Modelling the effects of railway implemented low-height noise screens; an investigation of train track ballast impedance
Typ
Examensarbete för masterexamen
Master's Thesis
Master's Thesis
Program
Sound and vibration (MPSOV), MSc
Publicerad
2024
Författare
Hildén, Marius
Modellbyggare
Tidskriftstitel
ISSN
Volymtitel
Utgivare
Sammanfattning
Conventional tall noise barriers (measuring approximately 3 meters and above) are commonly
employed to mitigate noise from railways in urban environments. They are effective and their
noise reduction outcomes can be accurately estimated using existing, low-order geometrical
ray-acoustic models, e.g. Pierce’s thin hard diffracting screen solution.
However, tall noise barriers arguably have an adverse effect on surrounding landscape, as well
as obscuring the sightlines of both train operators and passengers. In cases where noise levels
can be adequately attenuated using a low-height noise screen (LHNS) it can be a preferable
implementation in regard to aesthetic, cost, and maintenance aspects.
The current problem with implementing LHNS is that their noise reduction outcomes are,
due to fundamental design, difficult to accurately estimate. This is a problem in large-scale
urban development projects where the margin of error is small, often leading to LHNS being
disregarded in favor of conventional noise screens.
To improve the accuracy of insertion loss (IL) estimations from LHNS, a previously imple mented 2.5D boundary element method (BEM) model used for calculating railway LHNS IL
is revised. The main focus of the revision regards the surface impedance of the BEM-modeled
train track.
Measurements have been performed on ballasted train tracks to serve as validation data for an
impedance parameter study of ballasted train track surfaces. The resulting set of impedance
parameters have been used in 2.5D BEM-models simulating the sound pressure field of different
train shapes with and without LHNS, in other words estimating the IL of LHNS for different
railway applications. The IL results are compared with existing LHNS IL measurements from
other projects.
The simulated results demonstrate a generally accurate alignment with existing measurement
data for IL in third-octave frequency bands for passenger trains, however results differ between
different measurement comparisons. In the case of industrial trains, results are less promising.
This is hypothesized to be a result of the source model used in the simulations being inaccurate
for industrial trains.
Further investigation/development of source models used for different train types is a recom mended starting point for improving the reliability of the 2.5D BEM simulations. Access to
more LHNS IL measurement validation data is also considered necessary. Nonetheless, the
yielded results indicate that the revised impedance parameters have been an effective step in
improving LHNS IL estimation when compared with previous BEM-model results.
Beskrivning
Ämne/nyckelord
low-height noise screen, low-height noise barrier, ballast impedance, acoustics, insertion loss, BEM