In the early days of replacement windows, the sound was rarely an issue outside of the occasional bedroom that overlooked a busy street or highway. Times have certainly changed. Homeowners are in a war fighting increased noise pollution and their desire for their home to be a sanctuary and escape from… well from virtually everything!
I’m going to start this article with some terms that anyone who is concerned about sound levels will probably be confronted with as soon as they start to research technologies that provide serious sound reduction. (You may want to skip the next few paragraphs but save them for when these terms come up in your research!)
What is the meaning of ‘decibel’? A common definition is, “a degree of loudness”. So, reducing decibels is the same as reducing sound. However, this is not that helpful a term and rarely used in either codes or specifications.
STC means “Sound Transmission Class”. So, what does ‘Sound Transmission Class’ mean? Like many other rating systems, it is a single number that is intended to reflect how well a building partition limits conduction of sound through a surface.
So, what then does OITC mean? Outside Inside, Transmission Class is similar to STC, but a different formula. It should be the system used to measure sound going through windows and doors, but more often than not, STC is used. Anyone still reading, please take an aspirin before I continue.
The ability to throw around the above terms convincingly is how well-paid consultants justify their paychecks!
Let’s take the “Peabody Wabac(pronounced way-back) Machine” and return to the early days of thermal windows in the 1980s. After having their windows changed, many people remarked that their homes were quieter than before, particularly if they had replaced single pane windows. Thermal glass has a lower level of sound transmission than a single piece of glass. Add the improved seals and insulation around the windows and the effect can be quite dramatic.
Others were occasionally surprised to note that their homes were quieter before the new windows! It was not their imagination. A window with a storm window and both painted shut would likely have lower sound transmission than a typical thermal unit because the larger air space between the prime and storm window is more effective in reducing sound transmission than the smaller gap inside a sealed unit.
Over time, limiting noise pollution has become a more significant issue for many homeowners. Some companies take advantage of the value of a large gap between panes of glass by offering an internal storm window system, usually made from an acrylic panel and not glass. However, a major point of the industry going to sealed units was to prevent moisture from becoming trapped between the panels of glass and building upon the glass. Interior storms are not sealed, and condensation buildup is a significant, potential flaw.
Adding Argon or Krypton gas to a dual-glazed sealed unit was the next improvement to noise reduction. These two heavier gases do have a dampening effect on sound transfer in much the same way they reduce heat transfer.
Triple glass can have additional benefits in sound transfer reduction, however, the benefit depends on air space. Triple glazing with ¼” airspaces is barely better than a dual-glazed unit with a single 5/8” air space. Once again, air space size is a significant factor. Windows with sealed units that can utilize a 1 3/8” overall thickness can have two ½” air spaces and this does make a significant difference in sound transmission.
Going back to dual glazed units briefly, there is a technique that can have a noticeable effect in reducing sound transmission at a relatively low cost. Aha! This article has avoided the issue of cost. Yes, the cost will generally escalate as more effective methods of sound reduction are considered!
There is a benefit in manufacturing a sealed unit from the glass with different thicknesses. A 3mm thick piece of glass filters specific frequencies of sound. A 4mm thick piece filters a slightly different set of frequencies. Consider this as the effect of a filter. If you run something through two identical filters that are designed to filter the same thing, the second one is not overly effective compared to running something through filters designed to remove different things. Let’s say you have a filter designed to eliminate yellow and you run a rainbow through it. (Please just put up with me!). The first time a lot of yellow is eliminated. The second time, not so much. But, if you run the rainbow through a yellow filter and a blue filter, a lot more of the rainbow is eliminated!
So, even a sealed unit made with one layer of 3mm and another of 4mm has an effect reducing sound transmission. The same principle can work in a triple unit where one lite of glass is 4mm and the other both 3mm. Why not a 3mm, a 4mm and a 5mm? Wouldn’t that would work even better? Yes, it should, however, manufacturers will not do this because of the weight factor. It is simply too heavy for most operating windows.
We are down to our final and best option for serious sound transmission reduction. This is the use of laminated glass. Laminated glass is a sandwich made by bonding two pieces of glass with a soft mylar film between them. The film in the lamination tends to diffuse sound across the entire pane of glass, reducing its transition forward, into your home. A dual glazed sealed unit with one pane of laminated glass (usually a nominal 6mm thick piece) combined with a 3mm pane with a Low E coating has the same weight as a triple unit of 3mm glasses, but between the laminated glass and the single pane the airspace is significantly more effective at sound transmission reduction. If one laminated pane is really good, are two laminated panes even better? Absolutely, however residential windows are not designed for the weight of this kind of unit. Have we reached the end of our options?
Yes, and no. Not presently on the market but tantalizingly close is an entirely new product with tremendous potential for sound transmission reduction. Vacuum glass is lightweight, narrow and extremely effective. Why? For the same reasons that there is no sound transmission in outer space! Sound is a wave that requires a material to act upon and there is virtually ‘plenty of nothing’ between the glass in a vacuum sealed unit. (Not completely devoid of contact as there are micro-thin spacers, but close enough for a great improvement.)
Before you go running out to change your window order to vacuum glass, I must sadly tell you that it is not yet available in any practical way and can only be found in test labs. Ask again next year!
After all we’ve discussed, do any of these improvements guarantee that your home will be a quiet, serene paradise? Alas, no. Windows are only one component in your home’s outside envelope. Any flaw in the building envelope can be a problem. Badly insulated walls and roofs can significantly allow sound to leak into your home. If you choose to retrofit your windows into old frames, the old window frames may not be well insulated to the rough openings. Vents and flues can offer opportunities for sound penetration into your castle. These are the most likely culprits, but there could be others.
We live in a world of increasing noise pollution and homeowners like you want to be in control of their environments. At GEM we have the expertise to discuss your personal situation and requirements with you and can offer great solutions.