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Volume : 44

Part : 2

Proceedings of the Institute of Acoustics

 

 

A sustainable material for in-situ absorption in noise barrier walls

 

Andrew Cowsill1, Sealed Air Verpackungen GmbH, Alsfeld, Germany

 

ABSTRACT

 

Perforated macrocellular foams produced using recyclable polyolefin resins have existed as acoustic absorption panels for almost 20 years. They have broad acoustic absorption with good resistance to moisture, salt, and UV exposure. Adoption of In-Situ sound reflection standards, in the Rail and Road noise barrier industry; along with renewed focus on sustainable design has led to fresh interest in this technology. Traditional foam panel formats did not attain acceptable in situ sound reflection performance.

 

The purpose of this study was to evaluate an improved structure of foam to the industry norms required of absorptive components of the noise barrier wall; and then subject this to a lifecycle analysis. A new structure showed good reflection results in-situ, using a reduced mass of raw material. The novel panel was evaluated to a range of acoustic and mechanical testing standards to assess suitability for adoption in Noise Barrier Wall applications. The material was subsequently subjected to lifecycle analysis and the resulting product issued with an environmental product declaration.

 

This paper concludes that the new material is well suited to widespread use in in-situ category A3 noise barrier walls.

 

1. INTRODUCTION

 

Perforated Macrocellular foams (figure 1) have been used as acoustic absorption for twenty years(1). Macrocellular foams possess a combination of characteristics: high levels of sound absorption; useable transmission loss; a strong, fiber free, lightweight structure; UV resistance; fire retard ance; and sustained acoustic performance following long term moisture(2) and salt exposure (https://pages.sealedair.com/uk-whisper).


 

Figure 1: Perforated Macrocellular Foam (Whisper®) exposed to moisture

 

Perforated macrocellular foam absorption panels had not attained widespread use in noise barrier wall applications, despite one version being adapted specifically for outdoor use (Whisper® UV). Market feedback indicated a growing demand from noise barrier wall manufacturers for absorption materials that were lightweight, durable, met fire standards, and complied with sound absorption, and sound reflection testing, which also fit inside existing 40mm deep cassettes shown in figure 2.

 

 

Figure 2: A typical cassette format for a noise barrier wall

 

The construction industry demands sustainability as well with durability, low carbon emissions and recyclability, so new material should be suitable for these demands. From the existing products in the market, the 50mm, 30kg/m3 UV resistant materials were found to be unsuitable from a dimensional standpoint and feedback indicated that they did not attain sufficient sound reflection ratings. A new material was conceptualized, and prototypes made in 40mm product, at a more economic 25kg/m3density; key questions remained:

  1. How could the product be adapted to reach ‘highly absorbent’ classification to European noise barrier wall standards?
  2. Would a lighter, thinner material, still possess the durability necessary to be a wall cladding in the road and rail markets?

 

Several modifications were shortlisted by an external advisor to enhance sound reflection ratings, and the ultimate material selected for comparison was 40mm, in 25kg/m3density, with machined parallel slots, 30mm deep, at 20mm intervals. This slotted material is referred to as NB (see figure 3) in this paper; this was compared with a material from the same production run in 40mm and 25kg/m3 density, without the machined slots, this material is referred to as Standard in this paper.

 

 

Figure 3: NB with machined parallel slots

 

2. METHOD

 

No harmonized standards exist for CE marking the absorption material without testing the structure and material of the noise barrier as a system, so a range of tests based upon the recommendations by the ENBF (European Noise Barrier Federation),(3)and industry norms were used. Objectives were set based upon market feedback. The project intended to provide the industry with an absorption material, where the hard work and accredited testing had already been completed. A designer could subsequently build a structure around the absorption panels depending upon their desired parameters.

 

The test regimen was set as follows:

  1. Compare NB vs Standard alternatives to absorption standards EN 1793-1 and EN 16272-1 Only one material moves on to the subsequent stages.
  2. Subject new material to the in-situ noise reflection test EN 1793-5, and EN 16272-3-2.
  3. Subject new material to range of tests contained within durability standard EN 14388:2008
  4. Subject new material located inside a sample cassette to EN 1793-1, EN 16272-1, EN 1793- 5, EN 16272-5
  5. Subject new material to an EPD (environmental product declaration) based on the LCA prepared to ISO 14025/EN 15804

All testing was conducted in accredited, qualified laboratories to recognized standards.

 

3. RESULTS AND DISCUSSION

 

3.1 Compare NB vs Standard to Absorption Standards EN 1793-1 and EN 16272-1

 

These tests are based on the Determination of the sound absorption coefficient according to EN ISO 354:2003 (see figure 4) and calculation of the single integer specification DLɑ according to EN1793-1:2003 (figure5), and according to EN16272-1:2012 (figure 6).(5)

 

 

Figure 4: ISO 354 comparing two proposed alternatives, with and without posts in front.

 

The machining of slots to create NB, caused a significant reduction in absorption within the frequency range 250-500Hz; however, the machining also enhanced absorption in the range from 800 to 5000Hz. The results were relatively consistent, with or without posts in front of a portion of the absorption panels.

 

The standard material could not attain the design objectives. The new slotted material (NB) met the design objective of category A3 (figure 5). Both materials met the design objectives of at least 8dB DLɑ in the rail noise spectrum (figure 6).

 

 

Figure 5: Classification according to EN 1793-1

 

 

Figure 6: Classification According to EN 16272-1

 

Only the material with machined slots (NB) would move on to the next phase of testing.

 

3.2 Subject One Material to the In-situ Test EN 1793-5, Sound Reflection.

 

The new NB material provided acceptable performance in the sound reflection test(6) meeting 5dB(A) In-Situ (figure 7).

 

 

Figure 7: EN 1793-5, sound reflection results

 

3.3 Subject New Material to Range of Tests Contained Within EN 14388:2008

 

The NB provided surprisingly good performance in the tests required including impact, fire resistance, icing salt resistance, and water absorption and release, and UV exposure.(7) At this stage, a decision was made to further differentiate the NB study, into NBO (Outside) and NBI (Inside cassettes) variants by varying aging modifiers in the base polymer.


 

Figure 8: EN 14388:2008 test results

 

The results of the accelerated aging test have been reproduced below for the NBO variant, showing a comparison between 16m2 of new material, and 1m2 of material after being subjected to accelerated aging.(8)


 

Figure 9: Results of the acoustical measurement of the sound reflection on NBO plates

 

The aged NBO material did not suffer any significant loss of performance. This indicates that the life of the material should exceed 50 years in outdoor conditions. The gain of mass due to water, salt and limescale also did not significantly reduce the acoustic performance as would be expected of porous absorbers(2). At ten years, the material was supporting 0.79kg of salt and water, and despite a very short drying time, it did not move the reflection performance outside of the range seen in the full-scale 16m2 test. At all times, all samples over the 50-year accelerated aging test were within the expected performance range of the new samples tested in the 16m2 full scale test.

 

3.4 Subject New Material Located Inside a Sample Cassette to EN 1793-1, EN 16272-1, EN 1793- 5, EN 16272-5

 

A typical example of an aluminum noise barrier wall cassette was adapted by a customer for use, to evaluate whether the performance would remain consistent when covered by perforated metal plates (figure 10). The NBI version of the slotted material without enhanced aging modifiers (not essential sheltered inside a cassette) was used.


 

Figure 10: Image of customer aluminum element with NBI inserted for testing. Perforated area approximately 33.5%, 6mm hole diameter

 

NBI inserted inside a typical aluminum element attained acceptable performance to the design objectives of category A3, 8dB DLα(9) and 5dB(A) In-Situ(10,11) (figure 11). Please note that the perforated metal covers proved to reduce performance in sound absorption and sound reflection in three of the four measures comparing figure 7,8,9 results, with figure 11.

 

 

Figure 11: Summary of results of testing NBI inside typical cassette

 

3.5 Subject new materials to an EPD based on the LCA prepared to ISO 14025/EN 15804

 

For this portion of the study, NB product was again divided into two versions NBO (Outside) and NBI (Inside cassettes). NBO retained the high UV resistance formulation, and NBI contained reduced aging modifiers for a protected life inside a cassette.

 

An EPD is unique to a manufacturer, formulation, location, and material model therefore we must present it here including the commercial brand names. The results of the EPD(12) are presented per 1m2 of Whisper® panel for the applicable versions and thicknesses of the Whisper® panel family (figure 12). The indicator to express the impacts is the GWP-GHG with a neutral approach. The reader should take note that the density represents a finished product density after removal of swarf from the slotting process, rather than the extruded density of the macrocellular foam.

 

 

Figure 12: GWP results A1-A3 1m2

 

Evaluation of the final polymer formulation by a regulatory compliance team, proved the products are predominantly >95% Low Density Polyethylene (LDPE), they are compatible with mechanical or chemical recycling of LDPE polyethylene. Consequently, Whisper® NB Products are eligible to be labelled with a Resin Identification Code 4 (RIC #4), corresponding to low density polyethylene.

 

4. CONCLUSIONS

 

The perforated macrocellular foam, in 40mm thick with a surface modified with 30mm deep slots at 20mm intervals, named “NB” was well suited to use in the noise barrier wall environment surpassing all expectations set by the design objectives.

 

The variant referred to as “NBO” was well suited to outdoor aging conditions and an expected lifespan exceeding 50 years, significantly surpassing the sound absorption and sound reflection parameters. NBO offers further possibilities to enhance the sustainability of noise barrier walls by being designed for recycling and eliminating the need for perforated metal covers which demonstrably reduce absorption performance and can increase the carbon footprint. NBO also provides the potential to retrofit existing reflective walls with category A3 absorption, reducing the need to build entirely new structures. This retrofit potential can provide a source of significant carbon footprint reduction for acoustic projects and is worthy of further study.

 

The variant referred to as “NBI” was well suited to use inside perforated aluminum noise barrier wall cassettes and provides a recyclable option to replace aged interior materials, with a slightly lower carbon footprint.

 

Both materials were proven by virtue of their composition and light weight, to have relatively low carbon emissions, and be compatible with a wide range of mechanical and chemical recycling schemes in the European Union.

 

The range evaluated in this study provides acoustic engineers and designers with options for retrofit, maintenance, or new noise barrier walls, with a durable and sustainable solution.

 

5. ACKNOWLEDGEMENTS

 

We gratefully acknowledge the work of the European Noise Barrier Federation in providing advice, networking, and support for innovative materials. We also acknowledge the work, support and guidance of Claes Hermansson for making this material a commercial reality.

 

6. REFERENCES

 

  1. Park, C., Brucker, M., Remy, L., Gilg, S., & Subramonian, S. (2000, August). New sound absorbing foams made from polyolefin resins. In International Congress and Exposition on Noise Control Engineering (pp. 2-7).
  2. Subramonian, S., Remy, L., & Schroer, D. (2004). Acoustics and forming of novel polyolefin blend foams. Cellular polymers, 23(6), 349-367.
  3. The European Noise Barrier Federation. (2020). Technical note about CE marking of noise barriers used alongside road infrastructure. (Brussells 20/1/2020). https://www.enbf.org/technical-note-about-ce-marking-of-noise-barriers-used-alongside-road-infrastructure/
  4. DIN EN 14388, 2015 Edition, December 2015 - Road traffic noise reducing devices – Speci fications
  5. Stadt Wien (2021). Measurement of the airborne sound insulation characteristics of sound absorbing panels for noise barriers, types “Whisper® NBO-40” and “Whisper® NBO-40- A3” (grooved in post processing) with and without supports. (Report No. MA39-21-04706). Available upon request, Sealed Air.
  6. Kubisch Data GmbH (2020). Testing to determine the acoustic properties of noise protection barriers in accordance with Austrian Norm EN 1793-5 (sound reflection) On-site Testing. (Ansfelden 24-09-2020). Available upon request, Sealed Air.
  7. BTI Bautechnisches Institut GmbH (2020). A performance assessment of noise barrier ab sorbers: Whisper NBO-40-A3 noise barrier plates for noise barrier cladding and additional barrier equipment in accordance with ÖNORM EN 14388:2008-10-01.(Report No. 26692- C1A/2020). Available upon request, Sealed Air.
  8. BTI Bautechnisches Institut GmbH (2020). Investigation of the long-term behavior of Whis per®-NBO-40-A3 noise barrier wall types using time lapse test based on EN 14389-1:2015, EN 14389-2:2015 and EN 60721-3-4:1995 for 50 simulated years. (Report No. 26692- A2A/2019). Available upon request, Sealed Air.
  9. MFPA Leipzig GmbH (2021). Test of sound absorption according to DIN EN ISO354 and determination of the single number value acc. to DIN EN 1793-1 and acc. to DIN EN 16272- 1 on a noise barrier made of one-sided absorbing aluminium elements with the designation “AL-E PANEEL with Whisper Absorber”. (Report No. PB 2.3/20-212-44). Available upon request, Sealed Air.
  10. MFPA Leipzig GmbH (2021). Measurement of the in-situ values of sound reflection under direct sound field conditions and determination of the single-number value according to DI N EN 1793-5 on a noise barrier made of one-sided absorbing aluminium elements with the designation "AL-E PANEEL with Whisper absorber”. (Report No. PB 2. 3/20-212-45). Avail able upon request, Sealed Air.
  11. MFPA Leipzig GmbH (2021). Measurement of the in-situ values of sound reflection under direct sound field conditions and determination of the single-number value according to DIN EN/TS 16272-5 on a noise barrier made of one-sided absorbing aluminium elements with the designation "AL-E PANEEL with Whisper absorber". (PB 2. 3/20-212-46). Available upon request, Sealed Air.
  12. The International EPD System (2022). Environmental Product Declaration WHISPER® Acoustic Panel FR, UV, UV-ECO, NBO, NBI and NB-ECO. (S-P-05174). https://www.envi rondec.com/library/epd5174

 

1andrew.cowsill@sealedair.com