Soundproofing Windows
If you require any level of sound isolation through a wall that contains a window, then it is likely that, at least at some frequencies, the window will be the weakest link. As such, attention will have to be paid to the windows for best results. Indeed, even the best of walls – like a Green Glue double stud wall – may not perform brilliantly with common windows installed.
Windows are, of course, standard-issue on exterior walls. They are also critical components of studio control rooms. Let’s take a look at what can happen when a low-performance window is installed in a wall, and then lets take a look at how to build walls with window AND high soundproofing performance. At the end of this document, we’ll look at building a window plug for situations where your window doesn’t need to retain its visual function (i.e., basement home theater).
What happens to the performance of a wall when it includes a window? Well, if the window has isolation lower than the wall, then the overall performance of the wall will be reduced. For example, an STC=75 wall with a common window might have an STC of only 35 or 40, see below.
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The right graph assumes a wall of 160 square feet, with a window of 16 square feet. The enormous advantages of the superior construction are compromised quite a bit by the window.
But take heart, high performance partitions with windows can be accomplished!
The Basics of Soundproofing with Windows
The basic principles of isolation with windows are very akin to isolation with walls. Same basic principles apply. We should look for mass, decoupling, low resonance frequency if decoupled (mass and air space), absorption, and damping. Let’s take a look at some practical situations and the choices that will serve us best. We’ll start by taking a look at the basic goals of window design.
Mass
Mass is pretty straightforward – the heavier your windows are the better. Mass is particularly important at lower frequencies.
Decoupling & absorption. At middle and high frequencies, you can attain considerably better soundproofing by utilizing two panes of glass separated by an air space. The benefits of double pane windows will be maximized if you can include some type of sound absorption at the perimeter of the window, inside the air cavity if you’re building the window.
Low Resonance
When decoupling, low frequency soundproofing typically suffers. To maximize low frequency isolation, we need to drive the resonance frequency of our decoupled window as low as possible. To accomplish this, follow these basic guidelines:
1. Use as much mass on both sides of the double-pane window as possible
2. Use as large of an airspace inside the window as possible
3. If feasible, include absorption at inside the edges of the window
Damping
While products like Green Glue can’t be used in see-through windows, laminated glass offers some of the same benefits. Laminated glass is two layers of glass bonded by a polymer interlayer. The polymer interlayer considerably raises the damping of the windowpane. This has a considerable positive effect on soundproofing, particularly at resonant points such as the coincidence dip. Laminated glass is, unfortunately, considerably more expensive than conventional glass. Another drawback of laminated glass is that it’s damping benefits fade in cold weather – the polymer interlayer becomes too hard to help with damping.
Different thickness of glass on either side of the double pane window. Many people recommend using glass of different thickness on each side of the window. For example, 1/4” glass on one side, and 3/8” on the other side. This is helpful at a resonance called the coincidence dip – which reduces performance at higher frequencies important for speech and similar sounds.
However, if you opt to utilize panes of different thickness, and sacrifice weight in doing so, this will have a negative effect on low frequency isolation. Therefore, if your budget allows it, it is preferable to control the coincidence problem with damping (i.e., laminated glass).
In general, glass of different thickness is recommended if you are primarily trying to contain higher frequency noise, such as speech, but is not necessarily recommended (not recommended if it causes you to make a much lighter window) if you are in a situation such as a recording studio where low frequency noise is an important consideration.
Those are the basic goals and principles, now let’s look at some actual situations. First, an exterior window for a residential situation, and then a control room window in a studio.
Exterior window – making good choices and maximizing your value.
Exterior windows are exposed to many noise sources with a lot of low-frequency content. Traffic and aircraft are two examples. They are also typically utilized for thermal insulation, mandating double pane windows.
The best answer to exterior noise and windows is to find a commercial window, designed for sound control. When shopping for windows, don’t simply look for high STC’s, but also consider the weight of each side of the window, and the depth of the air space. All things remotely even, the deeper, heavier window is preferable.
Commercial sound isolating windows are available as complete units – typically with laminated glass on at least one side, thicker than normal panes, and well-designed seals and other considerations. In other cases, commercial products are sold as add-ons, to be installed behind your existing window with an air space between the newly installed window and the old. Typically, these add-on windows feature thick laminated glass. Either solution can work, and which is best for any given situation generally boils down to mass and air cavity depth.
If you want to construct your own windows, this can probably be done. Follow the guidelines given above.
Studio Control Room Window
This is a hotly debated topic, with several points being debated by experts everywhere. These include questions of expensive laminated glass –vs.– lower cost conventional glass, angling of one side of the window to reduce standing waves between the panes, and general questions of what criteria should be targeted to maintain the performance of the walls. Let’s look at the factual side of each of these points one at a time.
Angled windows or straight? Many sources recommend angling one of the window panes to eliminate internal standing waves and/or to reduce direct reflections from the window.
We can’t help you with the question of acoustics and reflections, but we can offer this certainty: while the angled window may help somewhat with standing waves inside the air cavity, standing waves will still occur. And angling the window will always result in a smaller net air space, which will have a negative effect on low frequency performance. How much of a negative effect depends on how much of the net air volume between the panes of glass is lost. Depending upon how much low frequency content is present in the music, it is very likely that the isolation of a window will be maximized without angled glass.
Choice of glass – different thickness in the panes?
The answer to this question is very much like the discussion of angled-vs-straight panes above. It’s a question of low-frequency isolation-vs-high frequency isolation. The advantage at low frequencies goes to the heavier window combination while the use of different pane thickness offer some advantages at higher frequencies.
Our answer is the same – for most studio situations you should opt for the heavier window and improved low frequency performance.
Laminated or standard glass? Here we don’t necessarily have a question of performance tradeoffs – the same weight of laminated glass is always preferable – but a question of cost. Is laminated glass worth the expense?
Well, the inclusion of laminated glass will have a considerable positive effect on performance, and if the use of laminated glass doesn’t involve a considerable compromise in weight, then it is preferable. It does far more at the coincidence dip than different pane thickness, and has benefits at other frequencies as well.
This, ultimately, boils down to your assessment of value. Is making one pane out of laminated glass a large percentage cost in your studio project? If it is, you might consider forgoing this expense and focusing on high-mass panes. If it isn’t, then you should consider utilizing it for all the benefits listed above.
Basic Design Overview:
Rules for construction of a high performance control room window:
1. The mass of the glass on each side of the window should be as high as the mass of the drywall on each side of the wall. More mass is better, always. Glass is about 3.5 times denser than drywall; so in general using glass of 1/3 the thickness of the drywall you’ve installed on the walls will yield good results.
2. Maximize the air space – ideally, you would attain as large of an air cavity as is attained in the walls.
3. Utilize two panes of glass, on the extreme outsides of the assembly. Utilizing two double-pane windows (making 4 panes of glass and 3 air cavities) will hurt isolation. First, you add a second resonance caused by the air space in each of the double-pane units, and second you detract from the useable air volume of the large central cavity for a given amount of mass. Stick to two panes, as heavy and far apart as possible
Absorbing material at the windows edges can improve STC by several points
4. If your budget allows, utilize laminated glass on one or both sides of the assembly
5. If you choose to angle the glass on one side, choose this for sound-quality reasons, and not for sound-isolation reasons
6. Include edge absorption if at all possible.
7. In these electronically-loaded times, you could bypass this problem and any performance loss that comes with it by replacing the window with a camera and monitor(s)
Building window plugs – for times when windows don’t need to function.
In some situations, existing windows can simply be covered. In some cases, they can be removed, and the wall rebuilt as if they were never present. In other cases, this is simply not convenient.
For situations where the windows must remain, but do not need to serve any visual purpose (common in basement home theaters), a simple window plug can be a great aid in preventing sound escaping through the window from disturbing your neighbors.
A typical window plug should be as massive as practical, and insulation should be included in the cavity between the existing window and the new plug. This sketch shows a typical design:
Operable Window Plugs
An operable window plug can also be fabricated. The seals should be substantial, and made from a compressible material such as neoprene weather stripping. Acoustic foam attached to the plug can serve as the sound absorbing material, and handles can be added if desired.
Summary:
We’ve taken a look at what to do with windows, both for typical domestic applications, and for specialized applications like recording studios.