Triple Leaf Effect and Windows

I am in the process of converting a room that is near a busy street and believe I may have created a triple leaf effect in trying to block the low frequency vehicle noise. I have searched this board in an attempt to find the answer to my question but have not been very successful. It seems that a triple leaf system (two air cavities) is always a bad thing for low frequencies and I am trying to confirm if that is still true when the leafs are of varying thicknesses (particulary when the middle leaf is only 1/8 inch thick). Here is the system I built for three windows in my room: 1/4 laminated glass on exterior facing noise source 2 inch air space 1/8 inch single pane (100 year old window double hung window) 2 inch air gap 3/8 inch laminated glass (interior) At first I just had the exterior laminated glass and the orginal window. I recently added the additional pane and although reduction at higher frequencies is noticeable I am wondering if I created a triple leaf effect (for lower frequency truck/motorcyle/bus traffic) and made the problem worse even though the middle pane is only 1/8 inch thick. I am planning to turn my attention to the walls which are the next weakest link but am not sure if I need to rethink my design for the windows first. I would appreciate any insight anyone has. Mark

It is still an incorrect procedure. If the middle leaf would have been the dominate one (the thickest) this would have been better than the introduction of a thin leaf.

Thanks for the reply. I understand that it is not the correct setup and wouldn't have designed it that way if it was a new construction situation. The middle leaf is the original window that is over 100 yrs old and I didn't remove it primarily due to historical zoning reasons. I added a storm window on the outside initially and it produced marginal results. I then added the 3rd leaf/pane thinking that it would improve the results prior to understanding the triple leaf issue. I guess my question is should I leave it as is or would I be better off removing the internal 3rd leaf and beefing up the middle leaf with a thicker pane? That would only result in approximatley a 2 inch air gap. I'm just trying to create the best possibile solution without removing the original window if at all possible. Mark

Hi Mark. Welcome! Simple answer: yes, you do have a triple leaf system, and like Brien said, you have the worst case for that system, since your thinnest layer is in the middle. Hard to figure exactly, since you have different surface densities on each leaf, but that assembly probably has a resonant frequency somewhere in the range of 120 Hz., meaning that it is basically transparent to sound up to about 170 Hz, and doesn't isolate well until around 240 Hz. If you would have removed the center pane, the story would be rather different: Resonance at 50 Hz, isolation starting at 70, good isolation above 100. So if you can figure a way to do so, you should take out the center pane.

I am trying to confirm if that is still true when the leafs are of varying thicknesses (particulary when the middle leaf is only 1/8 inch thick).

Yes, the rule is still true. Having different thicknesses doesn't change anything, except that it makes it harder to calculate the overall result! For panes of the same thickness and same spacing, it's not that hard, but with different thicknesses and spacing... well...

I am planning to turn my attention to the walls

For proper isolation, the entire room should be done to the same level: all six sides (four walls, floor, ceiling). Low frequency sound is not directional, and simply wraps around objects that are smaller than its wavelength, such as walls, houses, etc. So just putting one wall in its way is not useful: it will still get you by going around that wall as though it wasn't even there, and getting in from another angle. In order to isolate effectively, you have to do the entire room to the same level. The question is: What level of isolation do you need, and how much do you already have? What are you trying to accomplish? - Stuart -

I guess my question is should I leave it as is or would I be better off removing the internal 3rd leaf and beefing up the middle leaf with a thicker pane?

If you are going to remove the middle leaf, then don't replace it with anything! Leave it off! You could always keep the glass stored away somewhere, and put it back if you ever leave and have to restore the place to its original state.

That would only result in approximatley a 2 inch air gap.

Exactly. That thin gap is your enemy. The general recommendation is to never have an air gap thinner than about 4 inches, and that is for walls where you also have insulation in the cavity. For windows, it's best to keep it over 6", if you need good low frequency isolation. For most cases, with gaps above 4" there's a fairly linear relationship between mass and air gap: If you have to halve the air gap you can double the mass to compensate, or vice versa- But below 4" that linearity decays, and you have to add ridiculous amounts of mass (glass) to compensate for reduced air gaps.

I'm just trying to create the best possibile solution without removing the original window if at all possible.

Your other option is to increase your air gaps on both sides, and make both outer leaves as massive as you can afford. But it all depends on the rest of the structure: How are your walls built? Are they also decoupled MSM systems? - Stuart -

Thanks Stuart for your answers and comprehensive knowledge on this subject. I just wish I would have had your advice before I started. That said, I'm prepared to do what is necessary to improve the construction. Yes, my walls are traditional MSM systems with cedar beveled siding on the exterior and plaster on the interior. Cellulose insulation has been blown in the walls (house is over 100 yrs old) but I know that is not helping much. I have been weighing the idea of trying to treat the noise at the source by removing the siding and adding cement board + green glue + plywood as an under layer of sheathing before reinstalling the siding however that is going to introduce some issues with trim detailing (corner moulding, etc). The other option is to remove the plaster and double drywall with green glue and maybe add resilient decoupling mounts. I haven't been able to decide which way to go. I live in Atlanta GA so we don't experience many days of freezing temperatures howevver I know green glue doesn't work at below freezing conditions however I understand it reconforms once the temperature rises again. If I go the interior route I have I will lose some space with the extra wall thickness and I will have to remove a lot of old interior window trim and other molding in addtion to the plaster. I have a couple of other questions if you don't mind me bothering you a little more. I have a front entry door in the adjancent foyer which is virtually useless against the street traffic 30 ft away. I have attached a pic. As you can see it is pannelled (actually on both sides). I added a storm door with laminated glass a while back thinking that would block some of the sound but I don't think it had much effect. I was considering Rod's superdoor however I don't know if I could easily apply a veneer to create a more aesthetic appearance given it's an exterior entry door. Also, it must have a lock which I know is not preferrable. I have researched the web and this site and cannot seem to find a good source for a sound blocking exterior door that actually looks good if I decide not to build one myself. Are there any other options? Finally, one last question regarding the windows. I have attached a pic of my current construction. You can see the original window in between the exterior lami storm window and the thicker 3/8 lami (hard to see) on the side closest to the shutters. If I were to remove the glass only from the original windows (middle leaf) and keep the sashes without glass in place would that elimininate my triple leaf issue by only having one air cavity even though there would still be a partial obstruction? I'm not sure if you would be able to tell there was no glass in place or if it just appeared super clean. It might look horrible. I'm just trying to avoid any design issues given I live in an historical district with some restrictions. Thanks Mark

If I were to remove the glass only from the original windows (middle leaf) and keep the sashes without glass in place would that elimininate my triple leaf issue

Yup! That would work. It would be fun to take real isolation measurements before and after you do this, to help settle some long-running "discussions" as to what effect double vs. triple leaf really has! :)

Cellulose insulation has been blown in the walls...

Uh oh...!!! That may or may not be fine: Depending on how carefully (or not) it was blown, the cellulose itself might be creating flanking paths within the wall. That wall might be acting more like a single-leaf than a double leaf. Hard to say...

I have been weighing the idea of trying to treat the noise at the source by removing the siding and adding cement board + green glue + plywood

Not sure if I like that idea, from the practical point of view. Based on the highly scientific "If it ain't broke, don't fix it!" concept, researched by Professor Murphy and secretly encoded in his well-known law! :) You never know what you might find in there, as you start taking things apart: it might not be pretty.... But from an acoustical point of view: Sure! Go for it! : :mrgreen: :shot:

The other option is to remove the plaster and double drywall with green glue and maybe add resilient decoupling mounts.

That would be on the inside of the house, right? Sounds like a potentially less damaging approach. And acoustically valid, too.

I live in Atlanta GA so we don't experience many days of freezing temperatures howevver I know green glue doesn't work at below freezing conditions however I understand it reconforms once the temperature rises again.

But if we are talking about the interior walls of your building, those should (hopefully) never see sub-zero temperatures, surely?

was considering Rod's superdoor however I don't know if I could easily apply a veneer to create a more aesthetic appearance given it's an exterior entry door.

maybe you could take apart your existing door, and use the outside molding on your own new superdoor? That would have the added benefit of not changing the appearance of your house from the outside... - Stuart -

Soundman2020 wrote:

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Cellulose insulation has been blown in the walls...

Uh oh...!!! That may or may not be fine: Depending on how carefully (or not) it was blown, the cellulose itself might be creating flanking paths within the wall. That wall might be acting more like a single-leaf than a double leaf. Hard to say... - Stuart -

Shouldn't present any issue. When cellulose is blown into existing wall cavities, it isn't like the spray type that sticks to the cavity, it is lose. If anything, you get voids in the thermal break due to blocking in the walls, wiring, etc.