Why Foam? What 20+ Years of Research Taught Us About Acoustic Absorbers

CALMA-TEC uses an acoustic foam as the absorbing material. At first sight, foam may seem like an unusual material for infrastructure.

Noise barriers are expected to remain outdoors for decades, exposed to sunlight, rain, frost, heat, wind, dirt and constant environmental stress. So why would we choose a lightweight foam as the acoustic absorber at the heart of PIN?

The answer is quite simple: we did not start by looking for foam. We started by looking for the right acoustic absorber. And that search began more than 20 years ago.

It started with a research question

As early as 2003, we were intensively investigating different materials for noise-control applications. This included an EU-funded research project carried out together with international partners, in which different absorber materials and structures were examined in order to come up with better noise barrier solutions.

The fundamental question was already clear: What material can effectively dissipate sound energy and, at the same time, survive decades in an exposed infrastructure environment?

That sounds straightforward. In practice, it is not. Over the years, we learned that there is a fundamental trade-off between acoustic performance and long-term durability.

Good absorption alone is not enough

Fibrous materials such as mineral or stone wool can provide very good acoustic absorption. Sound can penetrate deeply into their structure, where acoustic energy is dissipated.

But infrastructure is very different from a protected indoor application.

An absorber installed next to a road or railway is continuously exposed to moisture, wind, temperature changes, vibration and mechanical stress. Protective layers can become damaged over time, and once the fibrous material itself is exposed, its structure can progressively deteriorate. The fine particles release are not particularly environmentally friendly or good for our health.

The other extreme: durability without enough dissipation

Hard absorber materials present almost the opposite situation. Materials such as wood concrete can be veryy robust and durable. They are well established in noise barrier construction and can withstand harsh environmental conditions for long periods of time.

But there is a physical limitation. The ability of an absorber to dissipate sound energy depends strongly on its internal structure and on how easily acoustic waves can penetrate into it. Hard porous materials can absorb sound, but their structure places limits on the amount of acoustic energy that can be dissipated.

This brought us back to the original engineering problem:
Could we combine the acoustic behaviour of a soft, highly porous absorber with the durability required for infrastructure?

Why we eventually chose this foam

Over the course of our research and product development, an open-cell polyethylene foam emerged as a particularly interesting solution. We tested a wide range of acoustic foams, but had to exclude many of them because they simply were not suitable for long-term outdoor use.

We were looking for something that is…

• accessible to sound waves,
• non-fibrous,
• lightweight,
• characterised by very low water uptake,
• suitable for permanent outdoor exposure when appropriately UV-stabilised,
• flexible and mechanically resilient,
• and capable of being shaped into defined three-dimensional geometries.

And that’s how we found our match.

That being said, the material itself is only part of the story. We knew that the absorber material is only one part of the acoustic solution. Geometry matters just as much.

So once we had found the right material, we had to shape it into something specifically developed to interact with the sound field at the diffraction edge of a noise barrier. This is how PIN became what it is today.

PIN is an engineered acoustic system. Material choice, properties, porosity, volume, geometry and position all work together.

But can a foam really survive outdoors for decades?

This is an obvious question – and one we wanted to answer with testing rather than assumptions.

We wanted evidence. That is why we initiated intensive UV-ageing tests with independent testing institutes. Initially, we drove this testing ourselves. Over time, our material suppliers also recognised that verified long-term durability was essential if this type of material was to gain wider acceptance in infrastructure applications.

The tests combined UV-B exposure with moisture and temperature cycles in a QUV chamber. Mechanical properties and, importantly for us, acoustic absorption were examined before and after ageing.

There was considerable scepticism around the use of foam as a permanent outdoor acoustic absorber. But the test results spoke for themselves. Even after extensive accelerated ageing, the acoustic behaviour remained comparable to that of non-aged material.

The conclusion for us was clear: this foam was ready for the long haul.

The material documentation also specifies testing of sound absorption, long-term water absorption and sound absorption behaviour under QUV exposure. For us, this is a crucial distinction.

A material used in infrastructure does not only have to provide good initial laboratory values. Its relevant properties need to remain stable as the material ages.

More than 20 years later

Our decision to use foam therefore did not come from a catalogue or from searching for the easiest material to process. It developed from research, testing and practical observation spanning more than two decades.

We have seen the strengths and weaknesses of different absorber concepts. We have seen what happens when materials that perform well acoustically begin to deteriorate. And we have learned that durability alone does not automatically provide high acoustic performance.

The result is the material concept we use in PIN today:

a lightweight, non-fibrous, open-cell absorber that combines acoustic performance with the requirements of long-term outdoor use.

And, importantly, one that allows us to engineer not only the material, but also the acoustic geometry around it.

The next generation is already being tested

There is, however, another part of this story. Using a polymer-based absorber naturally means that the environmental footprint of the material matters. So the next development step is already underway.

We are currently acoustically testing a new low-carbon-footprint variant of the absorber material.

The objective is clear: reduce the carbon footprint of the material while retaining the acoustic properties and durability that made us choose this absorber concept in the first place.

We will not make that claim until the testing confirms it. But if the results are what we expect, the next generation of PIN will combine the material characteristics we have relied on for more than two decades with a significantly improved carbon footprint.

Because good engineering is not about finding one solution and keeping it forever.

It is about understanding why a solution works – and then continuing to improve it.