Understanding Flanking

Flanking noise is noise reaching a room by an indirect path. A couple examples would be noise reaching the room above your home theater from some route other than through your ceiling or noise reaching the next room by a route other than through the wall that seems to divide the two areas.

To visualize what flanking noise is, take a look at this sketch:

The red arrows in the sketch above are direct noise, noise passing through the wall into the next room. The orange, blue, and green arrows reflect noise that can make it into the next room via paths not related to the wall. Noise coming from these paths is called “flanking noise”.

Here in the real world, flanking noise can spoil even the best-executed wall or floor/ceiling installation if prudence in planning is not exhibited.

Imagine the scenario above. You have a single wood stud Green Glue wall like the one reported on in Orfield labs test report TL05 0414 or TL05 1035. That wall twice tested to STC=56, a very good level of performance. That wall will stop about 55 dB of sound over much of the vocal range, which would mean that 80dB of source noise would become 25dB on the other side of the wall. 25 decibels of sound would pass through the wall. If 40 dB of sound passes through the ductwork, however, then the net sound reduction isn’t 55dB anymore, it is only 40dB. If noise makes it through other paths as well, the net sound isolation may fall even further. In the hypothetical case above, the net noise reduction falls from the 55dB potential of the wall to just 41 dB - all because of flanking noise.

No partition can ever perform better than the level of flanking noise. In the scenario above, no modification you can make to the wall will help improve sound isolation. This is because it isn’t the wall that is failing, it is the ductwork. The only way to improve the sound isolation is to improve the ductwork. Imagine that you improved the wall by adding 10 more layers of Green Glue and drywall, you will still have 41dB of noise in the receiving room because that is how much noise is making it through the ductwork + concrete slab, etc.

Laboratory measurements - vs.- the real world

Laboratory tests are immensely valuable because they are done in standardized ways, in certified labs that meet various requirements, and allow us to compare the sound-stopping potential of different walls. But in laboratories great effort is made to minimize flanking noise, and as a result (at least in many cases) the results reflect just the performance of the wall, with no flanking noise effects at all. It is therefore reasonable to consider laboratory results as reflecting the potential of a given construction, and not what that construction is certain to yield. If seal quality is good, and if flanking noise is suppressed, then you can most definitely attain laboratory results in your home or construction. If these things are not tended to, however, performance will be less.

Ductwork can be problematic for the simple reason that it can provide a direct air path for sound to travel. There are many products available for taming sound coming through ducts. Among these are:

• Duct liner is your ally. Used lined ductwork (insulated ductwork). Lined ductwork contains sound absorbing material inside the duct to help destroy sound as it bounces it ways down the “tunnel” created by the duct. Also consider "flex duct" which is flexible and insulated. INSULATION MUST BE INSIDE THE DUCT TO BE EFFECTIVE IN THIS MANNER.
• Long, complex paths are ideal. Ideally the duct path from where you will be making some noise to where you need it to be quiet would contain bends and be as long as possible. This creates a more complex path for the sound to travel, gives the duct liner more distance/time to do its work. 90 or 180 degree bends in lined duct can be helpful as they force airborne sound to interact with the duct liner more extensively. Flex ducts within a joist cavity should at least have a serpentine "S" shape.
• Flex duct can be useful. Use flexible duct, but only in areas where the duct will not be exposed to direct sound. Flexible duct over some part of the path will break the structural path that the duct provides. Ducts can radiate sound traveling through the air they contain, but they can also radiate sound that they carry in their thin metal shells.
• Use soffits to cover ductwork exposed to sound. If ductwork has to be exposed to sound, it is very preferable to cover it with a soffit. If there is no way to avoid having ductwork exposed to direct sound, round duct will perform better than rectangular duct, and coating the duct with a viscoelastic material can also help. Thin metal ducts are little barrier to airborne sound, and therefore they should be exposed as little as possible.
• Coating a duct: Coating a duct with viscoelastic material can help mitigate structural sound, but in general this shouldn’t be considered as valuable as lining the duct.
• Exposed duct downstream is not a good sound barrier. If a critical area elsewhere in your construction must have ductwork exposed to the air, make sure the joints in that ductwork are sealed, and also consider coating the ductwork with a dense viscoelastic coating to reduce noise transmission. Even more extensive measures can be taken if need be, such as covering exposed duct with mass loaded vinyl or insulation + vinyl barrier. Many products for this application exist and a web search may prove illuminating. Sound can pass out of thin metal ducts as easily as it can pass into them.

Noise can travel as mechanical vibration through the structure of your construction - studs, joists, sub-floors, walls - to remote locations where the vibration can stimulate wall, ceiling and floor panels to create noise. Structural noise can be controlled in a variety of ways. When you can, make one critical room different than the rest of the house.

If there is one room in your construction where noise will need to be contained more than others, it is helpful to make that room a different design than the other rooms. This is helpful because low frequency resonance points are the weakest link of any wall, and it is at these frequencies that sound can most easily enter the structure. Upon traveling through the structure to another place in the house, if the resonances are the same (i.e., if the construction is the same) then the same resonances in remote walls is again easily stimulated.

In situations where low frequency noise can be heard many rooms away from the source, structural noise is the culprit. Often, the worst cases are when all the walls in a building are the same. Changing the low frequency resonance behavior of a wall can be accomplished by any of the following:

• Use a fundamentally different wall design like staggered or double studs.
  
• Use resilient decoupling such as sound clips or resilient channel on the studs.
  
• Use an effective damping material such as Green Glue on the walls.
  
• Just adding a second layer of drywall or a layer of soundboard or MLV will not accomplish this goal.
  

Last, but by no means least, is seal quality and other small air cracks directly connecting two rooms. Even the smallest seal failure can have catastrophic consequences to a high performance partition, proper caulking practice is critical.

Poor seal quality will make a poor performer out of any partition, even one of the best possible walls like a Green Glue damped double stud wall. In fact, very poor seal quality will drag the performance of even the best walls down from the stratosphere to the level of the wall in a cheap motel. If your walls are not very well sealed, all the Green Glue and drywall in the world won’t do you any good. The graph below illustrates the effect of seal quality on a variety of walls.

As this chart shows, the huge performance gains that modern sound isolation technology can deliver are completely lost if seal quality is poor.

Do I need exotic specialized acoustic sealants? You should strive to use an acoustic sealant because acoustic sealants have been designed to be flexible, and to remain flexible. Flexibility prevents the seals from cracking over time and is an important component.

However, it is not necessary to utilize wildly expensive exotic materials often priced as high as $20 per tube or more. You can simply use common acoustic sealants such as Green Glue Sealant. Green Glue Sealant is water-based, low odor, easy to clean up, and dries to a non-sticky state.