SILENCE E-LEARNING FOR TRANSPORT & URBAN PLANNERS - MEASURES TO TACKLE NOISE
Low-noise road surfaces
What is it about?
Road surfaces influence the generation of noise by tyre/road interaction and the propagation of noise from the vehicle engine and transmission system. The relevant factors for noise emission are the texture of the surface, the texture pattern and the degree of porosity of the surface structure.
Low-noise road surfaces today are either thin layer surfaces or porous asphalts with one or two layers. Thin layers are different bituminous layers with a maximum thickness of 3 cm and a small aggregate size (4-8mm as maximum chipping size).
Porous asphalt has an open structure with about 20-25% air void inbuilt. As a result, it absorbs noise and drains water, thus increasing road safety. The noise reduction potential of porous asphalt is higher than for thin layers. However, for use in urban areas, the porous asphalt still shows significant disadvantages in terms of costs, durability, winter maintenance, ravelling caused by shear forces, drainage systems and difficult repair after trenching for pipes and cables and after accidents. Thus, the use of porous asphalt is only recommended for higher speeds (>60km/h), homogenous traffic flow, roads with only few crossings / traffic lights and without sharp bends.
There are also new, low-noise solutions for paving blocks that can be used as alternative to cobble stones, keeping a very pleasing and different (compared to asphalt) visual appearance. Paving stones normally cause increased noise levels of 3-5 dB because of their very uneven surface structure. In SILENCE, a special type of very smooth paving blocks has been developed and full scale tested. These paving blocks have about the same noise emission as ordinary pavements.
Benefits in terms of noise reduction
For thin layer surfaces, an initial noise reduction of up to 3 dB in relation to dense asphalt concrete with 11mm maximum aggregate size has been measured. However, the noise reduction effect decreases in the order of 0.1 dB per year (for light and heavy vehicles at low and high speed). In the SILENCE project, thin layers with an optimised surface texture for noise reduction have been developed and full scale tested in Denmark. An initial noise reduction of 4 dB was achieved.
Single layer porous pavements have an average noise reduction of 3-4 dB on highways (in relation to dense asphalt concrete).
Two-layer porous pavements have a noise reduction potential of around 4 dB or more (in relation to dense asphalt concrete).
For porous asphalts, the noise reduction effect decreases by 0.4 dB per year for light vehicles at high speeds and by 0.9 dB at low speeds. For heavy vehicles, this amounts to 0.2 dB at high speeds. No effect is assumed for low speeds.
What does it cost?
The cost of thin layers normally is about the same as the price for ordinary pavements. The price is to some extent related to the condition of the old pavement on the road. In Denmark, it is expected that the lifetime of thin layers is around one year less than of ordinary pavements due to their open surface structure.
Two-layer porous asphalt surfaces cost about 30 EUR/mē more than conventional surfaces.
Compared to other noise abatement measures (like barriers, sound proof windows), the costs for low-noise road surfaces remain relatively low.
Noise reducing pavements can be used in the ongoing pavement maintenance process and thus be a cheap and simple noise abatement measure to implement. In the SILENCE project, procedures for the integration of noise in Pavement Management Systems were developed.
The replacement of road surfaces can be done on short notice. No compliance of drivers is required to make this measure fully efficient. In most cases, low-noise surfaces reduce the rolling resistance, thus they might decrease fuel consumption as well.
Good craftsmanship and accuracy in the laying process are important to achieve the best results. No special maintenance has to be performed on thin layers. For porous surfaces, cleaning is necessary on a regular basis. Once the surface is strongly clogged, cleaning has no more impact on the noise performance. Attention has to be paid to maintenance and re-pair. Discontinuities reduce the noise reduction effect, at least locally.
A number of mechanisms are responsible for the generation of noise from vehicles passing over a road surface (Sandberg, U.; Ejsmont, J. A., 2002). One noise source is the engine and transmission system where the most important frequencies typically are smaller than 1,000 Hz. This noise propagates from the vehicle directly, and as reflected noise from the road surface. The surface structure is therefore important for the propagation and reflection. If the surface absorbs to some degree, the total noise may be reduced.
The second main source is the tyre/road interaction noise, which can be subdivided and described by different mechanisms:
The aerodynamic noise generated by air pumping, when air is forced out (and sucked in) between the rubber blocks of the tyre and the road surface as the tyre rolls by: this source is typically the most important in the frequency range between 1000 and 3000 Hz. If the road surface is porous with a high built-in air void, the air can be pumped down into the pavement structure, and the noise generated from air pumping will be reduced. If the pavement has an open but not porous surface structure, the air pumping noise will also be reduced to some extent.
Noise from vibrations of the tyre surface: the aggregate at the top layer of the pavement forms the pavement texture. When the rubber blocks of the tyre hit these stones, vibration is generated in the tyre structure. These vibrations generate noise typically dominated by the frequency range between 300 and 2000 Hz. With a smoother pavement structure, the generation of vibrations and noise is reduced. The vibration generated noise can also be reduced if the pavement is elastic.
In the driving direction, the pavement surface and the curved structure of the tyre forms an acoustical horn which amplifies the noise generated by the tyre/road interaction. If the pavement side of this horn is noise absorbing, the amplification by the horn is reduced.
The most effective low noise surfaces are currently porous asphalt and thin-layer asphalt. Thin layer surfaces either can be open graded asphalt concrete, stone mastic asphalt or a combination pavement. The noise reduction potential is based upon a low aggregate size of the mixture (e.g. a maximum aggregate size of 6mm on urban roads and 8mm on highways).
Porous asphalt reduces the noise generated by air forced out between the rubber blocks of the tyre and the road surface (air pumping effect) and reduces propagation of noise from the engine and transmission system of the vehicle (sound is not reflected but absorbed by the porous layer). In the SILENCE project, it was found that for highways single layer surfaces achieve the best noise reduction with a maximum aggregate size of 8mm, a built-in air void of around 20-23%, and a thickness of 40mm. For urban roads, this single layer is not suitable because the porous layer is clogged with dust and the noise reducing effect disappears after around 2 years.
For urban roads, two-layer porous surfaces have proved successful. The top layer should have an aggregate size of 8mm, the bottom layer of 16 to 22mm. The top layer reflects the dust, but lets the sound pass, which is absorbed in the bottom layer. These surfaces need to be cleaned regularly using high pressure water (e.g. twice a year).
In SILENCE, an experiment has been conducted in Copenhagen where an 8 year old and clogged top layer of a two-layer porous pavement has been milled off and replaced by a new porous top layer. This was done successfully. An initial noise reduction of around 6 dB was achieved. However, considering the disadvantages of porous asphalts for use in urban areas (as mentioned earlier), thin layer surfaces are normally to be preferred.