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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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**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 • 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?**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)**Greater and closer together**Fewer and farther apart As a room gets bigger, room resonances become…**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**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**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**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**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**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)**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.**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**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**Sample Problem cont.**I just designed a very “dry” hall; a good lecture hall but bad for music

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