As I discuss repeatedly in my blog articles, there is no such thing as perfect soundproofing, or soundproof windows. Windows cannot be made “sound proof”, however effective window noise reduction treatment is available.
How to measure window noise reduction?
Noise reduction effectiveness of windows is measured by standardized test procedure ASTM E966 and quantified as Outdoor Indoor Transmission Class (OITC). OITC is a single number that allows architects and builders to compare effectiveness of components of building envelope in reducing outdoor noise. A typical OITC for windows is in the range of 22-26, which is quite low. Windows are typically the acoustically weakest part of a building envelope.
In addition to OITC ratings (or instead) some window manufacturers provide Sound Transmission Class (STC) (specified in ASTM E336-16 and E413-16) ratings, because the STC number is higher than OITC number, and therefore misleadingly appears better, but also because more people are familiar with STC ratings. Typical STC for windows is in the range of 28 to 34. However, STC rating is intended to quantify sound attenuation effectiveness of interior partitions, not building envelope. OITC number provides more accurate representation of how much outdoor noise will be heard inside through the rated window.
In any case, higher OITC or STC number means better noise reduction effectiveness; just make sure that you are comparing the same classification rating number among different products.
How window noise reduction can be increased?
There are several ways how sound reduction of windows, or colloquially, soundproof windows, can be archived.
Install better windows
If you are installing new windows, it makes sense to specify windows that have the highest OITC rating. However, in most cases, replacing existing windows solely for the purpose of increased soundproofing is too expensive and insufficiently effective.
Install noise reducing curtains
You can reduce noise through windows by installing noise reducing curtains. These curtains are very heavy and do not admit light, therefore they are useful in situations where you want to reduce noise at night. These curtains can be moderately effective, reducing perceived noise by about 30% to 50% at a reasonable cost.
Install additional interior glazing
Installing an additional glass layer on the interior, some distance away from existing window, is the most effective method of achieving window noise reduction. Spacing between the existing windows and the new glass should be at least one inch, but preferably four to six inches [reference 1]. Larger spacing provides more noise attenuation. This solution can reduce perceived noise levels by about 50% to 75%. Most people find this improvement to achieve soundproof windows very significant.
Additional interior glazing is available in two versions:
- Acrylic windows
Plexiglas (acrylic) window glazing, the lower cost glazing option, is usually installed over special magnetized frames and is held in place by magnetic strip attached to the edges of the acrylic glazing. This type of additional window is easy to remove for cleaning or ventilation and yet it provides air tight seal.
- Laminated glass windows
Laminated glass window glazing is the best and most expensive solution. In most installations this window is more aesthetically pleasing than acrylic and provide the best sound reduction. The window is made from laminated glass in aluminum frames. It can be made to open to match the exiting window for ventilation.
Typical windows, besides being the weakest part of a building envelope from acoustical standpoint, are also usually the weakest part of a building for heat insulation. Therefore, if you install any of the sound reduction options described here your also significantly improve thermal insulation of windows, saving money on heating and cooling bills.
 Master Handbook of Acoustics by F.A. Everest and K.C. Pohlmann, Sixth Edition, McGraw Hill 2015, page 363
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