Interface - The Journal of Wheel/Rail Interaction

MASS-SPRING SYSTEMS

Using High-Performance Mass-Spring Systems to Reduce Noise and Vibration in Track
By Hans-Georg Wagner • July, 2008

The transmission of noise and vibration associated with the close proximity of rail lines to houses and office buildings has become a significant issue in metropolitan areas. The operators of rail transit systems have begun turning to measures that mitigate noise and vibration and the complaints from city dwellers and business operators.

Among vibration-mitigating measures, the so-called mass-spring-systems represent the most efficient method of reducing vibration directly at the source. Mass-spring-systems, which use soft steel coil springs in resiliently supported concrete track slabs, provide the lowest achievable system frequencies to attenuate vibration-related dB levels. The comparatively low depths of the concrete slabs allow for small tunnel diameters, simple installation, easy adjustment of the track level and low maintenance costs. Mass-spring-systems are used in tunnels, at grade and on elevated tracks on tram, metro, freight and high-speed lines.

Noise has been shown to negatively impact a community’s quality of life. Noise-sensitive buildings, such as those that are dedicated to cultural and technical activities, are often affected by the transportation infrastructure. Railway noise and vibration from transit systems are on the rise due to increases in traffic, train speeds and axle loads. Norms and directives (such as the DIN 4150, ISO 2631, VDI 2057) provide limits for mechanical vibrations and structure-borne noise for buildings and people. These and similar standards are already enforced in many countries, and are increasingly referenced in specifications for construction or renovation of railway tracks.

The sources and transmission paths of vibrations and structure-borne noise are complex and multifaceted. Methods to mitigate or eliminate vibrations and structure-borne noise are equally complex. They are usually grouped into either active (at the source) or passive (at the receiver) isolations. Active isolation is usually the more attractive solution, since vibration isolation that occurs at the source benefits all of the surrounding areas. This type of isolation can only be used, however, when new tracks or major track modifications are planned. Passive isolation is employed when a new building is constructed adjacent to an existing railroad (see Figure 2). In this case, the building is dynamically decoupled from the ground and thereby protected from ground-borne vibration.

Solutions beyond improvements to rails and wheels, such as the elimination of rail corrugation or wheel flats, have been developed to mitigate vibration. A common solution is the use of resilient materials in rail fasteners, sleepers or ballast. Such measures can reduce noise transmission by 2 to 15 dB. Resilient materials are standard in slab track systems to provide elasticity that otherwise would be provided by the ballast. Higher isolation requirements, especially for concentrations in the excitation spectra below 80 Hz, often demand the use of mass-spring-system technology.

Mass-Spring-Systems
Mass-Spring-Systems have been used for decades on all types of permanent way to effectively mitigate vibrations. Representing a special form of rigid slab track, they generally consist of a rail bearing plate or trough out of concrete that is placed on an elastic layer. The elastic material is often made of rubber, foam or steel coil springs. The material selection depends on the degree of isolation that is required or sometimes on the mass that can be used, due to construction restrictions. The cost of the isolation layer often limits the selection, as well.

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