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 (if you prefer) 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 A Triple Leaf Wall Is 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.

Well, 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 doesn’t 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 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 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 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 – half the mass. This will cause resonance to go up in frequency even more, and low frequency performance will be badly degraded.

Multiple resonances / possibly more severe resonances. 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.

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 – resilient sound clips and hat channel on joists, one big air space	Triple leaf – resilient sound clips and hat 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.

First Step: Build the Platform

Regardless of the sound isolation products you use, you will need to build a platform that is independent of the existing framing.  These platforms are typically built using 2″ x 4″  framing with 16″ on center spacing or 24″ on center spacing.  The less connection points that one material has with another the better the performance in sound isolation.  So 24″ on center is recommended, but when the platform is under 70 square feet then spacing really won’t make much of a difference at all.  The basic construction of the platform is shown in the gallery below.  Build a box frame using 2″ x 4″ studs and then connect studs 16″ oc or 24″ oc within the box.  Top this frame off with a minimum of one layer of 1/2″ or thicker sub-floor material.

Second Step: Add Something Beneath the Platform

Here’s where you have a few options:

Joist Isolators

Joist Isolator are U-shaped rubber pieces that friction fit on the bottom side of 2″ x wood products usually either 2″ x 4″ or 2″ x 6″ joists.  Using this product is cheap, simple, and depending on how much sound you need to isolate it may be effective enough to be the only sound isolation product you use if you want to keep the budget down for your floor.  A quick way to estimate the number of joist isolators needed is to take your total square footage and multiply by .65.  The smaller the area, the less likely that number will work so sometimes it is just better to draw the platform out and count how many you will need.  For small platforms you can space the joists 24″ on center, but for larger areas you will want to space the joists 16″ on center to help stabilize this floating floor design.  From there you will need one isolator on each end of the joist and then every 2′ in between.   As you can probably see if you have a platform that is only 6′ x 6′ then you will only need as few as 24 joist isolators to complete the job.  Joist isolators should be installed on hard surfaces so using this product over existing carpet will be pretty ineffective unless you were to first lay down a layer of sub-floor before setting up the joist isolator and platform system.

Rubber Underlayment

Recycled rubber underlayment is another option and typically the most common option because rubber underlayment can be applied in layers making it thicker than a regular joist isolator and because it is a more resilient product than joist isolators are.  The durometer (hardness level) of recycled rubber underlayment is almost half of what a standard joist isolator is.  Installation for this option is simple and is also pictured below.  Measure your total square footage to make sure you have enough QuietGround, cut the underlayment to size so it fits beneath the platform you create, and then lay it down on the floor before you place the platform.  The underlayment must cover the entire bottom side of the platform so that no part of the platform comes in direct contact with the floor.  Multiple layers of underlayment will improve performance and since the area you are covering is so small and underlayment typically only comes in 4′ x 25′ or 4′ x 30′ rolls we suggest using what you got.  : )

Other Options

You can add other products below this platform as well.  There are thick gym mat products, mass loaded vinyl (MLV), thick cork (6mm+),  and other ‘sound board’ type products that you can use.  We recommend the two options above over anything else because of the resiliency of rubber cannot be beat.  Dealing with deep vibrations you will need something resilient beneath the platform so the use of rubber is always recommended.  A thick gym mat can provide some decent results, but the mat will need to be over an inch thick which makes for a sometimes too cushy of an underlayment.

Third Step: Add Something On Top Of the Platform

To minimize costs you can avoid this third step.  This of course assuming you have chosen to use either joist isolators or rubber underlayment as discussed in the second step.  Like the bottom side, there are several options for the top side of this platform.  Here are a few that we recommend:

Rubber Underlayment

Rubber underlayment is superior at dealing with structure-borne noise so it is a great option for the underlayment that you install on top of the platform.  Like the bottom side, cut the underlayment to fit covering the entire top of the platform.  Multiple layers in this situation is always better than one layer so make sure to get enough!  Something thick like a 1/4″ rubber underlayment is always preferred to a 2mm or 3mm underlayment.  The rubber underlayment does not need to be attached to the top of the platform in any way.  To finish off the design you can lay a section of carpet over the platform and attach to the sides of the platform or just leave the underlayment as it is uncovered.

Mass Loaded Vinyl

A quality barrier will perform very well in this situation and since you are covering such a small area we recommend thickening up that barrier going with a 2 LB MLV over the more common 1 LB MLV.  The 2 LB MLV is as thick as one layer of QuietGround 250 rubber underlayment, but you do have the added weight with the 2 LB MLV which will help isolate more airborne noise than the QuietGround can.  The cost for the 2 LB MLV is a little more than QuietGround and if you layer the QuietGround then the 2 LB MLV will not perform better than the QuietGround.  But as a single layer underlayment, the 2 LB MLV is quite a performer!

Green Glue

The most popular sound isolation product online is without a doubt Green Glue.  To use Green Glue in this situation you will need a second layer of sub-floor screwed to the existing layer on top of the platform with Green Glue applied at a rate of either 2 tubes per 4′ x 8′ sheet or 3 tubes per 4′ x 8′ sheet.  This system will add a decent amount of mass from the extra layer of sub-floor and also the most damping that any product can provide between the two layers of sub-floor.  Performance for this option should be very similar to the QuietGround and 2 LB MLV options discussed above and also be very competitive in overall price as well.  The only drawback is if you add a second, permanent layer of sub-floor to your platform then it will become pretty difficult to move the platform if you ever need to.  If your platform is going to stay put for as long as you have it then this is of course not a problem.

Case Study

We had a customer in Minneapolis that likes to drum on an electric drum set in his apartment.  The only noise he really generated was the sound of the drumstick against the drum pads.  This sound was transferring directly to the neighbor’s below and he was receiving complaints about this.  So he contacted us to find a solution and the pictures below were taken by him showing what he had done and the final results.  Here is a quote from him about what he did and his results:

The QuietGround is working great!  I built a 5′ x 4′ platform out of 2″x4″s and a 0.5″ plywood sheet on top.  Since I didn’t want any of the roll to go to waste, I put two layers of the QuietGround between my carpet and platform, three layers on top of the platform, and some stock carpet on top of everything.

I have been rocking out pretty hard for some time now, and I haven’t heard a word from my floor neighbor.  I even contacted her and asked her to call me if there were ever any problems with the noise.  Still nothing!   I could have probably used less than 5 layers of the QuietGround to do the job, but I haven’t had any complaints so I have no reason to change the setup.  Thanks for the suggestion for building the platform by the way, I think that was key to isolating the sound.

Conclusion

Isolating sound transfer in a small area can be very simple and cost effective.  It also makes for a fun weekend DIY project–if you are in to that sort of thing!  With a basic platform and some sound isolation products beneath the platform and/or on top of the platform you will notice some dramatic results with minimal cost.  It is always recommended to treat the entire room rather than just the direct area that the sound is transferring through, but quite a bit of sound will want to flank through your floors so a simple platform will still reduce a serious amount of sound.

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Most often clips are used on the ceilings while the walls are built as a double stud wall or staggered stud wall.  Retrofitting for a double stud wall requires that you remove the existing drywall before building the second set of studs.  The second set of studs should be spaced at least 2″ from the existing wall framing.  Double that gap and you will raise performance by about 5 STC points, double the gap again and raise your results by another 5 or 6 STC points.  Resilient sound clips will perform as well as a staggered stud wall and the cost per square foot will definitely be less than staggered stud walls.  Read more about decoupling, Understanding Decoupling.

Damping with Green Glue
The most effective way to improve the damping for your project is to use a damping glue between two layers of drywall.  The most commonly purchased damping glue is without a doubt Green Glue.  Using a damping glue reduces the severity of resonance problems, and it reduces the ability of structures to conduct vibration.  Reducing vibrations will keep sound from traveling through your framing and to the rest of your house.  Using Green Glue is the same concept as products manufactured by QuietRock or Supress, but the formula in Green Glue is lab tested to be more effective in terms of sound isolation and more cost effective as well.  There are also other types of damping that are often used in sound isolation.  For instance, our pipe wrap which is used to dampen vibrations in drain pipes and also on anything made of sheet metal.  Pipe wrap is actually a very versatile product that provides basic constrained layer damping on any plastic or metal surface with basic peel and stick installation.  Read more about damping, Understanding Damping.

Increasing Mass with Extra Layers of Drywall or MLV
Adding more mass to the wall helps. To make a BIG improvement, you have to make a BIG change in mass. For example, to improve sound isolation by about 10dB you would have to quadruple the number of drywall layers on your wall – from one on each side, to 4 on each side. Mass makes a much larger improvement when combined with other techniques such as decoupling.  Products like our Mass Loaded Vinyl will be a much more effective solution for adding mass to your walls, ceilings, and floors compared to adding extra layers of drywall.  Each layer of 1 pound MLV (1/8″ thick) is estimated to be equal to the effectiveness of 4 layers of 5/8″ drywall in terms of sound isolation.

Absorption with Insulation
If there is no insulation in your walls, add some.  This will help only slightly if the drywall is not damped or not decoupled, but will still help.  The rule of thumb with insulation is to make sure and at least use some.  Think of an empty wall cavity as an empty drum, if you hit the drum sound will travel easily.  If you fill the drum with insulation, sound will still travel, but it will be muffled.  We recommend using standard fiberglass insulation that fits the wall cavity or ceiling cavity you are working with.  Additional improvements are seen from using R-19, or even thicker, but these improvements are progressively smaller, and generally don’t help with low frequencies.  Use something, but you can save yourself money by not spending exorbitantly on extremely thick insulation or exotic types of insulation.  Better yet, take the money you saved, and put it towards something else (like one of the other 3 areas of improvements).  In general, anything thicker than R-19 is into the realm of diminishing returns.

CONCLUSION

These four areas of improvement are interactive.  For example, adding mass to a decoupled wall is more effective then adding mass to a standard wall.  And using multiple layers of drywall (mass) with multiple layers of Green Glue will significantly increase performance of Green Glue’s damping abilities.  For the best results you should include all of the four principles in your sound isolation project.  View our site to learn more about Green Glue, Resilient Sound Clips, and other products you should consider for your project.

A typical exterior wall is made up of a layer of drywall, 2″ x 4″ framing, R-13 insulation, a layer of OSB sheathing, and either siding, stucco, or brick.  Exterior walls will also of course have windows and a door every now and then.  The STC rating of your exterior wall will not be much better than your interior walls.  An STC rating for a regular interior wall will be somewhere in the low to mid 30′s while your exterior walls are usually in the high 30′s to low 40′s.

The first thought by most customers is to purchase a ‘save-all’ sound isolation product to make all their troubles go away.  Whether you buy Green Glue, resilient sound clips, mass loaded vinyl, heavy exterior doors, ‘soundproof’ windows, specialty insulation, or whatever other product someone is willing to sell you; your efforts will not be successful if you don’t incorporate multiple methods to improve sound isolation.

Deal With Your Weakest Links First

Exterior doors typically perform decently because they are usually solid wood and sealed very well from the exterior elements.  So while the doors aren’t the best for isolating sound, they are not usually the weakest link.  Windows, on the other hand, can transmit sound almost as well as they transmit light.    The average STC rating of a window is in the high 20′s, about 1/2 the isolation properties of your exterior wall, ouch!

Replacing your windows with ‘soundproof’ windows may help performance, but you will have a major construction project to undertake.  You will need to essentially remove a 4″ or so perimeter around the window to remove the existing window and install a new window.  If the exterior of your house is rock, brick, or stucco then this can be a major project.  Avoid this headache and high cost by adding a second window on the inner part of your window frame.

Adding a second window will increase isolation in two ways: 1) by adding an air gap between the exterior window and the newly installed interior window and 2) by adding an extra path for sound to have to travel through.  Simply adding a second double pane window you will increase the STC rating between your two windows to the mid to high 40′s.  At that point your windows will perform better than your thick exterior walls!

There are several companies that sell windows that can be used for retrofitting on the interior side of your windows sill.  Run a Google search or humor the Microsoft guys and try their new Bing to find a company near you that sells retrofit windows.

Watch the video below to see the effect of adding a second window to your window installation.

Now, Let’s Improve the Rest of Those Walls of Yours

Your windows now have an STC rating of almost 50, yipee!  Now you need to continue on fixing your new weakest link, your walls…

As I mentioned before, your exterior wall STC rating is likely somewhere in the high 30′s to very low 40′s.  The most basic way to improve your walls at this point with minimal demolition would be to add a layer of 5/8″ drywall with Green Glue at a rate of 2 tubes per 32 square feet.  Adding the Green Glue is simple, the hardest part will be to hang, tape, and texture the drywall.  Adding the Green Glue and additional layer of drywall will bring your STC rating up to the low to mid 50′s and will improve low frequency isolation significantly.

Now your walls will perform just as well as your windows and without major investment into your project this is likely as good as it will get.  You can add decoupling to this scenario to increase isolation, but this will involve removing the wall’s current layer of drywall.

If you would like to go this far then you will need to remove the drywall to avoid a triple leaf effect and then add resilient sound clips with hat channel to your studs.  After removing the drywall, rather than using clips you can also build another 2″ x 4″ wall in front of the current exterior wall, insulate the newly framed wall, and then hang a layer of drywall on the new wall.  This will improve the performance of your walls quite a bit more than a layer of Green Glue ever could and even better than a resilient sound clip system could.  The problem with this though (other than having to sacrifice another 6″ or so of floorspace) is your wall will be big and beefy with a great STC rating, but your freshly installed second window will still be rating in the high 40′s.

If your double stud wall has a rating of around 70 STC and your windows are high 40′s STC, then your final STC value for your wall will be much closer to the window rating than the uber-high double stud wall rating.

Conclusion

If you want to keep exterior noise out of your sanctuary then start by improving your weakest links.  Improve your windows and make sure you have a quality exterior door that is well sealed.  If that does not satisfy your appetite for sound isolation then add a layer of drywall with Green Glue or a similar damping compound (ie, Decibel Drop, QuietGlue, QuietRock) in between.

Still have questions?  I guess we didn’t explain the topic well enough.  Let loose, ask us a question or two.