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PHYS 105

PHYS 105 Auditorium Acoustics Dr. James van Howe Lecture 20 Symphony Hall, Boston When did this song hit the charts, catapulting a new genre to mainstream listening? 1990 1991 1992 1993 Doppler Effect Wavelength gets shorter, Frequency lower Wavelength gets shorter, Frequency higher

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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. When did this song hit the charts, catapulting a new genre to mainstream listening? • 1990 • 1991 • 1992 • 1993

  3. Doppler Effect • Wavelength gets shorter, Frequency lower • Wavelength gets shorter, Frequency higher • Wavelength gets longer, Frequency lower • Wavelength gets longer, Frequency higher Stationary Christian Doppler The truck is moving to the left, how does Doppler think the wavelength and frequency change?

  4. 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)

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

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

  7. Free Field: Point Source -As sound spreads out, intensity gets weaker -Power is the same at each shell -So as shell gets bigger, power spreads out over larger sphere -Intensity is Inverse square Law Power at different times; bigger sphere is later time

  8. Free Field: Point Source Two ways of showing that expanding sphere on a graph 40 10000 6 dB Log Scale 35 8000 6 dB 30 6000 Intensity (dB) Intensity (Arbitrary Units) 25 4000 20 2000 15 x 2 x 2 10 0 -1 0 0.05 0.1 0.15 0.2 0.25 .04 .02 10 .01 Distance from Source (m) Distance from Source (m) Every time you double the distance, you loose four times the sound, or 6 dB

  9. Direct Sound If the sound source radiates like a point source, only the distance form the source determines sound level (6 dB less for every doubling) Not true of many brass instruments Overhead view of a trumpet radiating like a quadrupole not point source

  10. Multiple Reflections • Direct Sound: no reflection (typically 20-200 ms) • Early Sound: first group of reflections (50-80 ms) • Reverberant Sound: thick bunch of reflections In a room we get multiple reflections from speaker to listener: These three types of sound are crucial for determining the quality of a concert hall

  11. Early Sound (first group of reflections) -Multiple reflections of sound will trick the ear • Early sound arriving within 50-80 ms after direct sound is indistinguishable from the direct sound • Later arrival time of early sound = Echo -First reflection very important for “spatial impression” • If first reflection is less than 20 ms, “intimate” feel • First reflection from sides of concert hall (lateral reflection) found to be very important to quality of hall

  12. Reverberant Sound (Liveness) Reverberation time: The time it takes for the sound energy in a room to decay by 30 dB intensity (60 dB pressure)

  13. Calculation of Reverberation Time Bathtub analogy: The time it takes to drain a bathtub depends on how much water in tub and how large of a drain Drain time Volume of water Area of drain Drain time Reverb time is the time it takes to drain the sound out of a room.

  14. Enhanced Area Surface area of the drain includes walls, ceiling, floor, people, chairs, etc. The walls, ceiling, and other objects don’t just absorb all of the sound like a drain, but reflect some back into the room By using absorption coefficients, we can find how much of the area of the walls, etc, act like a perfect drain Note that perfect absorption corresponds to , Perfect reflection is In your book (table 23.1), we find that wood floor has at 500 Hz If I have a 20 m x 10 m wood floor, or 200 m2, it’s as if I have 20 m2 of perfect drain

  15. 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

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

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