TM Soundproofing The Triple Leaf Effect

The Triple Leaf Effect

During your research on the topic of soundproofing, you may have come across commentary on "triple leaf walls". In this article we will take a look at what a triple leaf wall is, and why they are very undesirable for any sound isolation application.

What is a triple leaf wall?

A triple leaf wall is a wall with 2 air cavities, and not just one. Similarly, a quadruple leaf wall would be a wall with 3 air cavities. A leaf in a wall is a solid layer like drywall and a triple leaf wall has 3 leaves, etc. Like this:

Here we see single leaf (no air cavity), double leaf (like most walls), triple leaf and quadruple leaf constructions. It is important to remember that if you use 2 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.

Why is a triple leaf wall a bad thing?

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 a

Solid mass & air space & solid mass & air space & solid mass & air space & solid mass.

That quadruple leaf wall is decoupled 3 times over! While 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 this makes sense, it is very false, especially at low frequencies. To understand why this is, we have to take a look at how decoupling works. Decoupling isn't effective at all frequencies. If you take two layers of drywall, and separate them with an air space, it does not improve things at all frequencies. The air in the cavity acts like a spring, and creates a resonance. Only well above this resonance do things improve (but then they improve very nicely indeed). This graph should make the point:

Effect of Decoupling a Solid Mass

What you see is the sound-stopping power of the wall, in decibels, at different frequencies. This is called "transmission loss". While this data is hypothetical, this is what occurs in real walls, the decoupling has a large positive effect at high frequencies, but a negative effect around the resonance.

Resonance must be as low in frequency as possible

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
1. Add mass to one or both sides of the wall
2. Increase the depth of the air cavity
3. Add insulation (if you don't have insulation)

Triple leaves 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 walls 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 & half the mass. This will cause resonance to go up in frequency even more, and low frequency performance will be badly degraded.

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 isn't simple or entirely predictable, and may be more severe than with a double leaf wall.

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 is 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 & resilient channel on joists, one big air space

Triple leaf & resilient channel between drywall layers, small air space

The STC values shown above are from a series of tests run by Owens Corning in 1972 at Geiger and Hamme laboratories. While STC doesn't tell the entire story, the point is clearly made.

Summary:

Triple leaf (or quadruple or higher # of leaf) constructions should be avoided like the plague. You will always get a lower level of sound isolation, and this loss may be most severe where you need performance the most, low frequencies.

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Customers Questions and Answers

1) Will: Hi, I am in the process of building a studio for a youth project and we are in a room which has a neighbouring stud wall. In other words, the wall between us and the room next door is a stud wall. It is one frame with one sheet of plasterboard either side. We are going to build a room within a room but we are not allowed to remove the plasterboard from our side. This means we will have a triple leaf effect. Is there anything we can do to reduce this? Many thanks for your help. Will.

Trademark Soundproofing Reply: Hi Will, The deeper the air space between the boards the less of an effect the triple leaf will have. So when framing your new wall move it in as far as possible and put boards on the inside only that should create enough air space for you.

2) Brian: We are renovating a bed/bathroom loud outside voices the concern. The Ext. wood stud wall will have 2.5'' spray foam in a 3.7'' stud bay then MLV applied,then RC channel,then 5/8 drywall. will this create a triple leaf effect?

Trademark Soundproofing Reply:

Hi Brian, As the MLV is a limp material you should not have much of a triple leaf issue there.

3) Brian: Neighboring unit in townhouse complex wants to upgrade her bathroom which is adjacent to our master bedroom. Currently we hear noise that we would rather not hear coming through the adjoining wall. Accodrding to the structural drawings, the shared wall is symetriical, consisting of: 5/8" gypsum, 3.5" space with acoustical insullation, 2 sheets 5/8" gypsum in the middle, 3.5" space with acoustical insullation and 5/8" gypsum. Contractor is proposing to put 1/2" QuietRock over neighbors gypsum using Green Glue. He is suggesting that he needs to use a different thickness of QuietRock than the gypsum in order for the QuietRock to work effectively. There is a significant difference in sound transmission going to 5/8" QuietRock. Can you advise, given what the wall consists of now, whether using 1/2" versus 5/8" QuietRock would be recommended to have best success for reducing sound transmission? Thanks.

Trademark Soundproofing Reply: A heavier wall is better than a thinner wall. So 5/8" is better than 1/2" however it is a very very slight difference.

4) Brian: Thanks for the reply. Follow-up question. What do you mean by "slight difference"? According to the QuietRock website 1/2" versus 5/8" QuietRock can result in 47-52 STC versus 52-74 STC respectively which is a big difference at the high end. Is there any rationale for using 1/2" instead of 5/8" in the described situation (5/8" gypsum, 3.5" space with acoustical insulation, 2 sheets 5/8" gypsum in the middle, 3.5" space with acoustical insulation and 5/8" gypsum); wood frame construction? Is there any concern for a "triple leaf effect"? The contractor conveyed that placing 5/8" QuietRock on 5/8" gypsum would be a problem for sound transmission, that you needed to use a different thickness of QuietRock overtop gypsum. Any truth to this? Thanks, Brian

Trademark Soundproofing Reply:

Hi Brian, In lab testing when using 2 layers of half inch vs 5/8 with the Green Glue I have not seen it to be more than 1-2 STC points difference. when using a pre damped panel like you mention, the 1/2" is really 2 quarter inch layers which may account for the bigger difference you see there. You can check with the company that produces the pre damped panels as to the results.

5) Herb: I've used your process of adding another layer of drywall to my ceiling with a generous coating of the green glue between. For whatever reason I seem to be hearing way more vibrational noise and conversations are clearer now from upstairs. Also there was rockwool insulation blown Ito the ceilling prior to the new drywall being added. It was completed last week . Can you shed some light on this epic fail? While reading your FAQs someone asked about using foam insulation between their walls. Your reply was that the foam insulation would adhere to the two pieces of drywall and ineffect make them one making it easier for sound vibrations to make it through. That seems very much like what's happening t the project I just finished. I will never sleep again and I am so disappointed! Help!

Trademark Soundproofing Reply: Hi Herb. That is very interesting as just adding a 2nd layer of drywall should reduce your noise to some degree. There are many different factors that can effect the sound transfer including the exact work that was done there and what the construction of existing walls and ceilings are etc. Try to contact the company that you purchased the product from to see if they can help you. Also give the GG at least 30 days to cure, especially if you used a "generous" amount. Thank You

6) curtis: in layman's terms what does single stud STC value of 37 vs quiet clip and GG STC value of 63 mean. Can I still hear normal talking, TV and radio on the other side of wall, or percentage wise how much of it is now "blocked"?

Trademark Soundproofing Reply: Hi Curtis. STC 37 you will still hear conversations. STC over 60 is luxury and you should not hear much unless it is very very loud.

7) Greg G: Hi. I have a 2 by 4 wall with fiberglass insulation. Between apartments. I can modify one wall. Would another layer of drywall ( what kind ) clips and green glue be my best option. Not sure of the combination here? Do you have a schematic?Thanks-----Greg

Trademark Soundproofing Reply:

Hi Greg, Yes that would be your best option. Remove the existing drywall, apply clips, channels, 2 layers of half inch drywall with GG. Seal with acoustical caulk and putty pads. You can read more details here soundproofing existing walls article.

8) Greg: I all ready have a finished wall I am working with, ( plastered and painted about two years old ). My thought was to either use the clips on the existing wall and another layer of drywall on the clips or use the green glue on the existing wall and then another layer of drywall. 1. can I use green glue over the plastered and painted wall ? Will it adhere ? 2. Are the clips attached to the existing wall and then drywall on them an option ? 3. Between the two if either is appropriate what would the difference in sould reduction be ? 4. Would cement board give more sound deadining ? Thanks , hope this clarifies my situation. -------- Greg

Trademark Soundproofing Reply:

We do not recommend adding clips and channels over existing walls due to the triple leaf effect. You can add another layer of drywall with Green Glue over your existing painted wall. You have to screw the new layer back to the studs as the GG is not meant to be used as an adhesive. Drywall should be as effective as Cement board. Thank You

9) Karen: In trying to avoid the triple leaf affect how far away should I build a second wall from the adjoining office wall. I am next to a bar with the subwoofer on our adjacent wall.

Trademark Soundproofing Reply: Hi Karen, As long as you can remove the drywall between the new wall and the old one then any amount of distance will be beneficial. If you cannot remove the drywall in between than we suggest a space of at least 6 inches.

10) Abraham: I am trying to sound proof a room that is 6 x 11, so it can be used as a studio. There are already two sheets of 5/8's drywall on the walls and ceilings. Would I be better off using green glue and another piece of 5/8's drywall or using 1/2 inch homasote and 5/8's drywall, or building additional walls for a decoupling effect. I am looking to block out the general noise of the house: Tv's, radios, washing machine, the kids playing etc... What do tou think?

Trademark Soundproofing Reply:

Hi Abraham, If there is insulation in your walls than I think adding another layer of 5/8 drywall with Green Glue should be enough. If there is no insulation then you should consider removing your wall adding insulation and at that point you can decouple your wall with resilient sound clips, channels and finish with double layer of drywall and Green Glue.

11) Karina: Hi, I have problems with sound transmission through walls/pipes in wall, and maybe some resonating in my room. So, I am wondering how would I figure out what kind of walls are between me and my side neighbor? I think the walls are hollow inside (they sound so) but how do I figure out if there are 3 drywall pieces between me and neighbor (bad triple leaf), or just 2 drywall pieces, or cement plus drywall on each side? I cannot tear down existing walls, don't have any approvals. If I hire a contractor to just green glue +mlv another layer of drywall on top of my room wall can that make the situation worse (if I don't know what's between me and neighbor)? Thank you, Karina

Trademark Soundproofing Reply: Hi Karina, You can try obtaining the original construction plans from the builder otherwise you would need to make a hole in your wall. Adding more drywall with Green Glue to your existing wall or adding MLV will only make it better.

12) Ed C: Trying to reduce sound emission from vacuum pumps under a table which will have sheet metal panels boxing in the space under the table. We are thinking of using a 2" layer of Owens Corning 705 against the panels and then hang 1/8" MLV in front of the 705 - sound hits the MLV first .....or....should we attach the MLV-PSA directly to the metal [to create a CLD, Dynamat type product??] and have the 705 as the first acoustic layer

Trademark Soundproofing Reply: Hi Ed, I would attach the MLV directly to the metal and have the 705 absorb the sound first.

13) Bob: Turning 120 year old house into two apartments,(upstairs and down).I plan on stripping the 1x6 t&g off ceiling and floor leaving me with full 2"x8" joist on 19" centers. For the floor 3/4 t&g osb green glue 1/2" osb. Ceiling hat channel and two layers drywall with gg in between. Insulation in cavity. (1)Is it best to to completly or partially fill with insulation. (2) I would like to add several can lights. Is there a good way to sound proof them. (MDF box gg and drywall on the inside glued to ceiling and not touching joist?). It's going to be a lot of work so any advice to get it right the first time would be appreciated. Thanks Bob

Trademark Soundproofing Reply:

Hi Bob, Minimum of 6 inch insulation, no problem with fully filling the cavity. Your idea of building boxes around the cans is a good one. Lately we have been advising customers to use flat LED lights that have the same look as recessed lights, those just use a a standard electric box which can be treated by using an acoustical putty pad.

14) Jo: Hi, I inadvertently created a triple leaf (maybe quadruple) wall before reading this article and learning about them. My wall assembly is as follows: drywall, 2x6 studs (filled with insulation), plywood, 2" air space (between the 2 wall assemblies), 2x4 studs, drywall, RC channel, drywall. What can I do to remedy this situation? I'd hate to tear down the drywall and redo since I just had it put up. Will filling the entire cavity (2" air space + 2x4 studs) with blown in insulation help?

Trademark Soundproofing Reply: At this point I would just leave it and see the results. It is still a good wall for many frequencies and you may be OK.

15) jason: I have a new build home (still in the framing stage, 2x6 studs) which is within 30 feet of a train. the train blows its horn right at the house. for insulation I plan on 2" of sprayed in polyurethane foam and then 4" fiberglass rolled in insulation. next I was going to put sound clips and two layers 5/8" drywall. I would also like to use the mass vinyl product either between the two sheets of drywall or on the studs. or would green glue be better? I am willing to build ANY type of configuration that you would recommend.

Trademark Soundproofing Reply:

Hi Jason, You are better off using the green glue between the layers of drywall. If you would like to add mass loaded vinyl in the nix than you should install it on the channel before adding the first layer of drywall. (you can also add it to the studs before applying the clips as there is no serious issue of the triple leaf with the mass loaded vinyl as it is a limp material). We do not recommend spray foam for soundproofing purposes.

16) Joel: Question: I have a finished basement ceiling with sheetrock, then glue up ceiling tiles fixed to the sheetrock. I'd like to add another layer of sheetrock for sound blocking purposes. Would the ceiling tiles qualify as an air cavity, in which case I would be creating a triple leaf? Or, would they qualify as a layer of somewhat rigid material? I'm trying to figure out if I should remove the ceiling tiles, or leave them in between two layers of sheetrock. Thanks in advance!

Trademark Soundproofing Reply:

Hi Joel, those are somewhat rigid and you can leave them in place if it involves a lot of work to remove them. You may want to consider removing them if you would like to apply Green Glue damping compound between the new layer of drywall and the existing one as that would boost the soundproofing significantly.

17) dougald: I have an existing ceiling. I plan to decouple with clips and add 2 layers of 1/2 drywall with green glue in between. What can I expect?

Trademark Soundproofing Reply: A very big difference over what you have now. Be sure that you install insulation in the ceiling as well. Also keep in mind that any recessed lights or HVAC vents will compromise the soundproofing.