September 21st, 2023
The Sound of Silence
With Energy Star 7.0 looming on the horizon, thermal performance ratings have window manufacturers and dealers wondering just how low U-values can go and how much lower makes a practical difference. As fenestration U–values have been pushed lower and lower in recent years, the National Fenestration Rating Council (NFRC) should be thinking about other important factors and corresponding test methods that may provide homeowners with additional comparative information to help them decide which door and window products are best for them. So, what about sound control?
Yes, we have spent a great deal of time talking about and comparing the energy efficiency of windows. But this summer I noticed something else. The sound of lawnmowers, screaming youngsters, barking dogs, motorcycles, trucks, planes, trains and automobiles and fireworks booming in my neighborhood had me pondering another important aspect of window design: the ability to provide peace and quiet! Perhaps it is time to focus on another great reason to replace those old windows—better sound control.
But when it comes to rating windows for sound control, the typical homeowner looking at an NFRC label does not currently have access to a rating method to compare the relative degree of sound dampening that a fenestration product can provide. Test methods do currently exist, but the testing is usually only done by window companies vying for commercial jobs with specific sound attenuation requirements in mind, such as high end residential or commercial buildings situated near airports, train stations or busy highways.
In these situations, architects are usually faced with a project that entails installing windows with a minimum Sound Transmission Classification (STC) value. This might be dictated by a building code or simply specified by the building owner. The STC value is the standard way to rate a window for sound transmission. Introduced in 1970 as ASTM E413, STC is a value based upon the attenuation that is required to reduce each octave level of the noise spectrum to match what is defined as the NC-25 contour. This is a weighting method that is designed to compare the relative attenuation of fenestration systems of office type noises (speech, radio, television and similar noises).
Now, STC ratings can be influenced in the design of windows by paying attention to the glass thickness, air infiltration, air gap, type of glass (annealed vs. laminated), type of spacer (non-metal and foam spacers are quieter), the use of insulating glass films, argon filling, edge dampening and the use of dissimilar materials to break up harmonic waves.
Laminated glass also offers significant improvements in sound reduction, while offering additional benefits such as improved security. There are specific interlayers that can be used to absorb sound waves. Also, by combining glass panes of dissimilar thickness within the IG unit, sound transmission can be reduced by preventing the buildup of harmonic waves. Windows can be designed so that harmonic waves are now cancelling each other out with peaks in the waveforms overlapping troughs as opposed to peaks overlapping peaks and troughs overlapping troughs. This results in reduced amplification which is the very definition of “sound deadening.”
In a previous life, I was a tire design engineer, and I used this same technology when designing tread patterns. The next time you happen to be putting air in your tires, look at the tread pattern around the circumference of your tires. You will notice that the tread elements are comprised of different shapes. They are designed to produce different wave patterns that cancel each other out, resulting in less road noise. If not for such designs, your tires would be wining during your entire journey and driving you absolutely insane!
Today’s architects are now digging deeper into sound data as opposed to just looking at single number rating systems such as STC or the optional Outdoor Indoor Transmission Classification (OITC) evaluation method. They are focusing on the fact that certain materials or technologies reduce sound transmission across a specific frequency range even though the STC value might not even change at all.
In addition to lowering U-values, Argon gas may also be used to reduce sound transmission across a specific bandwidth or sound frequency range. For example, if considering the bandwidth between 500 to 3,000 hertz, Argon may provide sound deadening even though, because of the way STC is calculated, it may not show up as a lower STC value. If one compared only the STC values of two windows with and without Argon, the conclusion might be drawn that the only benefit of the argon is that of slightly better thermal performance (lower U-value). But by digging deeper into the data, an architect may find that improved sound control in a specific frequency range is an added benefit that Argon brings to the table. In this situation, Argon could help in blocking out the sounds of car horns in urban areas. Indeed, architects can actually fine tune windows to absorb specific frequencies based upon the environment in which that window is to be installed.
Technologies for designing windows for improved sound control is likely to become increasingly important as our nation grows and population density increases. I envision the day when the NFRC label may also include a rating for sound control. This would help window customers decide which window may provide a greater degree of peace and quiet in their type of neighborhood.
So, have we reached the practical limits of rating windows for thermal performance? Many fabricators and dealers that I talk to think we have. If so, maybe it is time to focus on testing and ratings surrounding other factors that affect the wellbeing of homeowners. After all, after a long hard day out there in this crazy world, one can really appreciate returning home to the Sound of Silence!