Flanking Noise Through a Wall

This application note is about Flanking Noise Through a Wall. In a previous application note, Sound Transmission Through a Wall, we discussed how sound is transmitted through the structure of a wall.  Sound tends to take the path of least resistance through a wall, and can easily pass from one side of the wall to another, through so-called flanking paths.  A flanking path is any sound transmission path through a wall that has a lower sound transmission class (STC) than the wall structure, which will typically have STC values in the mid- to high-30s depending on construction.  Figure 1 shows a schematic of numerous noise flanking paths, including the following:

Flanking noise through a wall
  • Windows – Exterior windows account for a significant noise transmission path for outdoor noise, such as automobile or airplane traffic. Typical single pane windows will have STC values in the 26-28 range, meaning that they will pass 8-10 dB more sound than the wall structure.  Some of this noise passes directly through the window glass, and some of it may pass through the window pane or mullion.
  • Doors – Doors are a noise transmission pathwithin the interior of a home or office. Hollow core wood doors have STC values in the low-20s and solid core wood doors have STC values in the high-20s.  Other potential paths through which noise could pass include the gap through the door threshold and through windows in the door.
  • Ceilings – Walls will typically be built only to the height of the ceiling. Sound from one room may therefore easily pass through the ceiling and into the attic (or open space between floors).  This sound could then pass through the ceiling of adjoining rooms.  Other sources of noise in the ceilings could include exterior noise that may pass through attic ventilation louvers, or noise that may pass through ceiling lights and electrical boxes.
  • Ducts and Vents – Heating, Ventilation and Air Conditioning (HVAC) supply and return ducts, and vents may connect one room to another with no sound attenuation treatments within the ducts. These ducts may be a source of noise (from HVAC fans) as well as noise transmission paths between rooms.
  • Floors – The sound of people walking on the floors may transmit vibrations under the walls and into adjoining rooms. This is due to the fact that typical room construction involves the use of common floors onto which walls are built.  These vibrations are transmitted through the floor and converted into acoustical energy in adjoining rooms.
  • Electrical Outlets – Electrical outlets, lights, switches and other boxes are comprised of thin plastic covers, and light gauge metal or plastic boxes through which noise can readily pass.

It is therefore not a straight-forward matter to simply add an acoustical barrier material, such as mass loaded vinyl to a wall and expect significant sound attenuation.  The numerous potential noise flanking paths must therefore be considered and addressed together with the direct sound transmission path through the wall in order to develop an effective sound attenuation plan.

Sonic-Shield has an engineering staff with advanced technical capabilities who can assist individuals and building owners in the testing, analysis and solution of flanking noise problems.  We can perform the required acoustic tests and analyses to arrive at a solution, as well as develop and supply the appropriate sound abatement materials and products to significantly reduce flanking noise.