A recording studio in a converted warehouse in Brooklyn spent $180,000 building a control room with STC 65 walls. The client wanted complete acoustic isolation from the live room next door. The construction was meticulous — double-stud walls, resilient channels, triple-layer drywall, sealed penetrations. The contractor proudly told the acoustic engineer that the partition was the highest-rated assembly in the catalogue.
The studio opened. The first day of recording, the engineer in the control room could clearly hear the drummer in the live room. Not through the magnificent wall — through the structural concrete floor they shared, which ran continuously under both rooms with no isolation break. The $180,000 wall was performing exactly as specified. The floor, which no one had thought about, was undoing all of it.
This is flanking transmission. It is the acoustic equivalent of building a ten-metre-high security fence and leaving a five-centimetre gap under the gate.
The Definition: What Flanking Transmission Is
Flanking transmission is the propagation of sound from one room to another by paths that bypass the primary separating element — the direct wall or floor between them — by travelling through the surrounding building structure or through connected air cavities.
The word "flanking" is a military metaphor: an attack that goes around the side of the defensive line rather than directly through it. Sound does exactly this when a wall with an STC of 55 is surrounded by connected structural elements with effective STC values of 35.
Sound is energy, and like all energy, it takes the path of least resistance. In a building, the direct path through a well-designed, decoupled partition with multiple layers of drywall may represent an STC of 55 — a significant barrier. But the flanking paths — through the concrete slab that extends under both rooms, through the structural columns that penetrate the partition, through the shared ceiling plenum, through the HVAC ductwork — may represent an effective STC of 35-40. Sound preferentially travels via the flanking paths, and the high-performance partition becomes largely irrelevant.
The governing standard for predicting flanking in buildings is ISO 15712 (Building acoustics — Estimation of acoustic performance of buildings from the performance of elements), which provides a calculation framework for summing the direct and flanking transmission paths to predict the overall field performance. The field measurement standard is ASTM E336 (Standard Test Method for Measurement of Airborne Sound Attenuation between Rooms in Buildings) for airborne sound, and ASTM E1007 for impact sound.
The Five Main Flanking Paths
Understanding flanking requires knowing which paths are most likely to be problematic in different building types. The five most common flanking mechanisms are:
1. Structure-Borne Transmission Through the Floor Slab
In buildings with a continuous concrete floor slab, structural vibration induced on one side of a partition propagates laterally through the slab, continues past the partition, and re-radiates into the adjacent room. The concrete slab is a highly efficient waveguide for structural vibration — a frequency that the wall blocks at STC 55 may travel through 20 metres of slab with only 10-15 dB attenuation.
This is the mechanism that destroyed the Brooklyn recording studio example. The drummer's impacts entered the slab directly. The slab carried those impacts under the wall and into the control room, where the slab's underside vibrated and radiated sound into the space. The wall STC was irrelevant to this path.
Mitigation: Structural isolation breaks — rubber isolators, spring mounts, or resilient pads under floating floors — interrupt the vibration path at the partition line. In critical applications, the floating floor is extended across the entire room, isolating the floor surface from the structural slab entirely.
2. The Open Ceiling Plenum
In commercial office buildings with suspended acoustic ceilings, interior partition walls frequently terminate at the underside of the suspended ceiling rather than extending up to the structural slab above. The space between the suspended ceiling and the structural slab — the ceiling plenum — is an open air cavity shared by all rooms on a floor.
Sound generated in one room travels through the partition up to the suspended ceiling height, enters the plenum through the ceiling tiles (which have STC values of perhaps 30-35 in the cavity direction), propagates across the plenum as an airborne wave, and then comes back down through the ceiling tiles of the adjacent room.
The effective STC of this path is determined by the ceiling tile's STC rating — typically 30-35 — regardless of what the partition between the rooms is rated. A wall specified at STC 55 becomes effectively STC 35 when paired with an open plenum, because the plenum path completely bypasses the wall.
Mitigation: Extend the partition full-height from structural slab to structural deck (not from slab to suspended ceiling). Where full-height partitions are not possible, add a sealed plenum barrier — an acoustic blanket or board erected on top of the suspended ceiling at the partition line, creating an acoustic seal between the two plenums.
3. HVAC Ductwork
Supply and return air ductwork passes between rooms and carries sound through the building in two ways. Airborne sound can propagate through the duct itself — the voice raised in a meeting room travels into the supply duct and emerges through the supply grille of an adjacent room. Structure-borne sound transmitted to the duct by a fan or vibrating component travels through the metal duct walls and radiates into rooms along the duct path.
A single unlined rectangular duct can carry sound with attenuation as low as 0.5-1 dB per metre — meaning a conversation 10 metres away down the duct arrives at only 5-10 dB attenuated. In a quiet room with NC 30 background, a conversation at 10 metres down a duct is very audible.
Mitigation: Acoustic duct lining (fiberglass or mineral wool applied to the interior of duct sections), sound attenuators (factory-made in-line silencers with absorptive splitters), and careful routing of ductwork to avoid running a single duct branch between acoustically sensitive rooms. Where flexible connections are used at equipment, vibration isolating flexible connectors prevent structural vibration from the fan entering the duct system.
4. Back-to-Back Penetrations
Electrical outlets, data ports, light switches, and plumbing penetrations are frequently located on opposite sides of the same stud bay in a partition wall. When two boxes are placed back-to-back in the same cavity without a solid barrier between them, sound from one room enters one box opening, travels through the shared stud cavity, and exits through the other box into the adjacent room.
A single pair of back-to-back electrical outlets in an otherwise STC 55 wall can reduce the field performance to STC 40 or below. The acoustic engineer reviewing a wall assembly that underperforms in field testing will typically find these penetrations as the primary cause.
Mitigation: Offset electrical boxes so they are never in the same stud bay. Seal all penetrations with acoustically rated putty pads around the back of each box. Where back-to-back installations cannot be avoided, use a protective cap or acoustic putty pad on both boxes to seal the air gap.
5. Structural Columns, Beams, and Framing Connections
In steel-framed buildings, columns are typically continuous structural elements that penetrate partition walls at regular intervals. Because the column is rigidly connected to both the source-side wall framing and the receiving-side floor and ceiling structure, vibration entering the column on the source side is conducted efficiently to all connected elements.
Similarly, light timber or steel stud framing that connects directly to a floor joist or ceiling beam (rather than being isolated from it) creates a rigid bridge between the two rooms that transmits structural vibration around the acoustic partition.
Mitigation: Provide resilient isolation between the partition framing and the structural floor, ceiling, and column elements. In critical studio or recording applications, the partition is sometimes built as a "room within a room" — a freestanding structure that makes no rigid contact with the building structure except at isolated foot pads.
A Worked Example: The Office Partition That Did Not Work
A law firm fitted out two adjacent private offices with the following specification:
- Partition: Metal studs, two layers of 13 mm drywall each side, mineral wool cavity insulation, resilient channel mounting
- Specified laboratory STC: 54
- Plenum treatment: None — partition terminates at 2.7 m suspended ceiling height; structural slab is at 3.5 m
- Electrical outlets: Back-to-back positions in the same stud bay at desk height
- HVAC: Shared return air plenum above the ceiling; no duct lining
The STC gap is 17 points — from a specified 54 to a measured 37. At FSTC 37, loud conversation is clearly audible in the adjacent office. Confidential client discussions are not private.
Acoustic investigation identified three active flanking paths:
- Plenum path (dominant): Estimated effective STC of plenum path approximately 32. The open plenum above the partition was transmitting more sound than the partition was blocking.
- Back-to-back outlets (significant): Two pairs of back-to-back outlets were reducing the effective STC of the partition assembly.
- HVAC return duct (minor): A shared return grille connected both offices to the same duct cavity without any lining.
- Installing an acoustic plenum barrier on top of the suspended ceiling at the partition line
- Sealing back-to-back outlet boxes with acoustic putty pads and relocating one pair of outlets
- Adding acoustic duct lining to the common return duct section
- Total remediation cost: approximately 12% of the original partition installation cost
How Flanking Is Calculated: The ISO 15712 Framework
ISO 15712 provides a method for summing the contributions of direct and flanking transmission paths to predict the overall sound insulation between rooms. The framework uses a concept called vibration reduction index (Kij), which characterises how much structural vibration is attenuated as it crosses each junction between building elements (a T-junction where a wall meets a floor slab, for example).
The overall transmission is the energy sum of all paths:
R' = -10 log( 10^(-R_d/10) + sum of 10^(-R_fn/10) )
Where:
- R' is the predicted apparent sound reduction index of the complete assembly (equivalent to FSTC)
- R_d is the direct transmission through the partition
- R_fn are the sound reduction indices of each individual flanking path n
Common Mistakes That Create Flanking Problems
Specifying STC without specifying flanking control: A wall specification that states only "STC 55" without addressing plenum treatment, penetration sealing, and structural isolation is an incomplete acoustic specification. The STC number describes one path; a complete specification addresses all paths.
Building to code minimum without checking the complete assembly: Building codes typically specify minimum FSTC values. A contractor who achieves a high laboratory STC for the partition but leaves the ceiling plenum open is complying with the letter of the partition specification while leaving the FSTC well below the code requirement. Code compliance must be verified by post-construction field measurement.
Designing partition details without coordination with MEP engineers: The most common flanking problems — back-to-back outlets, open plenums, unlined ducts — arise from a lack of coordination between the architectural acoustic designer and the mechanical and electrical engineers. Acoustic requirements for penetration locations, duct lining, and plenum barriers must be explicitly coordinated in the construction documentation, not left to individual subcontractors.
Assuming that expensive assemblies eliminate the need for flanking analysis: High-performance assemblies cost more because they use more materials and more sophisticated decoupling. They are worth specifying for critical applications. But their performance advantage is entirely consumed if flanking paths are left unaddressed. The last 5 dB of partition improvement is useless if the plenum is transmitting sound at STC 35.
How AcousPlan Helps You Control Flanking
AcousPlan's Sound Insulation Calculator includes flanking transmission modelling as part of the overall partition performance assessment, using the ISO 15712 framework to give you a realistic estimate of field performance before construction begins.
Within the platform, you can:
- Model direct and flanking transmission paths simultaneously, including the ceiling plenum path, structural floor slab path, and HVAC ductwork paths, to predict the resulting FSTC rather than the idealised laboratory STC
- Identify the dominant flanking path in your specific assembly configuration — the tool shows which path is carrying the most energy so you know where to invest in mitigation
- Compare the cost-effectiveness of upgrading the partition versus addressing flanking paths — in most cases, flanking control delivers more FSTC improvement per dollar than additional partition layers
- Generate specification checklist output that details the flanking control requirements — plenum barrier locations, penetration sealing specifications, duct lining requirements — for coordination with MEP engineers during design
- Check resulting FSTC compliance against IBC 2021 Section 1207, WELL v2 Feature 75, and other applicable standards
Ready to model your assembly including flanking paths? Try the Sound Insulation Calculator at /insulation — input your wall assembly, ceiling configuration, and structural details, and get a realistic FSTC prediction before you build.