium

  20. Echoes Highly reflective flat or parabolic wall shapes Flutter echos from parallel walls Standing waves between parallel walls

  21. Resonances Parallel walls (flutter) Rectangular practice rooms Singing in the shower

  22. Double-valued T Playback room with reverberation Concert halls with side areas

  23. Auditorium Design

  24. General Design Considerations • Visual • Ventilation • Acoustical • a. seating • b. stage • c. room shape • d. room walls

  25. Absorptions (in sabins) Frequency (Hz) Material 125 250 500 1000 2000 Unupholstered seat 0.15 0.22 0.25 0.28 0.50 Upholstered seat 3.0 3.1 3.1 3.2 3.4 Adult person 2.5 3.5 4.2 4.6 5.0 Adult/upholstered seat 3.0 3.8 4.5 5.0 5.2

  26. Some Absorption Coefficients Frequency (Hz) Material 125 250 500 1000 2000 4000 Concrete/brick 0.01 0.01 0.02 0.02 0.02 0.03 Glass 0.19 0.08 0.06 0.04 0.03 0.02 Plasterboard 0.20 0.15 0.10 0.08 0.04 0.02 Plywood 0.45 0.25 0.13 0.11 0.10 0.09 Carpet 0.10 0.20 0.30 0.35 0.50 0.60 Curtains 0.05 0.12 0.25 0.35 0.40 0.45 Acoustical board 0.25 0.45 0.80 0.90 0.90 0.90

  27. Control of TRLet’s call it T T = 0.16 V / A Where: T= reverberation time in seconds V = room volume in cubic feet A = total room absorption in sabins

  28. Estimate the reverberation time of this lecture hall

  29. Should there be an enforced control of sound level in listening devices and public performance venues?a) yesb) no

  30. CNN ping refraction or total internal reflectionhttp://www.cnn.com/2014/04/11/tech/innovation/mh-370-underwater-sound/index.html?hpt=hp_t1

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