1. WAL Rail Damper for Track Noise Reduction
Wilson Acoustics Limited
Wilson Acoustics Limited

2. Contents Rail Damper Working Principles Damper Development History
WAL Rail Damper Characteristics
Manufacturing and Installation
Frequency Tuning & Rubber Selection
Laboratory Testing
Field Measurement
Wilson Acoustics Limited

3. Working Principles
Vibration bending wave propagate along the rail and radiate as noise
Track Decay Rate, dB/m
~ vibration attenuation per meter track (EN 15461:2008)
Lower track decay rate
 Longer vibration propagation distance
 Larger effective radiation surface
 Louder wayside / saloon noise
Tuned mass damper used to increase decay rate
Fn of the damper is tuned to rail vibration frequencies
 rail vibration is transfer to the damper
 dissipated by hysteresis
rail
damper
Wilson Acoustics Limited

4. Development History ↑1st Prototype (2006)
Developed after observing tonal saloon noise in MOL
Tuned Mass Damper by shear oscillation
Attach to rail by magnets
→ 2nd Prototype (2007)
Vibration absorption for both rail foot and rail web
Multiple resonance frequency
Wilson Acoustics Limited

5. Development History ← 3rd Prototype (2008) → 4th Prototype (2009)
Try to conduct test on operational rail.
Magnetic mounting is not endorsed by MTR due to signaling concerns
2 clamping clips + Wax adhesion
→ 4th Prototype (2009)
Vibration measured at operational rail indicates vibration level up to 80 g
Structural components are strengthened
Mounting mechanism is improved to provide higher and persistent mounting force.
Wilson Acoustics Limited

6. Wilson Acoustics Limited www.wal.hk
The mass is oscillating along the shear directions of the resilient layers.
provides effective vibration absorption in both vertical and lateral directions,
the noise radiating surfaces of the oscillation mass is significantly reduced,
the ratio of oscillation mass to mounting mass is maximized.
allows greater oscillation amplitude thus better vibration absorption.
Gap filling material with high viscosity and low compressibility is placed at the mounting interfaces.
Under static compressive mounting force, it behaves as flexible solid and deformed to fill up the movement gaps at the mounting interfaces.
Under high frequency dynamic train excitation force, it behaves as stiff solid which provide high adhesion force and allow vibration energy to be effectively transferred to the dampers.
Nylon nuts are screwed onto the top of conventional steel nuts.
The high thread friction prevents nut loosening after repeated train passage.
Resilient buffer layers are inserted at the mounting components to provide persistent mounting forces.
prevents structural damage of mounting components for exceptional high vibration >100g
Wilson Acoustics Limited

7. Damper Assembly
Before transportation to site for installation, damper components are assembled into 3 parts
Wilson Acoustics Limited

8. Damper Installation
10-15 dampers can be installed per hour by 1 team comprising 2 people.
Wilson Acoustics Limited

9. Each rail section has its own noise frequency
Rail damper design frequency shall be specifically tuned
Tuned for KTL curve and down-slope track section
Wilson Acoustics Limited

10. Saloon Noise Critical Frequency 315-1600Hz
Hz  Tuned Mass Damping Mechanism
The oscillation masses in each damper are tuned at specific frequencies within 300Hz to 1000Hz to absorb the vibration energy.
>1000Hz  Viscous Damping Mechanism
The damper employs a thin layer (~0.2mm) of viscous damping material applied at the rail foot bottom face to provide effective energy dissipation by viscous damping.
Wilson Acoustics Limited

11. Damper Frequency Tuning
800Hz
400 & 800Hz
315Hz
630Hz
1000Hz
where
G is dynamic shear modulus of the resilient layer
A is the surface area of resilient layer
b is the thickness of the resilient layer
M is the oscillation mass
Frequency tuning can be achieved by adjusting the shear stiffness and dimensions of resilient rubber.
Frequency
Rubber Type
Dimension
315Hz
550 Neoprene
40 x 45 x 1.25 mm
400Hz &
800Hz
50 x 55 x 1.25 mm
50 x 56 x 1 mm
630Hz
40 x 40 x 1 mm
55 x 58 x 1 mm
1000Hz
650 Neoprene
63 x 65 x 1 mm
Wilson Acoustics Limited

12. Rubber Selection Appropriate Dynamic Shear Modulus
 Affects vibration absorption frequency
Appropriate Mechanical Loss Factor
 Affects vibration absorption bandwidth
Durability to repeated dynamic stress
Aging resistance under operation conditions
Neoprene was selected.
(A common material used in trackbed isolation)
Wilson Acoustics Limited

13. Laboratory Testing
General test and detail test are conducted to check the damper resonance frequencies.
Wilson Acoustics Limited

14. Track Decay Rate Measurement
400Hz, Lateral
1000Hz, Lateral
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15. Track Decay Rate Measurement
Estimated Rail Noise Reduction 4-8 dB
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16. The End
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17. Wilson Acoustics Limited www.wal.hk
Oscillation
Mass D1 & D2
Oscillation
Mass C
Oscillation
Mass E
Oscillation
Mass B
Oscillation
Mass A
Resilient Layers
Wilson Acoustics Limited