Advanced engineering of acoustic treatments in the Airport Rail Line tunnel

Challenge

The Airport Line is a relatively new electrified railway line which connects the Perth suburb of High Wycombe to the city via Perth Airport. Managed by the Public Transport Authority (PTA), the majority of its 8.5 km length consists of tunnels.

When trains operate in tunnels, a much higher sound field is generated around the perimeter of the train as a result of the build-up of reverberant noise. As a result, in-car noise levels in tunnels are higher than when operating on open track. These increases in noise levels have the potential to affect passenger comfort, ease of conversation, and the enjoyable use of personal devices.

Accordingly, in-car noise criteria were written into the design and construct contract of this project, applicable at driver and passenger cabin positions. Traditional methods rely on predicted reductions in bulk reverberation times to estimate reductions in noise levels experienced by passengers.

In practice however,

• The noise field incident on the passenger cabin is controlled by direct and ‘single reflection’ pathways, which are short in terms of wavelength (with geometric near field effects)
• The speed of the train (and therefore the spectra of noise emitted) and installation of treatments vary with track distance, introducing time domain effects
• Local features to the tunnel (as well as absorptive treatments) are shown to locally block, reflect or focus sound, e.g. rails, drains, walkways and services, creating non-diffuse sound fields incident on the train.

Solution

Our experts developed a novel and advanced engineering method for the prediction and optimisation of sound absorptive treatments. The technique is the first to directly resolve 3D sound pressure level spectra distributions within the tunnel, and then integrate those results with tunnel treatment extents, speed profiles and local track features to predict rolling-average noise levels throughout each journey, at each key in-car position – within one model.

It is unique in that no other model has been constructed which can predict the distribution in rolling-average in-car noise levels ‘on-the-fly’ with changes to speed, train, trackform, dampers and/or absorptive panel arrangements with chainage. It has been validated using hours of field data obtained using the same rollingstock, trackforms and tunnel geometry, leading to very high confidence levels in the results under specified conditions.

Impact

As a result of the methodology employed, the material savings achieved compared to the original estimates are estimated to be in the order of tonnes, with significant long-term safety and maintenance benefits.

The innovation led to more effective treatment, whilst minimising the amount of material involved. The more effective treatment led to reduced noise exposure, and reducing the amount of material involved improves safety and reduces ongoing maintenance costs. The project enabled material savings of the order of tonnes compared to what would have been the minimum recommended using traditional design approaches.

Each installed acoustic panel represents a maintenance (lifting) and safety (trip / movement) risk, so therefore the reduction in the number of panels improved the safety of the asset. The documentation of the analyses undertaken was effective in communicating the risks involved and providing confidence in outcome. This underpinned decisions which achieved material savings and other improved outcomes for the project.