What’s the Difference Between Direct and Flanking Sound Transmission in Building Design?
Most real estate projects start with a well-detailed demising wall or floor. That’s essential, but in the real world, building noise often finds other circumventing routes. Sound can reach a neighbour in two ways: directly through the separation shown on the drawings, and indirectly by slipping around or through the surrounding structure. If there’s a single takeaway for architects to learn from an acoustical perspective it’s this: design the partition, then think about how sound could flank around it and design to prevent that leakage as well.
Direct Sound Transmission
Direct transmission is sound passing through the demising wall, floor, or ceiling. Heavier, stiffer, well-sealed assemblies do better because mass, stiffness, damping, and airtightness all matter. In practice that looks like added drywall, insulated cavities, and resilient clips or channels to decouple layers. In laboratory sound testing, this performance is reported as STC (Sound Transmission Class). Labs purposefully suppress any other sound transmission paths, so STC represents the best-case acoustic behaviour of the partition itself. Out on site, of course, experience tells us buildings walls and floors don’t function so cooperatively.
Flanking Sound Transmission
Flanking is sound that bypasses the partition via connected building elements—continuous subfloors or strapping, slab edges, gaps in the caulking, ceiling plenums, façade rails and mullions, structural steel, or ductwork. It shows up in two ways you’ll actually encounter on drawings and during coordination.
The first is sound leakage. Think of water sneaking around a dam. Unsealed head-of-wall, a missing bead of sealant at the base track, back-to-back device boxes, or a sloppy penetration are all straightforward air paths. When those gaps are sealed with the right acoustic sealant, rated firestopping, putty pads, and gaskets, sound leakage drops dramatically and the partition behaves closer to its lab promise.
The second is structural sound flanking. Here, sound energy vibrates a continuous element—say a plywood subfloor running under the demising wall, an exposed CLT panel ceiling running above the demising wall, or thin concrete layers above steel deck continuous below the demising wall. That element carries vibration laterally and re-radiates noise on the far side. It’s why an immaculate STC-60 wall can still yield complaints if the flanking element never breaks at the suite line. In the event of excessive structural sound flanking, the fix is to cut continuity or introduce decoupling at the demising line.
STC vs ASTC Testing
Field tests report ASTC (Apparent Sound Transmission Class). Unlike STC, ASTC captures the total experience—direct transmission plus both types of flanking. Because real buildings include these side routes, codes typically set ASTC minimums lower than lab STC values.
Water as Sound Analogy
The water analogy in the video below maps cleanly to real-world building site conditions. Water sliding around a wall is a good stand-in representation for sound leakage, which is why a proper caulked head- and base-of-wall makes such a difference. Water moving through the foam in the demo represents structural flanking, where the “material” itself carries energy from one side to the other. Cutting that foam at the wall is identical in spirit to interrupting a continuous subfloor or strapping at the demising line: you’ve broken the bridge, so far less sound energy can cross over.
