MithraSIG - Optimizing Noise Impact Assessment: A Comparative Analysis of CNOSSOS 2012, CNOSSOS 2020, and NMPB 2008 Methods

In June 2020, Geomod and CSTB introduced a significant evolution to the CNOSSOS method in response to the initial trials of the 2012 approach. This update, officially implemented from 2021, became part of the Annex II modification of the Directive. Simultaneously, the harmonized European method CNOSSOS-EU was also updated in December 2020, marking a crucial milestone in environmental noise management. 

The New Method: CNOSSOS 2020

CNOSSOS 2020, a novel approach optimizing the consideration of the affected population, builds upon the concept introduced by the CNOSSOS-EU 2012 method with a notable nuance. CNOSSOS 2020 considers only the half of a building's most exposed facades, enabling a more evenly distributed impact on the population. This adaptation, stemming from the Geomod/CSTB proposal, appears to be a prudent compromise, especially when the distribution of housing within buildings is not fully known.

Population Calculation Methods by MithraSIG

In its pursuit of precision, MithraSIG offers three distinct methods for calculating populations affected by noise:

1.    NMPB 2008 Method : Focuses on the most exposed facade of each building, attributing the calculated noise level to all inhabitants of the building. This French method (maximum level per building) is conservative in terms of the exposed populations.

2.    CNOSSOS-EU 2012 Method : Considers all facades of buildings, distributing inhabitants based on the length of each facade relative to the total length of facades. This method (proportionality between the number of receptors and the number of inhabitants) is optimistic, assigning 50% of the population to low noise levels. Note: In the case of individual dwellings (single housing unit), it associates all inhabitants with the level of the most exposed facade.

3.   CNOSSOS-EU 2020 Method : Behaves similarly to the CNOSSOS EU 2012 method, except that it considers only half of the most exposed facades of the building. Inhabitants are then proportionally distributed based on these remaining facades. This method, originating from the Geomod/CSTB proposal (100% of inhabitants associated with 50% of receptors), seems to strike a good balance when the distribution of housing within buildings is unknown.

Practical Illustration: Differences Between Methods

Using calculation receptors placed by the software on building facades, MithraSIG determines the number of inhabitants exposed at each noise level interval. Each receptor represents a facade.

Example 1: Collective Building Case

To illustrate differences between methods, consider a collective building measuring 24 m by 18 m housing 100 inhabitants. The NMPB 08 method assumes that all inhabitants are exposed to the loudest facade, while the CNOSSOS-EU 2012 method assigns inhabitants to the less exposed facade. In contrast, the CNOSSOS-EU 2020 method shows a more homogeneous population distribution.

Here are the results obtained for each method:

Example 2: Urban Area Case

Explore method differences in a densely populated urban area where 6073.7 inhabitants are assigned to residences.

Here are the results obtained

Example 3: Suburban Area Case

Examine variations in methods in a suburban area near a highway, where 1309.9 inhabitants are assigned to residences.

Here are the results obtained

Conclusion: Striking a Balance with CNOSSOS 2020

This analysis underscores the significant impact of population calculation methods on results. Opting for the CNOSSOS-EU 2020 method appears to strike a balance, reducing the population exposed to high noise levels. Whether in urban or suburban environments, the NMPB 2008 method overestimates exposures, while the CNOSSOS-EU 2012 method exhibits the opposite trend. CNOSSOS-EU 2020 emerges as a balanced solution, emphasizing the importance of judiciously choosing the calculation method for an accurate assessment of noise impact.