What is the Triple Leaf Effect In Soundproofing?
Mar 25, 2024Over the years of teaching people about soundproofing there is one topic that causes the most fear and that is the dreaded Triple Leaf Effect. However, it is really not something to fear and in this article I will go over what it is, how to avoid it and a couple tricky situations you might encounter during your design and build.
1) The Mass Spring Mass System
To understand soundproofing and the triple leaf effect you first must understand how a soundproof system works. The foundation of soundproofing is that you have a barrier like a wall, ceiling or floor and you use a mass spring mass system in that barrier.
What is the mass spring mass system? It is a physical property that works to block sound more efficiently than just mass. Say you have a solid concrete wall. It is massive and will reflect a lot of sound, but if you were to tap on it with a hammer the sound would fly right through the wall. For this reason we need two walls and an air gap to increase the sound isolation.
In the prior example if we add another concrete wall with say 8" between the walls then we will have mass (concrete wall 1) spring (the air gap) and mass (concrete wall 2). This creates a much better sound isolation system than just one massive wall.
Now if we decrease the air gap in half to 4" then the isolation will be worse. Why is that? The spring in this case is air, but it behaves like any normal coiled spring we can see. If you stretch out the spring it is looser and vibrates freely and easily up and down like a slinky. If you compress the spring it becomes rigid almost like a solid object and does not vibrate easily or move up and down. It behaves more like a closed slinky.
Here is the main and very important point. The looser the spring, aka the bigger the air gap, the more isolation you will get because sound will dissipate from the vibration of the air molecules and convert into heat. The tighter the spring, aka the smaller the air gap, the less isolation you will get because the sound travels through the spring more easily transferring to the other wall.
Simply put: loose spring good, tight spring bad. Big air gap good, small air gap bad.
In the above diagram the wall on the left would be more soundproof than the wall on the right because the spring is twice as wide meaning twice as loose, which leads to more transmission loss in the spring.
2) What is the triple leaf effect?
So now that we understand the mass spring mass system we can now tackle the triple leaf effect. If we are building a wall with wood and drywall then we can encounter a counter intuitive problem. In the diagram below you can see that the two walls side by side with drywall on both sides of each wall only gets you an STC rating of 40. Wow! that doesn't seem to make sense right?
Then look at if you remove one layer of interior drywall, you get an STC of 50. If you take those two interior layers and put them on the outside of both walls, you suddenly get an STC of 63. That is a huge jump in soundproofing.
You now understand why this happens. The spring (air gap) increases when we remove the inner drywall layers, thus increasing our sound isolation.
The triple leaf effect is when you add a mass layer, in this case drywall, between your double wall system and therefore decrease the air gap making the spring more rigid and less efficient at converting sound waves into heat.
As a side note, the pink insulation in the wall diagrams increases the isolation as well and works to convert sound to heat through the vibration of the tiny fibrous strands as sound passes through the web of fiberglass or mineral wool.
3) Do not fear the triple leaf effect
Now that you understand the mass spring mass system you have a much greater understanding than most about this triple leaf effect. As long you have an isolation system that gets you to your desired STC or sound isolation needs you can build walls within walls within walls.
For example, let's say you want to avoid using the exterior walls of your garage because they are very flimsy and have almost no mass. Instead you want to build a new double wall system in your garage and never attach it to the exterior wall.
As you can see in the diagram above, you have three walls. Many people would call this a triple leaf effect and say it is bad. They would be wrong. The reason is that you still have a double wall system that gets you an STC of 63. If you add more walls around that system it will only increase the isolation. The garage wall doesn't have enough mass, so it is easier and cheaper to build a new wall away from the garage wall. This happens all the time with garage doors.
You could leave the drywall off of the inside of the garage wall, but it may reduce the mass, which would be bad. If the air gap is already big enough than why reduce mass for the sake of adding more air. You can see that the triple leaf effect is not so simple and that understanding the underlying mechanics of the mass spring mass system gives you much more freedom in your designs.
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