Definition
Flanking transmission is sound that travels around a partition rather than through it — through junctions, ceiling voids, ducts, penetrations and structural connections. It is the main reason a partition that tests well in the laboratory performs worse on site.
Common Flanking Paths
- Ceiling and floor voids — sound passes over or under a partition through the cavity.
- Junctions — rigid connections at wall/floor/ceiling intersections transmit structure-borne sound.
- Penetrations — pipes, ducts and electrical boxes create direct air paths.
- Doors and windows — gaps and leaks sharply reduce effective insulation.
Why It Matters
Field insulation (apparent R′w / FSTC) is usually lower than the laboratory rating because of flanking. Controlling flanking is as important as the partition itself — see How Sound Insulation Is Measured.
How to Control It
- Break the path — seal ceiling and floor voids, and continue the partition full height.
- Decouple — resilient connections reduce structure-borne transmission at junctions.
- Seal penetrations — acoustic sealant around pipes, ducts and boxes.
- Upgrade doors and windows — solid-core doors with perimeter seals; well-fitted windows.
Relevant Standards
- ISO 10848 — laboratory measurement of flanking transmission.
- ISO 16283 — field measurement of sound insulation.
Limitations
Flanking is assessed in the field because it depends on the whole building construction. A laboratory rating of the partition alone does not guarantee on-site performance.
Related Yamamah Solutions
For flanking-control detailing, contact us for a free acoustic consultation.
Frequently Asked Questions
Can a high-STC wall still perform poorly? Yes — if flanking paths are not controlled, on-site performance can be far below the laboratory STC.
How do I find flanking paths? Through field measurement and inspection of junctions, voids and penetrations.
Verified Sources
- ISO 10848 — Flanking transmission measurement.
- ISO 16283 — Field measurement of sound insulation.