Noise reduction solution bends sound waves upwards: Less impact on environment

noise reduction solution, environment
UK WHIS®wall trial at Transport for London

Here, Bart Willems discusses the Dutch noise reduction solution using diffraction to decrease the impact on the environment

The Dutch know a thing or two about innovation. Dikes, windmills, dredging and now: noise reduction via diffraction. A noise solution as simple as it is elegant.

With a perennially growing population and a matched growth in demand for mobility, traffic noise has become Europe’s second-largest environmental problem. According to research by the World Health Organization, it leads to hazardous levels of stress and lack of sleep. Other direct consequences include cardiovascular disease, slowed cognitive development in children, and tinnitus.

4Silence, the company behind a new innovation in noise reduction, believes that no one should suffer from the population’s growing need for mobility, and negative effects such as noise disturbance. They have therefore developed a completely new method of traffic noise reduction.

Bending sound waves upwards

As a spin-off of the University of Twente, the Dutch innovators have taken a scientific principle called ‘diffraction’ and used it to develop noise reducing products that are cheaper, (socially) safer, more durable and more esthetic than current options. How does this work? Astonishingly simply, and cleverly.

Sound moves like water: it always seeks the path of the least resistance. Therefore, when you create resonance beneath sound waves, the noise deflects upwards. 4Silence’s diffractors make sound waves resonate in a grooved element, which ‘push’ other sound waves upwards. As a result, the area behind it is much quieter.

In more detail:

1. Traffic noise moves off the road horizontally.

2. The horizontal sound waves meet the WHIS®stone: an element with grooves of varying depths. Inside these grooves sound resonates, creating pressure.

3. This leads to a resistance in the air, hindering the horizontally moving sound waves. These waves ‘dodge’ the WHIS®stone’s waves by deflecting upwards.

4.This happens at an angle of about 25 degrees, creating a quieter area behind the WHIS®stone.

Groundbreaking results

Can such a simple method, – no electricity or high constructions are needed, just a few cleverly placed grooves varying in depth – produce real results? The answer is yes. Independent research has found that the WHIS®stone, a concrete diffractor placed directly next to the road at ground level realises the same amount of noise reduction solution as silent asphalt, 2.5 dB. However, the WHIS®stone does this at least 30 years, without losing its effect or needing to be replaced every 7 years. And even greater noise reduction solutions are possible when two rows of WHIS®stones are placed next to each other. The environment keeps its views, while reduction can lead up to 4 dB.

noise reduction solution, environment
Traffic noise (800hz) no reduction method
noise reduction solution, environment
Traffic noise (800hz) deflected by WHISstone
noise reduction solution, environment
Traffic noise (800hz) deflected by WHISwall






The results with a 1 metre WHIS®wall are even more impressive. This innovative noise barrier produces the same noise reduction solution as a 3 metre conventional noise barrier, at a third of the height. The WHIS®wall requires no foundation and is easily installed. Stand-alone it already realises a reduction of 9 dB, but combined with existing reduction measures like silent asphalt, even much higher reductions are possible.

Especially for existing situations where more reduction is needed than the planned noise barrier or the barrier already in place, 4Silence created the WHIS®top. This is a light-weight aluminum diffractor which can be placed on top of all kinds of noise barriers, realizing an added noise reduction of 4 to 5 dB at only 40 kg/m1 and without adding wind load. That’s the same as raising the current screen by 2 metres, but without adjustments to the existing structure.

The company is currently based in The Netherlands, but is rapidly growing and is doing pilot projects all over the world. In 2019 4Silence was awarded an Innovate UK “First of a Kind”- grant. This programme, funded by the Department for Transport, funded the first application of a WHIS®wall alongside a UK railway track. In this case Transport for London helped achieving this grant, by providing a suitable testing location. In January 2020 the WHIS®wall installation was placed near Chorleywood Station (metropolitan line) and testing began. End of April 2020, the final results will be available.

Interested in 4Silence’s noise reduction solution products? Regularly check their website or stay in touch via Linkedin:


*Please note: This is not a commercial profile

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