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What Is the Triple Leaf Effect?
What Is A Triple Leaf Wall?
A triple leaf wall is a wall with two air cavities rather than the typical one air cavity. Similarly, a quadruple leaf wall would be a wall with three air cavities. This applies to both walls and ceilings alike and is also referred to as a retrofit installation when discussing the installation of resilient sound clips in an assembly that will create a triple leaf. A leaf in a wall is a solid layer, like drywall or plywood, and a triple leaf wall has three leafs. Example below:
Here we see single leaf (no air cavity), double leaf (like most assemblies), triple leaf and quadruple leaf assemblies. It is important to remember that if you use two layers of drywall directly against each other, it still only counts as one leaf because there is no air cavity between the layers. In the sketch below we show again single through quadruple leaf walls, but this time they all have the same number of drywall layers. To apply this concept to assemblies with resilient sound clips or resilient channel, the user would take a double leaf assembly, install resilient clips or channel over the drywall, and then hang a layer of drywall from the clips. Effectively creating three separate leafs of material.
It is not illogical to presume that the quadruple leaf wall above would have the best sound isolation. After all, the sound has to go through drywall, air space, drywall, air space, drywall, air space, and then again drywall often with insulation within each air space. In a double leaf wall it has to make it through only one air space, and in a single leaf wall there is no air space at all. While it makes sense that the quadruple or triple leaf assemblies are better than double leaf assemblies, it is unfortunately not the case. To understand why this is, we have to take a look at how decoupling works. The first issue issue is dealing with resonance.
The most common triple leaf assemblies in real world construction are when resilient channel, resilient sound clips, or furring strips are installed over existing drywall. Another common assembly is building a new stud wall in front of an existing wall with drywall.
Resonance Must Be Low In Frequency
To attain good low frequency performance, this resonance must be as low in frequency as possible. Otherwise the weak point of your wall will fall at an unfavorable location, and low frequency noise will have little trouble passing through the wall. The goal of any decoupled wall should be to drive resonance down in frequency. To do this you have to:
- Add mass to one or both sides of the wall.
- Increase the depth of the air cavity.
- Add insulation (if you don’t have insulation).
Triple leafs are bad because for a given amount of mass and space they always have a higher resonance point than a double leaf wall. One of the criteria that were given above for getting a low resonance point, and good low frequency performance, was a deep air space with a lot of mass on either side.
The double leaf wall might have an air cavity depth of 8”, but for the same overall net wall depth, the triple leaf wall’s cavity will be just half that, and the quadruple leaf’s cavities will be only 1/3 of the depth of the double leaf wall. To make matters worse, each leaf in the double leaf wall is very heavy, but each leaf in the quadruple leaf wall is far lighter resulting in half the mass. This will cause resonance to go up in frequency even more, and low frequency performance will be badly degraded.
Multiple resonances and possibly more severe resonances are created. To further complicate things, a triple or quadruple leaf wall may exhibit more than one low frequency resonance. And if one is bad, then two or more are surely even worse. Finally, the resonance behavior of multiple leaf walls is not simple or entirely predictable, and may be more severe than with a double leaf wall.
Comparing Single, Double, and Triple Leaf Walls
Next, let’s take a look at just how bad triple leaf walls can perform relative to their double leaf counterparts.
These are tests TLF-95-107a (double leaf) and TLF-95-153a (triple leaf), taken from IR-811, a document published by the National Research Council of Canada. This data is copyright NRC Canada and shared with permission.
The difference here is stunning to say the least. It’s even more remarkable when you consider that the same type of construction, and same type and amount of materials were used in each case.
Double Leaf Assembly
Triple Leaf Assembly
Actual Testing Results For Double, Triple, and Quadruple Leaf Assemblies
The STC values shown above are from a series of tests run by Owens Corning in 1972 at Geiger and Hamme laboratories. While STC does not tell the entire story, the point is clearly made to avoid triple leaf assemblies whenever possible.