What is flanking noise and why it should be carefully assessed

According to ISO 12354-1, flanking noise can be defined as “the transmission of sound energy from an excited element in the source room to a receiving room via structural (vibration) paths in the building construction, e.g. walls, floors, ceilings”.

If a building is not properly analysed and flanking paths are left “untreated”, even when the main separation wall/floor offers high acoustics resistance performance, the overall sound insulation between two spaces may be substantially lower than initially predicted.

Figure 1 refers to examples of junction configurations which can be problematic if not carefully assessed.

Figure 1 - Junction configurations which may result in an overall low sound insulation between spaces

Calculating flanking transmission

Flanking transmission can be determined in accordance to the methodology laid out in ISO series 12354. These standards set out a detailed and a simplified model to carry out predictions, based on the performance of each element involved in the sound transmission.

Figure 2 illustrates the typical sound transmission paths to consider between 2 rooms. For spaces with a simplified geometry this translates to a minimum total of 13 different transmission paths.

Figure 2 - Sound transmission paths between 2 rooms

The calculation approach requires the following input data:

— sound reduction index of separating element: Rs;

— sound reduction index for element i in source room: Ri;

— sound reduction index for element j in receiving room: Rj;

— sound reduction index improvement by additional layers added to the separating element in the source room and/or in the receiving room: ΔRD, ΔRd;

— sound reduction index improvement by additional layers added to the element i in the source room and/or element j in the receiving room: ΔRi, ΔRj;

— structural reverberation time for an element in the laboratory: Ts,lab;

— vibration reduction index for each transmission path from element i to element j: Kij;

— normalized direction-averaged velocity level difference between element i to element j: Dv,ij,n

— flanking normalized level difference Dn,f;

— area of separating element: Ss;

— area of element i in source room: Si;

— area of element j in receiving room: Sj;

— common coupling length between element i and element j as measured from surface to surface: lij.

NOTE: The complete calculation method is described in detail in the aforementioned relevant documents.

Consequently, the flanking sound reduction index for each of the transmission path can be determined and solutions can be employed to minimize transmission where required. Common solutions consist of: wall linings, floating floors and dropped ceilings.

A detailed assessment of sound transmission between spaces will allow to minimize the use of build materials which may result in substantial construction cost savings.

Flanking transmission for CLT elements

Regarding CLT elements, ISO 12354-1/2:2017 has introduced empirical data for junctions characterized by vibration reduction index, Kij, since the structural reverberation time is in most cases principally determined by the connected elements.

This information significantly improves sound insulation predictions between spaces for CLT building structures which are becoming more relevant nowadays due to their small carbon footprint when compared to a concrete/metal frame building.

Figure 3 - Junctions between CLT elements (characterised by vibration reduction index, Kij)

Final considerations

Flanking transmission should be carefully assessed to make sure sound insulation between rooms achieves the required criteria. The methodologies included in ISO 12354 standards allows for significantly accurate predictions which in turn make up for better design decisions leading to reduced construction costs.


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