By Jon Paul Faulkner
School of Architecture Planning and Environmental Policy
University College Dublin
The Noise-Adapt Ireland Project
In accordance with Art. 6.2 of the Environmental Noise Directive (END), the European Commission recently developed Common Noise assessment methods (CNOSSOS-EU) for road, railway, aircraft, and industrial noise, to be used after adoption by member states for the purpose of strategic noise mapping (SNM) as required by Article 7 of the END. The CNOSSOS-EU model will come into effect for the 2022 noise mapping round for all EU nations. The Noise-Adapt Ireland project aims to identify Ireland’s adaptation needs for transitioning to the CNOSSOS-EU standardized noise model and the standardized approach for population-exposure estimation. This article highlights some interesting findings from a data needs assessment report that aimed to identify Ireland’s key transitioning needs under CNOSSOS-EU.
Findings from the Report
CNOSSOS-EU introduces five categories of vehicle for road-traffic noise, where formerly there were only two categories, for light and heavy vehicles (CRTN Methodology). The authority responsible for Ireland’s national road network is Transport Infrastructure Ireland (TII). This authority’s method of classification has limited accuracy in identifying certain classes of vehicle and currently suffers from cross-category overlap between existing TII classification and CNOSSOS-EU classification. If data gaps exist, a default decomposition of heavy vehicles into two categories can be implemented based on the type of road under analysis. Alternatively, if one of the two categories dominates the flow strongly, this category is recommended. However, it is possible that more accurate information may be garnered from regional statistics acquired through traffic counts by regional authorities (e.g., Dublin City Council) or estimated from commercial vehicle registration data collected by the Central Statistics Office (CSO).
The CNOSSOS-EU model recommends the calculation of average speed for each vehicle category. Ireland currently uses the legal speed limit on respective roads as a measure of average speed. Therefore, average vehicle speeds will need to be recorded using vehicle-speed monitors. In the context of agglomerations, Dublin City Council (DCC) has vehicle-speed monitors in place. The National Roads Authority (NRA) and DCC will need to translate its road categories to spectral α and β coefficient octave bands. Other specific data needs include road-surface gradient, roadside topography, and more detailed spatial information, particularly related to the positioning of traffic lights and stop signs.
In the context of railway noise, Irish Rail will need to gather data on the types of track composing the Irish rail network. It is imperative for practitioners to focus on track base and railhead roughness in the initial stages of CNOSSOS-EU implementation. Data must also be gathered in relation to track-base and sleeper-type as well as brake-type data. In relation to brake-type data, stock category assumptions will have to be made based on vehicle type and vintage and examination of publically accessible photographs (e.g., where brake discs are visible). In the scenario where vehicles have a combination of disc brakes and cast-iron tread brakes, only the latter is recommended for classification. Data sets for elevated structures such as bridges are required. In order to identify elevated structures, data sets may need to be manually generated through field surveys and analysis of aerial photography.
In the context of industrial noise, there has never been any SNM of industrial sites in Ireland. The first stage of SNM should begin by cataloging all applicable industrial sites within the agglomeration under analysis. The Environmental Protection Agency (EPA) of Ireland has a register of International Plant Protection Convention (IPPC) and waste licensed industries in Ireland. Since these plants have sound emission control conditions attached to their licences, this data can be extracted from reports submitted under the licensing requirements. This data is measured rather than modeled, however, it could potentially be used to carry out population assessments once the sound levels at the boundary of industrial sites are known.
In relation to sound propagation, geographic information systems (GIS) are proficient at generating accurate spatial dimensions for buildings along a horizontal plane but not along a vertical plane. In Ireland, precise building-height parameters need to be integrated into future SNM. DCC has a database on all building heights in DCC area, however, for other agglomerations in Ireland building height data will need to be retrieved from Ordinance Survey LiDAR data (a high resolution elevation dataset yielded from a Light Dectection and Ranging technique of data collection.. Accurate information regarding the position of noise barriers is also required, as even minor inaccuracies in height can result in major discrepancies in noise calculation. Noise barrier data may be acquired using Google Maps and Google Street-view.
Road Vehicle Propulsion Noise
CNOSSOS-EU applies the same correction coefficient for respective road surfaces to both propulsion and rolling noise. This methodology is problematic because no distinction is made between variation in rolling noise caused by the roughness change associated with the road surface and the variation in rolling noise caused by absorption (Pallas and Dutilleux 2018). It is more relevant to apply a single measure of absorption effect to propulsion noise (Pallas and Dutilleux 2018).
Road Noise Emissions
At the present time, the current calculation of road noise emissions using the 2015/996 Directive is incorrect. This is because the CNOSSOS sound-power coefficients were drawn from the IMAGINE project using the Harmonoise propagation model. The majority of the frequency range (i.e., 63 Hz–8 kHz) predictions by Peeters and van Blokland (2018) calculate that CNOSSOS methodology underestimates noise levels of up to 3 dB due to transference in propagation models. Peeters and van Blokland found that after calculating the sound-emission level using sound-power coefficients presented in the Directive 2015/996 table F-1 and the CNOSSOS propagation model, outputs did not reflect the actual noise emission generated from roadside measurements.
Furthermore, Peeters and van Blokland highlight an inaccuracy in relation to road-surface reflection presented in Directive 2015/996. Directive 2015/996 states, “In this method, each vehicle (category 1, 2, 3, 4 and 5) is represented by one single point source radiating uniformly into the 2-π half space above the ground. The first reflection on the road surface is treated implicitly” (7). The directive assumes a “semi-free field” or “hemispherical” point source, and this assumption does not correspond with the IMAGINE/Harmonoise project. On the other hand, sound-power coefficients have been acquired for a free-field point source radiating into the 4-π space. Hence, “the first reflection on the road surface” (7) is not treated implicitly and should therefore not be treated as part of the propagation model. Directive 2015/996 relating to road surface reflection should be disregarded or modified to reflect a free-field (4-π) point source, whereby the first reflection on the road surface should be treated using the propagation model. Peeters and van Blokland modify the current coefficients in the Directive 2015/996 table F-1 (i.e., IMAGINE) to the CNOSSOS propagation model in order to calculate the correct sound-power coefficients for the CNOSSOS methodology.
There is also a discrepancy between the road-traffic emission and the propagation models within the CNOSSOS methodology. As such, the emission model applies a source in a free field, and the propagation model applies a source in a semifree field (i.e., above ground surface) (Salomons and Eisses 2018). This discrepancy can result in an error of 3 dB but may be resolved by a relatively elementary formula of correction (Salomons and Eisses 2018).
Railway Emission Calculation
In implementing the CNOSSOS-EU model in the context of Finland, Kokkonen (2018) expresses concern associated with the fact that railway emission calculation is fundamentally based on railway and wheel roughness. Finland has railway-roughness data for only a single point, and results indicate that there may be up to a 10 dB error when sound power is calculated using the CNOSSOS methodology. Kokkonen emphasizes the lack of guidance in relation to how national measurement of emission values should be considered within the CNOSSOS model, citing the example of absent guidance on how to adjust inaccurate speed correlation.
When default aircraft-noise data sets were reviewed, modified, and verified in relation to empirical data, Trow and Allmark (2018) found that noise outputs could vary by +/− 2 dB. More research is required in relation to aircraft-noise modelling, and models should move away from default datasets.
CNOSSOS does not provide a ground parameter value, G, for porous asphalt. In their investigation of the CNOSSOS propagation model in the context of the Netherlands, Salomons and Eisses (2018) use a ground parameter value of G = 0 for porous asphalt but suggest that G >= 0.5 is possibly more applicable. Furthermore, in relation to the actual CNOSSOS propagation model, an error exists in how modified heights from equations VI-19 should be applied to VI-20 (Kephalopoulos, Paviotti, and Ledee 2012, 89). Hence, alternative unmodified heights should be used.
In the context of diffraction, the CNOSSOS description of path difference -λ / 20 between the ground model and the diffraction model applicable to flat ground/low barriers and high barriers, respectively, is ambiguous within the CNOSSOS methodology. Screening attenuation in the context of multiple diffraction is also highly problematic. Salomons and Eisses (2018) demonstrate that placing a second noise screen at 500 m from source (with the first screen 1020 m from source) results in a completely unrealistic increase of up to 20 dB in sound at high frequency. This problem derives from the CNOSSOS method of calculating acoustic path length differential under favorable conditions, whereby such an approach does not translate to more than a single diffraction point. Salomons and Eisses propose a solution for multiple diffraction under favorable conditions.
The CNOSSOS-EU process is an ongoing procedure. The Commission Directive (EU) 2015/996, on which CNOSSOS-EU is based, will be modified appropriately in accordance with any future technical and scientific developments required for the successful implementation of the CNOSSOS-EU model, and, similarly, will make any adjustments deemed necessary, based on the experience of Member States. As such, the report reflects this continuous procedure and will be amended appropriately in parallel with future phases required under the CNOSSOS process.
The Noise-Adapt Project is being conducted by University College Dublin and Trinity College Dublin and is funded by the Environmental Protection Agency Ireland.
Kephalopoulos, Stylianos, Marco Paviotti, and Fabienne Anfosso Ledee. 2012. “Common Noise Assessment Methods in Europe (CNOSSOS-EU).” Common Noise Assessment Methods in Europe (CNOSSOS-EU).
Kokkonen, Jarno. 2018. “CNOSSOS-EU Noise Model Implementation in Finland and Experience of It in 3rd END Round.” Proceedings of Euronoise 2018.
Pallas, Marie-Agnѐs, and Guillaume Dutilleux. 2018. “Experimental Confrontation of Medium–Heavy Vehicle Noise Emission to the CNOSSOS-EU Prediction Method.” Proceedings of Euronoise 2018.
Peeters, Bert, and G. J. van Blokland. 2007. “The Noise Emission Model for European Road Traffic.” IMAGINE deliverable 11 (11).
Trow, James, and Claire Allmark. 2018. “The Benefits of Validating Your Aircraft Noise Model.” Proceedings of Euronoise 2018.