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Noise Attenuation Provided by Highway Noise Barriers Email: [email protected]
Lecture 10: Noise Attenuation Provided by Highway Noise Barriers
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Presentation Overview
n Definitions of barrier attenuation
n Performance of noise barriers
n Design considerations and materials
n Examples of noise barriers
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What is a noise barrier?
- Any form of solid obstacle between source and receiver can comprise a noise barrier
- Unlike building insulation, noise barriers are designed to protect the external as well as the internal environment at a dwelling
- The majority of barriers are installed in the vicinity of transportation and industrial noise sources to shield nearby residential properties
- Noise barriers are cost effective only for the protection of large areas including several buildings and are rarely used for the protection of individual properties
- Noise barriers of usual height are generally ineffective in protecting the upper levels of multistorey dwellings
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What these barriers look like?
Absorbing lining
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Are noise barriers subjectively effective?
0
1
2
3
4
5
6
7
8
9
54 64 74 84
A-weighted SPL (LAeq dBA)
Ave
rag
e n
ois
ines
s ra
ting
Barrier No barrier
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Objective Performance of Noise Barriers
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Definition
2
10 20
10log ,bpIL dB
p
= −
sound pressure at a receiver in absence of the barrier
sound pressure at a receiver behind the barrier
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Basic Prediction Method
A common prediction method of noise barrier performance is outlined in ISO 9613-2
12 2
3
5 1 51
3C
w wλ λ
− = + +
double diffraction term
receiversource
rr
w
d 0r
sr
receiversource
rrsr
0r
3 1C = single noise barrier
insertion loss
10 310log 3 20 metIL C Kδλ
= +
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Other Prediction Methods
Theoreticaln Keller J. B. J. Opt. Soc. Am., 1962n Embleton T.F.W., J. Acoust. Soc. Am. 1980n Hadden W.J. and Pierce A.D., J. Acoust. Soc. Am. 1981
Empiricaln Maekawa Z., Appl. Acoust. 1968n Tatge R. B., J. Acoust. Soc. Am. 1973
Numericaln Hothersall D.C. and Chandler-Wilde S. N., J. Sound Vib. 1991n Duhamel D., J. Sound Vib. 1996
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Boundary Element Method
Requires the discretization of the boundary, Γ, with the step / 5s λ∆ <
r0r
acoustic surface admittanceGreen's function
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Nomograms: single barrier
receiversource
noise barrier
rrsr
0r
01 rrr rs −+=δPath difference,
H
practical limit
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Nomograms: double or wide barrier
01 rwrr rs −++=δPath difference,
receiversource
rr
mw 0.1=
d0r
sr
practical limit
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Propagation effects
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Maximum performance: 20 dB (single), 25dB (double)
Turbulence scattering effect
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Refraction
Atmospheric refraction effect
wind speed
heig
ht
wind direction
shadow zone
ground
sound rays
Receiversource
NB
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Combined atmospheric effects: light wind
Measured and predicted excess attenuation at a receiver 18m from 2.44m wide, 2.55m high barrier showing the effect of atmospheric turbulence.
Source is 8m from barrier (by J. Forssen, Appl. Acoust. 2002)
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Measured and predicted excess attenuation at a receiver 18m from 2.44m wide, 2.55m high barrier showing the effect of atmospheric turbulence.
Source is 8m from barrier (by J. Forssen, Appl. Acoust. 2002)
Combined atmospheric effects: strong wind
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Ground effect
piknp
β=∂∂
' ' '( , )d dx y=dr
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Effect of foliage
Scattering by branchesImproved flow stability and reduced turbulence
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Noise Barrier Design
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Barrier Length
The length of a noise barrier should considerably exceed the barrier height.
LR80θ ≥ o
L4
R≥
NB
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Transmission of Sound through Barrier
Noise contribution transmitted through the barrier material should be insignificant in comparison with the diffracted sound, i.e.
According to the ISO 9613-2
Range of materials from 4.5 kg/m2 to 244 kg/m2
10 dBd tL L− ≥
210 kg/msρ >
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Categories for airborne sound insulation (EN 1793-3:1997)
Category DLR , dB
B0 Not determined
B1 < 15
B2 15 to 24
B3 > 25
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Material Absorption
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Categories of absorptive performance defined in EN 1793-1
Category DLa , dB
A0 Not determined
A1 <4
A2 4 to 7
A3 8 – 11
A4 > 11
It is generally accepted that noise barriers with high surface absorption offer superior performance only when the distance between a noise barrier and the source is less or comparable to the barrier height.
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Effect of barrier shape
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Modification to barrier top edge
Noise Attenuation Provided by Highway Noise Barriers Email: [email protected] 31∆L (dB) is given here relative to a 3m high plane screen NB
Ishi
zuka
and
Fuj
iwar
a, A
ppl.
Acou
st. 20
04
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Modification to Barrier Profile
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Noise barrier materials
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Concrete noise barriers
Concrete wall with transparent elementsSolid concrete wall
Concrete barrier blended with plants
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Timber noise barriers
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Transparent noise barriers
acrylic panes
glass panes
composite barrier
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Barrier with absorbing lining
Transparent barrier
Transparent noise barriers
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Metal sheet noise barriersMetal barrier with aestheticstructural elements
Perforated absorbing panels
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Other types of noise barriers
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Earth berms vs Noise barriers
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Eco-designs
Based on the combination of a structural frame, earth wall and living plants
Height limited to 4-6m
Fit well into urban, sub-urban or rural areas
Most of bio-barriers require irrigation and general maintenance
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Eco-designs - Examples
Wooven-willowPlanting in coated steel pockets
Planting over concrete pipes
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Eco-designs – Wooven-willow
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How barriers are installedEarth work
Concrete work
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Good reference
Kotzen, B. and English C., (1999)Environmental Noise Barriers – A Guide to their Acoustic and Visual Design, E&Fn Spon, London and New York.