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PHYS 105

PHYS 105 Auditorium Acoustics Dr. James van Howe Lecture 20 Symphony Hall, Boston Who is the Artist? Modest Mouse Flaming Lips Radio Head Shins True or False Reverberation times over 5 seconds muddies the sound too much in a concert hall (not very useful for most kinds of music)

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PHYS 105

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  1. PHYS 105 Auditorium Acoustics Dr. James van Howe Lecture 20 Symphony Hall, Boston

  2. Who is the Artist? • Modest Mouse • Flaming Lips • Radio Head • Shins

  3. True or False Reverberation times over 5 seconds muddies the sound too much in a concert hall (not very useful for most kinds of music)

  4. Which hall would you guess has better acoustics? A B

  5. Greater and closer together Fewer and farther apart As a room gets bigger, room resonances become…

  6. Sample Problem What is for the concert hall below (shoebox design) at 500 Hz? Walls: plaster on lath; a=0.06 Ceiling: Acoustic Tile; a=0.83 Floor: Carpet on pad; a=0.57 10 m 15 m 25 m

  7. Sample Problem cont. I just designed a very “dry” hall; a good lecture hall but bad for music

  8. What is for the modified concert hall below? Walls: plaster on lath; a=0.06 Ceiling: plaster on lath; a=0.06 Floor: Carpet on pad; a=0.57 10 m 15 m 25 m If I changed my ceiling to plaster rather than acoustical tile, just part 3 of drain changes: Much better hall, good for music and speaking!

  9. Ideal reverb time for size of room

  10. What makes a good concert hall? Study of 22 European concert halls • Long reverberation up to 2.0 s; however > 2.0 s, in general is less pleasing • Narrow halls -early reflection from side walls very important (concert halls good acoustically often shoebox design) -drawback is you can’t see performance as well; often sacrifice sound for ability to see performance (fan-out design) • High binaural dissimilarity (one ear hears something slightly different than other) • Less definition: sound energy in the first 50 ms lower in good halls

  11. Things to avoid in auditorium design • Echoes -usually culprit is the rear wall • Flutter Echoes -rapid series of echoes from two hard parallel surfaces • Sound focusing -curved walls (concave) focus sound causing shadows in some spots and loud spots in other locations • Sound shadows -often occur under balconies because reflections are blocked • Background noise -Heating and cooling systems usually culprit

  12. Shower Modes Resonant modes of a box H Shower strengthens the specific frequencies given by fl,m,n and kills the others W L Example: Tahiti Sand Shower Stall from Home Depot

  13. Frequency Response Concert Hall Frequency Response Shower discrete spikes continuous Amplitude Amplitude Frequency Frequency Holes Why are Shower Modes Bad? -The room acts as a filter that takes away frequencies you might want! -Not so good for your home theatre or recording studio Why doesn’t this happen in a Concert Hall? -It does happen in rectangular halls, but since the volume of the hall is so big it really isn’t noticeable

  14. Ways to avoid strong room resonances • For rectangular room, make or choose one whose ratio of dimensions is non-integer • Make the room irregular shape (avoid the rectangle) -crucial for recording studio • Electronically compensate • Spread sound with sound diffusers -crucial for recording studios

  15. Non-integer ratios A room whose dimensions (L x W x H) give a ratio of 5:3:2 has a more continuous frequency response than 1:2:3 This is because for integer ratios, many fl,m,nare the same number (degenerate), making these peaks stronger in the response curve A cube has a ratio of 1:1:1 is the worst possible choice If you want your home theatre to sound the best, put it in a room whose dimensions are way off from integer ratios

  16. Irregularly shaped rooms, walls, surfaces -Lack of parallel surfaces reduces constructive and destructive interference that leads to box resonances -Irregular surfaces help smooth out spectral response -Also eliminates flutter echoes Recording Studio Schematic Utusan Malaysia Irregularly shaped reflecting surfaces

  17. Electronically compensate Intelligent sound field tuning system for home theater systems IEEE Transactions on Consumer Electronics 51, pg 635 – 639, 2005. System responds to acoustics of room by adding/subtracting the appropriate sound field, like very fancy noise canceling headphones Can make the room sound less like a box without changing the structure of the room!

  18. Sound diffusion -Same reasoning behind irregular shape of room -Want sound to spread in many directions to avoid resonance -Additional effect is to allow sound to “reverberate” and mix Sound diffusers are angular shapes or convex; often rough surfaces

  19. One of the better concert halls in the U.S Why? What do you notice? Symphony Hall, Boston

  20. True or False Reverberation times over 5 seconds muddies the sound too much in a concert hall (not very useful for most kinds of music)

  21. Which hall would you guess has better acoustics? A B

  22. Greater and closer together Fewer and farther apart As a room gets bigger, room resonances become…

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