non-traditional noise and vibration mitigation … noise and vibration mitigation strategies ......
TRANSCRIPT
Non-Traditional Noise and Vibration Mitigation Strategies
Christopher Layman, Ph.D.Shannon McKennaJudy Rochat, Ph.D.
ATS Consulting
Pasadena, CA
• We can take into account the condition of the rail and vehicles operating on the system– Maintenance (rail grinding and wheel truing)
– Low-noise vehicles
• We can capitalize on elements already included in the project:
– Extending the width of the ballast to reduce noise
– Vibration reduction due to noise barrier foundation
Non-Traditional Mitigation Measures
• There could be noise and vibration reduction measures already included in the project that are not being accounted for.
Key Presentation Take-Aways
• Noise and vibration levels can rise by 10-15 dB for severely corrugated rail or wheels
• Acoustic rail grinding and wheel truing can be used to reduce noise levels
Maintenance as a Mitigation Measure
• Mitigation recommendations will depend on the reference noise level used in the analysis
• Important to document conditions of the existing system, including rail roughness
Maintenance as a Mitigation Measure
System Lmax*, dBA
FTA Reference Level 81
Sacramento 82
San Diego 75
Maintenance was recommended
as a mitigation measure
Less mitigation is required* Lmax for a 2-car train at 50 ft, 40 mph on
ballast-and-tie track
• Maintenance could also be used as a vibration mitigation measure
Maintenance as a Mitigation Measure
Up to 15 dB difference in Force Density Level before and after re-profilingSource: Wilson, Ihrig & AssociatesNorthgate Link Extension Final Design
About 10 dB difference in vibration level before and after grindingSource: Ben Lawrence, proceedings of Acoustics 2004
• Some systems, like San Diego Trolley, have low-noise vehicles
• If you are procuring a vehicle, you can request a low-noise vehicle. Features may include:
– Wheel skirts
– Under-car absorption
– Others?
Specify a Low-Noise Vehicle
• Ballast is a sound absorbing material
• Can use a simple reflection model to determine the region of influence affected by ballast
Extending Ballast
Region of Influence
Source
Absorptive ground
• Can measure effective flow resistivity to quantify absorption
• Can model ground absorption in TNM, CadnaA, or SoundPlan
Extending Ballast: Predicting Effects
Width of ballast
beyond ties (ft)
Estimated extent
of influence*
Reduction*
3 52 ft ~1 dB
5 64 ft ~1.5 dB
10 94 ft ~2 dB
*Assumes receiver is 5 ft above ground level
Extending Ballast: Example Project
Source
Extra Ballast
Existing noise (dBA)
Allowable increase due to
project (dB)
Predicted noise (dBA)
Predicted increase (dB)
Predicted increase including ballast
effect (dB)
58 2.4 61.8 3.8 2.3*
*Assumes 1.5 dB decrease due to ballast
Ballast with Berm
• Ballast with small berm next to tracks can provide 10 dB reduction (Attenborough, Inter-Noise 2005)
Note: 38 cm = 1.2 ft, 110 cm = 3.6 ft, 530 cm = 17.4 ft
• Train vibrations measured on BART at the Walnut Creek/Pleasant Hill region.
• Wayside vibration measured with (2012) and without (2005) sound wall in place.
Vibration Mitigation with Sound Walls
B&T Track on
Embankment
• Measured Train Vibration at Two Similar Locations
Difference in Measured Vibration
-10.0
-5.0
0.0
5.0
10.0
15.0
20.0
6.3 8 10 12.5 16 20 25 31.5 40 50 63 80 100 125 160
dB
1/3 Octave Band Center Frequency, Hz
Site 3/5
Site 4/6
Sound Wall Insertion Loss
Site 4/5
Site 3/6
Stiff Foundations Block Waves[…usually]
What Have Others Found?
Jet Grouting Wall
cement–bentonite
mixtures
Track
El Realengo, Spain
Measured Insertion Loss (Red Curve)
Soil Dynamics and Earthquake Engineering 77 (2015) 238–253
50 ft
10m 14m
24m 32m
• Modeling Approach
– 3D Finite Element – Solid Mechanics
– Ignores the track and moving vehicle
– Random point sources are located at axle locations of a typical 3-car consists.
– Frequency domain – linear analysis
Can We Simulate Our Measurements?
Modeling Geometry and Details
PML (grey regions)
Soil
Concrete
Sound Wall
Soil
Probe
This entire face
is Symmetry
Vertical Insertion Loss
20*log10(|u_zref|/|u_zwall|)
• Moderately Stiff Soil
• Geometry and mesh scales with
wavelength
• Hysteretic damping throughout
• Given measurement locations and
frequencies of interest, model is limited
to f <= 70 Hz
Insertion Loss Results
Data
Model
(115ft)
• Model qualitatively agrees with measurements.
• Agrees better with farther measurement(?)
• Does embankment area have softer soil?
• Is there a coincidence frequency effect?
Spatial Patterns of Vertical Displacement
Wall+Embankment
Embankment
Sources
50 Hz
• We don’t see significant amplification.
• General trend: moderate distances from
track have highest IL
Sensitivity To Wall and Embankment Geometry
• Reducing Embankment Height
• Embankment has little effect on IL
• Reducing Foundation Depth
• Stronger dependence on foundation depth
• Take into account the noise and vibration conditions of the existing system during environmental assessments
• Identify any noise and vibration reduction measures already included in the project:
– Extending the width of ballast could provide a noise benefit
– Properly designed sound walls may reduce vibration impacts significantly. However, the optimal range appears narrow.
Conclusions
THANK YOU!
END
Modeling Parameters
• Noise and vibration can be mitigated at the:
– Source
– Path
– Receiver
Background
Noise propagation through air:
Vibration propagation through soil:
• Rail dampers are tuned to absorb specific vibration frequencies which reduces the amount of noise radiated by the rail
• Measurements at SacRT show reduction in wayside noise levels by 2-3 dB
• Rail dampers can be used where sound walls are not feasible
Rail Dampers
• A study at BART show rail dampers reduce noise over time and limit corrugation growth
• Rail dampers are easy to add to existing track
Rail Dampers
y = 1.49x + 88.2
y = 0.25x + 86.5
R² = 0.96
86.0
88.0
90.0
92.0
94.0
96.0
98.0
100.0
102.0
104.0
0 10 20 30
12
th O
Ct.
Co
rru
ga
tio
n (
dB
A)
Months after Grinding
Growth of Corrugation Noise around
450 Hz
No Rail Dampers
Rail Dampers
Linear (No Rail
Dampers)
Linear (Rail Dampers)
Date Difference*
Apr 2013 5.8
Oct 2013 2.7
Apr 2014 1.7
Sept 2014 3.0
Apr 2015 3.3
Oct 2015 2.1
* Difference in noise level for a track section
without dampers minus a section with dampers