Stop the Sound

Robert Grupe / October 2021

Can you explain the term “flanking” relative to acoustically rated partitions?

“Flanking” in a military sense means to circumvent the enemy’s front line. The same is true in acoustics. Focusing on an acoustically rated partition, this would translate to sound energy circumventing what otherwise could be high performing design. Flanking paths can have a devastating impact on acoustics. Sound energy can be seen as water in a bucket. If there is any way for the bucket to leak the water, the water will find it. In a past “Wachuwannano” article the important role of acoustical sealant in maintaining the intended level of performance was discussed. In that case, the attention was on sound leaking around the perimeter of the wall, but flanking can occur in other ways as well.
The presence of a common interior door can be a major source for flanking. Both the composition of the door and its installation are major culprits. Tests run on a baseline 48 Sound Transmission Classification wall revealed that a hollow core door can drop the overall STC to the upper teens. Even going to a solid core door with its increased mass brought the STC to only 20. Sealing the door in place, essentially making it inoperable, still kept the STC below 30. The location of doors in relation to other doors can have a negative effect on the proposed acoustical design.
Getting back to the wall itself, the location of outlet boxes that services the adjacent spaces is critical. Outlet boxes should never be directly opposite each other in a wall. Taking that concept one step further, they should not be located within the same stud cavity. Outlet boxes should be fitted with specially designed pads that help minimize sound transmission. Bathtub and kitchen cabinet alignment can be a source for flanking paths. Also, how plumbing is supported in a stud cavity may have a negative impact on the required STC performance. Plumbing brackets can serve to couple the two sides of a chase wall and produce the unintended consequence.
Another source for flanking paths depends on how the HVAC is laid out. A forced air system with a trunk line that directly feeds adjacent units that require acoustical privacy will create an unacceptable result. The more circuitous the duct work extending from the trunk to independent registers, the better. There are also acoustical mufflers that can be installed within the duct to further reduce sound transmission within the duct.
Partition layout also plays a part. An acoustically rated partition that terminates into another partition is of concern. The acoustical partition should penetrate into the stud cavity of the wall it’s intersecting. A gypsum panel should not be continuous from one side of the acoustic partition to the other side, or sound will flank the acoustical partition. This is true at the ceiling plane as well. The ceiling gypsum panel should not be continuous over the top of the acoustical partition. It will produce the same result.
In multifamily conditions, the floor and wall framing become critical where they intersect. Tests run at the National Research Council of Canada revealed major weaknesses in impeding sound transmission through the interface. In the test, four rooms are separated by a common vertical wall and a common floor ceiling assembly. It can be seen as two rooms stacked on top of two others. Sound transmission was then measured through the assemblies. For the rooms directly adjacent to each other and with floor joists running parallel to the wall, it was determined that there were three flanking paths if the subfloor ran continuously between the two rooms. The wall itself provided a 52 STC, but the flanking paths reduced it to an apparent 45. When the test was run with the floor joists perpendicular to the wall and the subfloor was continuous, the apparent STC dropped to a 38.
The tests revealed two flanking paths when investigating sound transmission vertically from the upper room to the lower one. Here the STC through the floor assembly was a 56, but the two paths reduced the number to an apparent 52. That particular test was run with parallel running floor joists and the subfloor running continuously. This testing led to the concept of the apparent STC, which is when the total STC is equal to the sum of all paths.
It is incumbent on all trade factors, including the designer, to understand their individual roles in obtaining the desired acoustical performance. Adjacent room layout and the installation of all mechanical, electrical and plumbing services are all critical components. A successful project for the contractor not only relies on proper materials and installation techniques of a single assembly, but also the correct overall design and placement of materials that are known to facilitate flanking paths.

Robert Grupe is AWCI’s director of technical services. Send your questions to, or call him directly at (703) 538.1611.