September 18th, 2019
Sound Control: The Wave of the Future
I recently replaced some windows facing my back yard and one of the things I noticed after the installation is how much quieter it is in that room. The new windows are doing a better job of blocking out unwanted noise such as lawnmowers, barking dogs and even planes taking off or landing at the nearby airport. I am really appreciating the added sound control.
The funny thing is that I was not thinking about sound when making my window buying decision. If I would have been comparing sound control when I was in the buying process, the windows that were on my shortlist would not have been on my list at all. The reason? Most window manufacturers are not marketing sound control.
Well, we spend a great deal of time talking about and comparing the energy efficiency of windows, and I had these windows installed last December so U-Values were a key consideration. But this summer, I was reminded of the importance of sound control. Unwanted neighborhood sounds such as lawnmowers, screaming youngsters, barking dogs, motorcycles, trucks, planes, trains and automobiles can all be reduced by new windows.
When it comes to rating windows for sound control, a homeowner looking at an NFRC label does not currently have access to a rating method to compare the relative degree of sound deadening that a fenestration product can provide. Test methods currently exist but the testing is typically only done by window companies vying for commercial jobs with specific sound attenuation goals in mind. These usually entail high-end residential or commercial buildings situated near airports, train stations or busy highways. In these situations, architects will specify 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).
So how do we design windows with better (higher) STC ratings? The STC rating 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 is also used to improve 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 different thicknesses within the IG unit, sound transmission can be reduced by preventing the buildup of harmonic waves. Each glass thickness produces a different set of harmonic waves when sound waves hit each pane. So, when different glass thicknesses are used, the resulting waveforms are different and will cancel each other out when peaks in the waveforms overlap troughs as opposed to peaks overlapping peaks and troughs overlapping troughs. This results in reduced amplification or “sound deadening.” I spent some time as a tire design engineer and used this same principle when designing tread patterns. Look at your tread pattern around the circumference of your tires and you will see the tread elements are of different shapes, which are designed to produce wave patterns that cancel out each other resulting in less road noise.
Architects are now digging deeper into sound data as opposed to just looking at the single number rating systems such as STC or the optional Outdoor Indoor Transmission Classification (OITC) evaluation method. They have found that certain materials or technologies can reduce sound transmission across specific frequency ranges. The STC may be the same because it covers broader frequency ranges but the windows response to specific, more narrow ranges may be improved. Perhaps the window offers more sound deadening with typical neighborhood sounds such as lawnmowers or maybe it cuts down on airplane sounds or train sounds. Specific frequency ranges can be targeted depending on the desired results.
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 3000 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, a savvy architect may find that improving sound control in a specific frequency range is an added benefit that the argon provides which could help in blocking out the sounds of car horns in urban areas.
Technologies for designing windows for improved sound control are likely to become increasingly important as population density increases. With the fenestration market possibly slowing down somewhat, window manufacturers are looking for others to differentiate. Designing windows with improved sound control can surely give you a competitive advantage!