Maxx Minutes Blog

Solving the Floor/Ceiling Sound Puzzle -
Flanking Paths

In last month’s MaxxMinutes, we discussed components of a floor/ceiling assembly as they relate to sound control. In order for all the components to work together, it is necessary to prevent sound from circumventing these components by eliminating as many flanking paths as possible. The term “flanking paths” refers to any sound path around a building element. Examples of flanking paths are recessed and canister light fixtures, openings around furnace ducts, plumbing runs, wall/floor junctions that are not caulked, sprinkler heads, etc.
Flanking Paths
  • Bathrooms – Bathrooms are a principle source of noise complaints. Invariably, the problem is related to openings around the bottom of the toilet, bathtub, and large amounts of plumbing running through the walls of a relatively small space. When investigating noise complaints large open spaces below the bathtub and around the toilet plumbing are usually found. These openings should be treated with flexible fire stop materials to close off the sound path. Ideally, bathtubs and showers should be set on top of a completed floor. The Maxxon Underlayment should be poured throughout the entire bathroom area and the fixtures set on top. If the fixtures are installed first and the Maxxon Underlayment poured afterward, there is greater potential for sound flanking. 
  • Heavy Timber Renovations –Old mills and factories that are converted to multifamily housing (heavy timber renovation projects) particularly struggle with flanking paths. Existing penetrations, gaps between wood planks, large cracks in structural beams that pass through multiple units, etc. are all flanking paths.  Special attention is necessary to thoroughly address flanking paths in heavy timber renovation projects.
  • Structural Flanking Paths – When items like resilient channel are installed rigidly as opposed to being properly installed (like a spring), impact vibration travels past the channel without having to interact with it. This creates a structural flanking path. Care must be taken to make sure all vibration breaks (sound mats, resilient channel, etc.) are installed correctly.


Relative to flanking, any penetration that is not properly treated acoustically in a wall or floor/ceiling system is going to substantially reduce STC and/or IIC ratings below the acoustical design of the building. Workmanship cannot be over-emphasized in the areas mentioned above. If openings are not treated or assembly components are improperly installed, it makes little difference if the Maxxon Underlayment is 3⁄4" or 3" thick as sound will pass through these areas regardless of depth.