Transcript Fundamentals of Acoustics

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Environmental Noise
Architectural Acoustics II
March 17, 2008
Outline
• Barriers
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Basic insertion loss
Location relative to S and R
Edge geometry
• Source and receiver conditions
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Close vs. far
Moving vs. stationary
• Distance effects
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Ground cover
Grazing incidence
Temperature inversions
• Traffic and railroad noise
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Sources/Resources
• Long, Architectural Acoustics
• J. Foreman, Sound Analysis and Noise Control, Van Nostrand
Reinhold, 1990.
• Kurze and Anderson, “Sound Attenuation by Barriers,” Appl.
Acoust. 4, 35 (1971).
• S. Ho et al., “Noise reduction by a barrier having a random
edge profile,” JASA 101 (5) 1997.
• Z. Maekawa, ‘‘Noise reduction by screens,’’ Appl. Acoust. 1,
157, 1968.
• D. N. May and N. M. Osman, “Highway noise barriers: new
shapes,” J. Sound. Vib. 73 (1), 1980.
• Berkhoff, “Control strategies for active noise barriers using
near-field error sensing,” JASA 118 (3), 2005.
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Noise Barrier Performance
http://www.ashraeregion7.org/tc26/pastprograms/Outdoor_Noise/barriers.pdf
Math Review
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e z  e z e2 z 1
• Hyperbolic tangent: tanhz   z  z  2 z
e e
e 1
-2π
• tanh jz   tan(z)
-π
0
π
2π
Barrier Geometry
b
S = source
a
R = source
θ
d
R
a + b = shortest path from S to R
over the barrier
θ = angle between SR and the
barrier normal
S
Barrier
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d = source-to-receiver distance


2N
IL  20  log10 
  5 dB,
 tanh( 2N ) 
Fresnel Number: N 0 
2a  b  d 
for - 0.2  N  12.5
 24 dB,
N  N0  cos 
Kurze and Anderson, “Sound Attenuation by Barriers,” Appl. Acoust. 4, 35 (1971).
for N  12.5

Barrier Insertion Loss
• Assumptions
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S is a point source
No sound is transmitted through the barrier
The barrier is infinitely long
Ground reflections and other secondary propagation paths
are negligible


2N
IL  20  log10 
  5 dB,
 tanh( 2N ) 
for - 0.2  N  12.5
 24 dB,
N  N0  cos 
Kurze and Anderson, “Sound Attenuation by Barriers,” Appl. Acoust. 4, 35 (1971).
for N  12.5
Theoretical Barrier Performance for a
Point Source
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What does the
increasing trend
with N suggest
about the optimal
placement of a
barrier given the
source and receiver
positions?


2N
IL  20  log10 
  5 dB,
 tanh( 2N ) 
 24 dB,
for - 0.2  N  12.5
for N  12.5
Theoretical Barrier Performance for a
Line Source
• Integrate the point-source IL equation

R
Barrier
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Line Source
ILLine
1
 10  log10 

 /2
 10
 /2
 ILPo int
10

d 

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Comparison of Point Source and Line
Source IL
Foreman, Sound Analysis and Noise Control, Figure 4.28, p. 104.
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Comparison with Measurements
Ho et al., “Noise reduction by a barrier having a random edge profile,” JASA 101 (5) 1997.
Homework Assignment
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• Come up with a new barrier design
• Explain why you think it will out-perform a
standard barrier
• Due Thursday 3/27
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Jagged-Edge Barriers
“…the edge [at the top of a noise barrier] acts as a line source. For the
traditional straight-edge barrier, the line source is coherent. Since a crooked
line source is less coherent, we propose to improve barrier performance by
making the edge randomly jagged.”
Ho et al., “Noise reduction by a barrier having a random edge profile,” JASA 101 (5) 1997.
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Jagged-Edge Barriers
Improved performance at high frequencies, worse at low frequencies. Why?
Ho et al., “Noise reduction by a barrier having a random edge profile,” JASA 101 (5) 1997.
Other Barrier Designs
5
5
3.5
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2
X
Reported broadband increase in IL (dBA) over a straight-edge barrier
D. N. May and N. M. Osman, “Highway noise barriers: new shapes,” J. Sound. Vib. 73 (1), 1980.
Other Barrier Designs
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2.5
X
Reported broadband increase in IL (dBA) over a straight-edge barrier
D. N. May and N. M. Osman, “Highway noise barriers: new shapes,” J. Sound. Vib. 73 (1), 1980.
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Other Barrier Designs
And THNAD is for Thnadners
And oh, are they sad, oh!
The big one, you see, has the smaller one's shadow.
The shadow the small Thnadner has should be his.
I don't understand it, but that's how it is.
A terrible mix-up in shadows! Gee-Whizz!
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Absorptive Barrier Surfaces
Long, Architectural Acoustics, Figure 5.9, p. 167
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Absorptive Barrier Surfaces
Long, Architectural Acoustics, Figure 5.10, p. 168
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Active Barriers: Theory
Berkhoff, “Control strategies for active noise barriers using near-field error sensing,” JASA 118 (3), 2005.
Active Barriers: Experiment
Microphone
Array
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Sources
Barrier
Berkhoff, “Control strategies for active noise barriers using near-field error sensing,” JASA 118 (3), 2005.
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Active Barriers: Simulation Results
Berkhoff, “Control strategies for active noise barriers using near-field error sensing,” JASA 118 (3), 2005.
Environmental Effects
• Air absorption due to

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Viscosity
 Thermal conductivity
 Molecular relaxation
Sources of energy loss
Environmental Effects
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• Air absorption
Environmental Effects
• Excess attenuation in forests
1/ 3
 f   r 
L f  10
 
 dB
 1000  100
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f = frequency
r = distance through the forest
• Grazing attenuation
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Reflection of sound from a soft surface at shallow angles
(close to parallel incidence) often results in a phase shift
The reflection destructively interferes with the direct sound
to cause excess attenuation
This also occurs in concert halls with grazing incidence
sound over audience seats. The attenuation is known as
“seat dip”.
Environmental Effects
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Wind velocity
increases upward
Long, Architectural Acoustics, Figures 5.20 and 5.21, p. 178
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Truck Noise
Long, Architectural Acoustics, Figures 5.28, p. 189
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Truck Noise
Long, Architectural Acoustics, Figures 5.29, p. 190
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Car Noise
Long, Architectural Acoustics, Figures 5.30, p. 190
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Train Noise
Long, Architectural Acoustics, Figures 5.32, p. 191
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Train Noise
Long, Architectural Acoustics, Figures 5.36, p. 195
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Train Noise
Long, Architectural Acoustics, Figures 5.37, p. 195
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Aircraft Noise