A Sustainable Option for Absorption Materials in Noise Barrier Walls

At least one in five people in the EU are exposed to long-term noise levels considered harmful to their health. Health issues related to these exposures include annoyance, sleep disturbance, and cardiovascular and metabolic issues. Noise pollution also affects wildlife. It can cause various physical and behavioural issues in animals. According to the European Environment Agency, the impacts of railway noise could potentially increase in the coming years due to higher network volumes, and faster trains. (1)

“Given negative impacts on a large portion of Europe’s population, environmental noise has become a significant concern for citizens and policymakers”.

Rail noise travels to humans through structures via vibration and, through the air as airborne noise. The most common intervention for airborne noise is to build noise barrier walls along rail corridors, and these noise barriers are typically lined with sound-absorptive materials. Absorptive materials reduce the amount of sound bouncing off the walls and scattering into the environment. Noise barriers are an effective way for rail operators to improve their impacts on people and animals. 

According to the EU Commission on Energy, Climate Change, and Environment, construction, and demolition waste accounts for more than a third of all waste generated in the Eu so there is a growing need for noise barrier wall producers to improve sustainability of these important systems.(2).

Noise barrier walls on the rail network must first meet acoustic standards, but also remain durable over time, withstanding water, wind, weather, dynamic loads, and de-icing salt. The ongoing performance of absorptive walls can be monitored using new “in-situ” techniques and specialised equipment.

What are the options for sound-absorption?

Mineral wool-based cassettes have evolved over the years to be very durable, highly engineered systems.  Mineral wool is a non-metallic inorganic fibrous product formed by spinning or drawing molten stone or silica and binding it with resins and oils. Typically noise barrier wall cassettes use high densities to attain extended durability in outdoor conditions. “Despite being the most widely used form of insulation in the EU, it is generally not recycled at a building’s end-of-life” (3).

Natural materials exist as well, such as processed plant fibre derivatives, however gradual natural degradation limits the durability of biologically sourced fill materials. 

A relatively new option is the Sealed Air® brand Whisper^® NB range of Acoustic Panels.  Whisper^® NBO is proven to be resistant to weathering without a cassette, passing an independent 50-year assessment (4) including freeze thaw cycles, UV and heat exposure, salt and water exposure, and is comparatively light weight.

Whisper^® offers a compelling option for noise barrier wall producers looking to enhance durability, reduce environmental impact, offer a re-use service, and improve recycling or resource recovery efforts.

1. What is Whisper^®?

Whisper^® Acoustic Panels are a honeycomb-like matrix of networked closed cells which efficiently absorb noise; the technical term for this type of material is Macrocellular foam. All Whisper^® panels use zero ozone depleting substances.

2. How does it work?

Whisper^® uses membrane absorption as its primary method of absorbing sound. The membrane absorption phenomena is similar to that of a trampoline mat absorbing the energy of a falling body. Most membrane absorbers are two dimensional, while Whisper^® is produced as a three-dimensional structure, enabling the use of additional acoustic mechanisms including friction losses, and absorption within the strained solid cell matrix.

Whisper^® NBO was the first material in the product range.  From the start, the Whisper^® NB range was designed to meet the absorption category A3, while using as little material as possible. Testing for absorption to the rail standard EN 16272-1, resulted in 11-12dB (material only) which is category A3 absorption at only 40mm and 1kg/m2. After qualification for absorption, the new in-situ reflection method was performed to EN 16272-3-2. The result was a pleasing 6dB of reflection.

The material was then subjected to independent testing for acoustic performance, brushfire resistance, stone impact, pendulum impact, and resistance to frost and de-icing salt. Whisper^® NBO was subjected to an accelerated aging test by the BTI in Austria, where the material was subjected 50 years of simulated outdoor conditions. The result showed acoustic properties were maintained for the duration of the test to EN 14388:2008.

Further testing was conducted by adding the material to an existing ‘representative’ aluminium cassette where A3 absorption, and 5dB of in-situ reflection were also obtained through testing by a notified body. We now had two options, NBO-40-A3 for use as a cladding material without metal covers, and NBI-40-A3 for use inside cassettes as a retrofit material.

3. Environmental Product Declaration

 Sealed Air proved our commitment to sustainability with an independent EPD from the International EPD^® system to provide reliable and verifiable claims for customers to use for comparisons to other acoustic solutions and methods. The EPD was conducted to ISO 14025 and EN 15804. Sealed Air achieved a Climate change – total (neutral GWP-GHG) A1-A3 figure of 4.13kg CO2eq; along with the significant benefit to our customers of independent certification of emissions.(5)

As the Whisper^® NB range is predominantly >95% Low Density Polyethylene (LDPE), it is formulated for compatibility with mechanical or chemical recycling of LDPE polyethylene. LDPE is a robust, non-polar plastic with a low surface energy, making it highly resistant to water, salt, and graffiti. The degree of recyclability will vary depending on scope and availability of polyethylene foam collection and recycling programs in your area. Services such as ENF Recycling can help businesses connect with recyclers to ensure that valuable plastics are recycled; and SEE is also able to take Whisper^® back for reprocessing into packaging materials.

The outcome of further R&D resulted in Whisper^® NB-Eco-40-A3.  NB-Eco-40-A3 gives us the ability to supply Whisper^® NB materials in colours, such as grey, to match the appearance of concrete.

4. Applications

• Retrofit retaining walls, sheet pile, concrete walls and other reflective surfaces
• Acoustic absorption on kerbs and station platforms
• Cladding without metal covers on tunnel ramps and entrances
• Replacing failed or slumped materials inside existing cassettes
• Filling new cassettes
• Enhanced durability of wooden wall systems.

References:

(1) https://www.eea.europa.eu/en/topics/in-depth/noise#:~:text=In%20Europe%2C%20at%20least%20one,levels%20considered%20harmful%20to%20health.

(2) https://environment.ec.europa.eu/topics/waste-and-recycling/construction-and-demolition-waste_en

(3) (Cordis Europa project #821000) https://cordis.europa.eu/project/id/821000#:~:text=Despite%20being%20the%20most%20widely,wool%20is%20an%20important%20step.

(4) Performance assessment to EN 14389-1 and EN 14389-2

(5) https://www.environdec.com/library/epd5174

Andrew Cowsill has had an extensive career in solving engineering challenges using plastics. He launched Whisper® into the Australia and New Zealand region in 2014 with the first major sale being a tall noise wall around a large electrical substation.  Andrew has also advised numerous customers on resolving their noise issues using Whisper® acoustic panels.  From Asia Pacific to Europe, Andrew is now widely regarded as an expert on this material and its applications.  Andrew is a member of the European Noise Barrier Federation, and gained a BMS degree from the University of Waikato.