Longitudinal transfer

Professional Definition

Flanking transmission is the phenomenon of transmitting acoustic energy from a source room to a receiving room via paths other than the direct barrier separating these rooms. It includes the transfer of both airborne and structure-borne sound through structural elements connected to the separating barrier (side walls, ceilings, floors) and through service ducts.

In acoustic engineering, this phenomenon is crucial in calculating the overall sound insulation ($R'$). It often turns out that even if the barrier itself has a high insulation rating ($R$), flanking transmission drastically reduces the actual protection against noise. Acoustic energy induces vibrations in adjacent elements, which then radiate sound into the neighboring room.

Acoustics in Simple Words

Imagine you want to stop water flowing down a wide river by building a solid dam in the middle of it (your partition wall). However, if you forget to seal the edges of the river and the bottom, the water will simply flow around the dam at the sides and underneath. In building acoustics, flanking transmission is exactly those "unsealed edges."

Sound striking a wall or floor in the neighbor's apartment behaves like a vibration running along railway tracks. Even if you place an obstacle on the tracks, the vibrations of the rails (i.e., the ceiling and side walls) will transfer underneath it and be audible on the other side. That is why modern construction uses expansion joints and floating floors – to "cut the tracks" and prevent sound from traveling through the building's structure.

Summary

Flanking transmission is the main reason why the insulation of a partition wall can often be ineffective. Sound bypasses the obstacle, traveling through ceilings and side walls. Effective noise control therefore requires a holistic approach, taking into account the insulation of all surfaces bordering the given room, not just one wall.