One of the least talked about aspects of sound isolation is something called flanking noise. What is flanking noise? Flanking noise is noise reaching a room by an indirect path. For example noise reaching the room above your home theater from some route other than through your ceiling, noise reaching the next room by a route other than through the wall, and so on.
Part 1 - What is flanking noise?
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.
Part 2 - Understanding the importance of flanking noise
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 duct work, 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 39 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 duct work.
The only way to improve the sound isolation is to improve the duct work. Imagine that you improved the wall by adding 8 more layers of Green Glue and drywall…. you will still have 41dB of noise in the receive room because that is how much noise is making it through the duct work and 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. However in laboratories great effort is made to minimize flanking noise, as a result (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 the performance will be less.
Part 3 - Potential flanking path sources and how to deal with them
In this section, we will take a look at some of the many things that can cause the performance of sound-isolation partitions to suffer. These are things that can compromise an otherwise excellent design.
If you are trying to stop sound from going from one room to the next, and a door connects the two rooms, the door is very likely to be the weak link. In other words, more sound is likely to come through the door than through any high quality soundproofed wall.
Even in situations where a door doesn’t directly connect the two rooms, doors can still be problematic. Sound going out one door, down a hall, and in another door to the neighboring room can often exceed the sound going through a high quality wall connecting the two rooms.
Mass and seal quality
Some simple and low-cost guidelines that will help you get good performance out of your doors are these
1. Use heavy solid-core interior slab mounted in an exterior jamb. Mass is one of the key components of any sound isolation scheme, and it’s important in doors as well. The solid core contributes mass, and the exterior jamb has weatherstrip all around for a reasonable seal. Do not use hollow core doors where sound isolation is important under any circumstances.
2. Concentrate on ensuring that seal quality is good. Specialized seals should be considered when performance is critical. Such soundproofing door seals are available on our site.
Specialized acoustic doors
Various manufacturers offer specialty, engineered acoustic doors with elaborate seal systems and other goodies that allow performance to be much higher than normal doors. Trademark Soundproofing would advise you to not just look at STC ratings, but to ask the manufacturers for transmission loss data at lower frequencies as well. Click here to see some more detailed tips on how to soundproof a door.
If a door directly connects two rooms, the best option is a communicating doors system - two doors that form an “airlock” between the two rooms. This configuration yields higher performance than any conceivable single door, and it also grants us the most tolerance of somewhat compromised seal quality. Still, seal quality is critical, even for communicating doors. It is completely reasonable to consider two heavy communicating doors preferable to any realistic single door, engineered or not. If you opt for communicating doors, it is a great idea to put some type of sound absorption in the cavity between the two doors. Cloth-wrapped rigid fiberglass or acoustic foam are two excellent choices.
Upgrading a door
Whether you select a DIY solid core choice or communicating doors, the addition of Green Glue + mass in the form of a sheet of MDF, plywood, etc., can increase the performance of your door considerably. Green Glue benefits doors by damping the resonances that typically limit the STC of these assemblies.
Electrical outlets can cause sound isolation problems as well. Outlets create holes in the wall. Instead of having two layers of drywall and Green Glue, now you have just a tin can with a thin plastic lid. Outlets also tend to be poorly sealed.
To avoid compromising sound isolation with electrical outlets, follow these basic guidelines:
1. Never put outlets back to back, always put them in separate stud cavities
2. Seal the outlets well with acoustical caulk. Seal quality around the outlet is critical.
3. Use an acoustical pad around the outlet.
4. Use insulation in the wall (something you should anyway do if you care about sound isolation). Insulation helps absorb sound as it travels from one outlet box to another.
DuctworkDuctwork 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:
1. Duct Liner.
Used lined ductwork (insulated ductwork). Lined ductwork contains sound absorbing material inside the duct to help destroy sound as it bounces its way down the “tunnel” created by the duct. Insulation must be inside the duct in order to be effective in this application. Also consider "flex duct" which is flexible and insulated.
2. 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.
3. 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.
4. 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. Our favorite is to frame out the soffit and then wrap it with 2 layers of drywall with Green glue sandwiched in between.
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
5. 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.
6. 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. 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, subfloors, 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.
Among these are:
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 construct 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 whisper 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.
Mechanical decoupling can help
The use of modern sound clips, or constructions such as staggered or double studs can be effective by keeping sound (vibration) from reaching the structure in the first place. That is how these systems are effective - they provide breaks that keep drywall on the inside of a room from being in mechanical contact with the rest of the structure.The ability, however, of decoupling schemes to reduce structure-borne sound is just like their ability to reduce airborne sound: they are effective only at frequencies well above their primary low frequency resonance. At and around this resonance, they will worsen the situation for both airborne and structure-borne noise, not help it. That is the nature of the beast when it comes to decoupling.
Since most walls are not decoupled, the construction of a decoupled wall usually meets the criteria above for non-homogeneous construction as well. Decoupling options include room within a room, resilient channel, staggered studs, resilient sound clips, and other options.
By far the most effective form of decoupling is the “room within a room”, where double rows of studs are utilized in combination with separate ceiling joists to create a situation where almost no mechanical connection exists between the room and the rest of the structure.
Structure-borne noise is reduced, as there is no mechanical path from noise to the main structure of the construction.
Viscoelastic damping can help. Viscoelastic damping materials such as Green Glue can contribute greatly to reducing structure-borne noise. This is because damping, by definition, is energy dissipation. As energy travels along an undamped structure, very little of it is dissipated, and it can travel great distances. When energy travels over a highly damped structure on the other hand, the energy is quickly dissipated - the damping materials convert the energy to heat.
The use of Green Glue in a structure can dissipate structure-borne noise more than 10 times faster than undamped structures. Quite simply, the noise cannot travel far enough through a very well damped structure to cause disturbance at long distances. Unlike decoupling, the effect of damping is not frequency dependent.
These graphs show the impact of damping on the dissipation of energy over distance in a structure.
The tan graph shows the decay over 8 feet in sheet of standard drywall that was stimulated at one end by a mechanical shaker. The energy level essentially doesn’t decline over this much distance.
The green graph shows the decay over 8 feet in 2 sheets of drywall laminated with Green Glue. The decay is enormous, in just 8 feet the level falls to just above the noise floor of this measurement (noise floor is denoted by the red line).
The best choice is, as always, a combination of damping and decoupling. For example, the low frequency resonance of a heavily damped double stud wall is so much less severe than a conventional double stud wall that this problem can be simply disregarded.
Structural breaks can help
While we can’t list every opportunity for minimizing structural connections, we hope that the diagram above illustrates the point.
Floating floors can help. The use of floating floors can help mitigate flanking noise via structural paths as well. A floating floor is a floor that has a surface “floating” on top of some type of resilient layer. The floating surface may be wood, multiple layers of wood, gypsum concrete, or various other materials. The resilient layer comes in many forms, from rigid fiberglass to rubber mats to other elaborate engineered systems.
It is wise to leave a mechanical break between the floating element and the neighboring structure, as shown here.
A floating floor can be extremely expensive, and if you are attempting to soundproof a basement room you should consider sound-treating the concrete floor via creation of a floating floor only after prudent measures have been taken on the walls, ceiling, ductwork, doors, etc. While concrete slabs can be flanking problems, typically they limit only the highest performance walls, and a reasonably good wall can be built without elaborate and often extremely expensive floating floors. But remember, for the highest level of performance that slab will have to be dealt with. Upgrades of some type to the floor of a 2nd level room are compulsory for good quality sound isolation.Upgrades of some type to the floor of a 2nd level room are compulsory for good quality sound isolation.
Structural Noise Summary:
The comments above will get you started on the right track with your plan. It’s not necessary to take every possible step in many cases, but if an ultimate level of performance is to be attained, then a very involved design is necessary.
Seal quality and other direct air leaks.
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 with acoustical caulk, 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.
Common Wood Stud Wall,
Common Wall with poor Seal,
Green Glue Wood Stud Wall,
Green Glue Wood Stud Wall
with poor seals, STC =27
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 The SilenSeal Acoustical Caulk sold on our site, it is water based, low odor, easy to clean up and dries to a non-sticky state.
How should I use the caulk?
Bad - Sealant under frame only. Sound can easily enter and exit the wall cavities.
Good - A good start for remodeling jobs where you are adding drywall to an existing wall.
Very good - Beads under drywall to keep sound from leaking into the cavity, and under frame for redundancy
Perfect - A heavy bead under the walls plates plus a bead under each drywall layer.
You should strive for multiple caulk layers on partitions where sound isolation is critical. This helps ensure that a quality seal is attained by “doubling up” on seals - if one layer has compromised quality, it is backed up by the other layers.
Many different sealant patterns can be utilized with success. And 10 perfect seals aren’t much better than 2 perfect seals (one on each side of the wall), however redundancy ensures that sealant performance will be good, and we strongly recommend it.
Caulk with resilient mounts.
Resilient mounts such as resilient channel or more modern, high performance engineered sound clips call for special sealant practices. To get performance, particularly low frequency performance, as good as the lab tests it is generally recommended to leave 1/8” to 1/4” between the drywall and any other surface, and then fill that gap with very flexible acoustic sealant. This creates a “floating” wall in the truest sense.
We have taken a look at flanking noise and how to cope with it in this document. The topics contained herein are as much a component in the success of your project as is the selection of wall or ceiling type. Ultimately the success of any isolation project relies on competent planners, designers, and installers. Click here to read our How To Soundproof article, or here to return to our Soundproofing and Acoustical Material homepage.