outdoor noise barriers: design and · pdf fileoutdoor noise barriers: design and applications...
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Noise Barriers
Outdoor Noise Barriers:Design and ApplicationsTom Paige, P.Eng.Products Application EngineerKinetics Noise Control, Inc.Mississauga, [email protected]
Return to www.enoisecontrol.com
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Topics
• Barrier Fundamentals• Attenuation Calculations• Practical Limitations• Design Considerations• Structural Issues• Case Studies
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Barrier Fundamentals
• A sound barrier is a solid structure that intercepts thedirect sound path from a sound source to a receiver.
• It reduces the sound pressurelevel within its shadow zone.
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Barrier Fundamentals
THE EFFECT OF A BARRIER ON SOUND WAVES
BARRIER
BRIGHT ZONESOUND W
AVE
REFLECTEDSOURCE RECEIVER
STRAIGHT PATH
DIFFRACTED PATH
SHADOW ZONE
TRANSMITTED
2
0
3
1
4
5
6 DE
CIB
EL
S O
F N
OIS
E R
ED
UC
TIO
N
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Typical Noise Barrier Wall
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Fundamental Issues
• How high does the barrier have to be?
• How close should the barrier beto the equipment?
• How much sound attenuation will we get from the barrier?
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Barrier Attenuation
• Based on mathematics involvingFresnel integrals (borrowed fromoptical diffraction theory).
• The theory agrees well with measurements.
• Tables and graphs are availablefor practical use.
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Path Length Difference
• PLD is determined from basic geometry.
• Affected by barrier height and location.
• Also affected by source and receiver heights.
• A larger PLD will result in higherattenuation.
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Path Length Difference
5 ft 5 ft
A = 6.4 ft
B = 5.1 ft
D = 10.4 ft
POSITION 1: PLD = A + B - D = 1.1 ft
SOURCE
RECEIVER
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Path Length Difference
1 ft 9 ft
A =
4.1
ft
B = 9.1 ft
D = 10.4 ft
POSITION 2: PLD = A + B - D = 2.8 ft
SOURCE
RECEIVER
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INSERTION LOSS VALUES FOR AN IDEAL SOLID BARRIERINSERTION LOSS VALUES FOR AN IDEAL SOLID BARRIER
Path-LengthDifference, ft 31 63 125 250 500 1000 2000 4000
0.01
0.02
0.05
0.1
0.2
0.5
1.0
2.0
5.0
10.0
20.0
50.0
5
5
5
5
5
6
7
8
10
12
15
18
5
5
5
5
5
7
8
10
12
15
18
20
5
5
5
5
6
9
10
12
14
17
20
23
5
5
5
6
8
10
12
14
17
20
22
24
5
5
6
7
9
12
14
17
20
22
23
24
6
6
7
9
11
15
17
20
22
23
24
24
7
8
9
11
13
18
20
22
23
24
24
24
8
9
10
13
16
20
22
23
24
24
24
24
Insertion Loss, dB
Octave Band Center Frequency, Hz
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Barrier Attenuation
• High frequencies are attenuatedmore effectively than low frequencies.
• The maximum theoretical limit for barrier attenuation is 24 dB.
• The actual attenuation will always be less due to practical limitations.
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Attenuation WorksheetSound Barrier Attenuation WorksheetSource: Return air intake louvreReceiver: Second floor bedroom window at 100 feet distance (30 m)
S/R distance - d (m) 106S/B distance - Dsb (m) 6B/R distance - Dbr (m) 100Source height - Hs (m) 33Receiver height - Hr (m) 15Barrier height - h (m) 36PLD (m) 2.9
Octave-Band Centre Frequency (Hz) 63 125 250 500 1000 2000 4000 8000 dBA
Return fan discharge PWL (dB) 94 94 92 92 94 94 90 86 99Adjustment for 2 similar fans (dB) 3 3 3 3 3 3 3 3Plenum attenuation (dB) 3 3 3 3 3 3 3 3Directivity index on axis (dB) 8 8 8 8 8 8 8 8Distance attenuation (dB) 41 41 41 41 41 41 41 41Receiver noise level - no barrier (dB) 61 61 59 59 61 61 57 53 66
Fresnel Number - N 1.058 2.100 4.200 8.399 16.799 33.598 67.196 134.391Barrier attenuation (dB) 8 10 12 15 18 20 22 23Receiver noise level - with barrier (dB) 53 51 47 44 43 41 35 30 48
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Practical Limitations
• Barrier attenuation is generally limited to 10 to 15 dBA.
• There are usually practical limitson barrier height and width.
• Sound flanking around barrier edges.
• Reflections from near-by objects.
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Reflecting Surfaces
REFLECTION FROM WALL BEHIND BARRIER
BUILDING
TO RECEIVERSOURCE
BARRIER
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Reflecting Surfaces
REFLECTION FROM TREES OVER TOP OF BARRIER
TO RECEIVER
SOURCE
BARRIERTREES
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Reflecting Surfaces
REFLECTIONS AROUND ENDS OF BARRIER
TO RECEIVER
SOURCE
BARRIER
TREES
BUILDING
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Design Issues
• Materials to be Used• Sound Absorption• Equipment Accessibility• Equipment Ventilation• Structural Issues• General Design Guidelines
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Materials
• Barriers should be constructed of solid, non-porous material.
• Minimum density of material should be 20 kg/m2 (4 lb/ft2).
• Sound transmission loss of barrier material must be at least10 dB higher than the barrier attenuation.
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Materials
• Useful materials include steel,pre-cast concrete, wood and composition boards.
• Steel panels are particularly suitable for mechanical equipment noise barriers (easy to cut holes and openings for pipes and duct penetrations).
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Sound Absorption
• Sound absorbing material is often used on the source side ofthe barrier to reduce the buildupof sound pressure level.
• Prevents sound reflection from barrier surface.
• Improves overall acoustic performance of barrier system.
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Equipment Access
• Doors and access panels can be provided in barrier walls.
• Adequate clearance must be maintained between barrier andequipment.
• Follow mechanical equipment supplier’s recommendations.
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Equipment Ventilation
• Follow mechanical equipment supplier’s recommendations forventilation requirements.
• Acoustic louvers or silencers canbe provided on ventilation openings in barrier walls.
• It may be necessary to provide openings at the base of the barrier.
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General Design Guidelines
• The “line of sight” between the source and receiver must be cutoff completely by the barrier.
• A barrier should be at least5 times wider than it is high.
• The barrier should be built as close as possible to either the source or receiver.
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Structural Issues
• Wind Loading• Seismic Restraint • Concrete Footings• Tie-in to Existing Walls• Drainage
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Case Studies
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Rooftop Condensing Unit
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Dust Collector Fan
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Ventilation and Access
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Steel Structure
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Base Anchor Bolts
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Structural Bracing
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3-Sided Barrier
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Barrier/Enclosure