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F. Anfosso-Lédée, J. Haberl, G. Watts:
"Combination of low-noise road surfaces with road and building equipment";
Report for EU Projekt SILVIA (Sustainable road surfaces for traffic noise control); 2005; 32 pages.

SILVIA is a collaborative RTD project part-funded by the European Commission under its Competitive and Sustainable Growth (GROWTH) programme. The project is of three year duration and commenced in September 2002. The final deliverable will be a guidance manual on lower noise surfaces covering topics such as integrating low-noise surfaces with other traffic noise control measures including vehicle and tyre noise regulation, traffic management and road and building noise protection equipment.
The SILVIA project has been divided in to a number of work packages. Work Package 5 (WP5) studies the effects of combining low noise road surfaces with other noise reducing measures. The work is based mainly on existing results although some new modelling work is included.
The research in SILVIA WP5 is divided into 4 tasks. The current report is based on the results of Task 5/4. The aim of Task 5/4 is to provide indications on the noise reduction of low-noise pavements when combined with noise barriers and earthworks, when used on bridges, and when combined with facade insulation.
The main conclusions and recommendations for each topic are summarized hereafter :
About combination with noise barriers.
Most of the knowledge on this subject is based on predictions from theoretical models. These models often overestimate the efficiency compared with real data.
On site data of combined effects are rare, probably because it is very difficult to evaluate the relative contribution of the road surface and the barrier. More experimental data should be obtained.
There is a strong evidence that noise barriers and low noise road surfaces used together can provide an optimised solution for noise abatement. The later contributes to reduce noise emitted at the source, whereas the previous acts on the sound propagation. The combination of both techniques can bring the ultimate decibels that are missing with a specific barrier.
In general, due to the frequency dependence of sound wave propagation, diffraction and absorption, the global efficiency of the combination is lower than the addition of respective efficiencies. This is confirmed by theoretical predictions as well as on site measurements. In principle, when combining a low noise surface and a sound barrier, a great attention should be paid on the spectral efficiency of each in order to optimise the combination.
Thus, the advantage of low noise road surface (porous asphalt) is smaller in the presence of a noise barrier than in the case of unobstructed propagation.
Because they reduce noise emission at the source, low noise road surfaces bring sound reduction in the zones where noise barriers are inefficient (non shaded zones, opposite side of the road...).
The use of low noise road surfaces can, in principle, reduce the barrier cost by reducing its height or length for equal performance.
About the use on bridge and combination with bridge joint treatment.
There are no standardised measurement method, and consequently the comparison of experiments are difficult. The availability of such a standard would probably increase the development of innovative joint solutions.
If pass-by measurements are performed near the joints, an important increase in sound pressure level can be observed, up to 10 or 15 dB(A), depending on the quality of the joint.
Results are often expressed in terms of noise increase when passing on the joint. But this indication is relative to the tire-road noise level, i.e. to the road surface performance. In the case of a noisy joint, the emergence of the peak will be bigger on a quiet road surface than on a noisy one, thus probably making it more annoying. In the case of a silent joint, the road surface performance will dominate.
Very silent joints have been developed recently, that do not cause any peak when vehicles are passing on, even with for a low noise road surface.
If long term LAeq measurements are performed in the vicinity of the joint, the effect of noise increase will be attenuated. But then the evaluation of joint effect is more difficult because many things are mixed for example the effect of traffic, road surface, surrounding noise.
It is often suggested that physical measurements may be supplemented by an evaluation of noise annoyance. The global dB(A) value is not an appropriate scale to evaluate the annoyance due to repetitive impulse noise caused by the vehicles passing on bridge joints.
About combination with façade insulation.
There is a serious lack of knowledge on the effect of low noise surface on indoor sound pressure levels. This should be a subject for research.
Low noise road surfaces can be used in combination with improvement of façade sound insulation. The benefit for the inhabitant will be optimum. However, it is expected that due to the variation of façade sound insulation with frequency, the benefit of road surface is lower indoor than outdoor.
Road traffic noise may have a significant energy content in low frequencies, especially when the percentage of trucks is high. Because the façade sound insulation has less performance in the low frequency range, it is suggested that a C-weighting scale or a loudness measure (e.g. Zwicker loudness) is used to supplement the A-weighted measure in order to have a better estimate of inhabitant´s perception. But further research should be made on this proposition.