Last week at Glass Build I wondered into the Soundproof Windows Inc. booth to check out their soundproof “recording studio assembly,” featuring sliding glass doors with STC ratings of 65. As I stood inside the studio and closed the soundproof doors, it was as if I had been listening to a loud television show in a library that was suddenly turned off. You can experience this for yourself by watching the latest [DWM] video coverage of the show Glassbuild Provides a Glimpse at the Future.
The sudden transition from noise to complete silence is quite impressive. It reminds me of a trip that I made many years ago to visit the manufacturer of UVEKOL acrylic resins, which are used to create interlayers between glass layers to produce laminated glass. The resin is filled in between glass layers and then cured with UV radiation. The result is laminated glass with improved impact resistance that also serves as a sound deadening barrier so that the doors and windows so produced offer dual benefits of safety and sound reduction. The windows in their offices were made with this product. Their on-site advertisement was simply to open the window so that I could hear the street noise below and then to close it which resulted in utter silence. I was quite impressed!
PVB films are another option. Companies such as PRL Glass Systems and Eastman Chemical Company, with its Saflex Acoustic PVB interlayer, produce laminated glass by laminating PVB sheets in between glass. Modern building designs like to emphasize liberal use of glass, but the downside is that sound transmission can be significant through ordinary glass, and there is a lot of surface area transmitting it. The answer to noise concerns isn’t less glass, but the use of these innovative laminated safety glass products. The Saflex Acoustic PVB interlayer, for example, is a patented, tri-layer system which targets sounds in the 1,000–4,000 Hz range (an area sensitive in human hearing) for superior sound damping in the range that humans hear the most.
PVB (polyvinyl butyral) interlayers slow down and reduce sound transmission in laminated glass primarily through a mechanism called constrained layer damping and by decoupling the glass panes.
When sound waves hit a standard sheet of monolithic glass, the rigid glass easily vibrates and transmits the sound directly to the other side. Introducing a PVB layer then changes how the system responds to sound energy in three main ways:
1. Viscoelastic Energy Conversion (Damping)
PVB is a viscoelastic polymer, meaning it possesses both viscous (fluid-like) and elastic (solid-like) properties. When sound waves strike the outer glass pane, they create flexural (bending) vibrations. As these vibrations reach the flexible PVB interlayer, the polymer molecules slide against each other.
The result: The internal friction within the PVB layer acts like a microscopic shock absorber. It converts the mechanical, kinetic energy of the sound vibrations into trace amounts of thermal energy (heat), which safely dissipates rather than passing through to the next pane of glass.
2. Acoustic Decoupling
In a solid piece of glass, vibrations travel uninterrupted. A PVB interlayer acts as a structural break. Because the plastic is much softer and more pliable than the surrounding glass, it decouples the two glass panes.
The result: The outer and inner glass panes are prevented from vibrating in perfect synchronization. When the outer pane flexes under sound pressure, the interlayer absorbs the shift, forcing the sound waves to travel through a mismatched medium, which severely disrupts the transmission path.
3. Flattening the “Coincidence Dip” (Resonance)
Every rigid material has a coincidence frequency—a specific weak point where the material resonates perfectly with the frequency of incoming sound waves, causing the glass to become almost transparent to noise. For standard glass, this dip usually falls squarely within the 1,000 to 4,000 Hz range, which matches the frequencies of human speech and traffic.
The result: The mechanical dampening provided by the PVB interlayer breaks this resonance loop. It “fills in” and effectively flattens the coincidence dip, ensuring the glass maintains a consistent level of sound isolation across the entire spectrum
Standard PVB vs. Acoustic PVB
While the UVEKOL resins and standard PVB films, both designed for safety and windshield impact resistance, offer decent sound reduction, specialized acoustic PVB, such as the Saflex product previously mentioned, use a modified tri-layer design. It features an exceptionally soft, extra-pliable core sandwiched between two stiffer skins. This architecture optimizes the internal friction, allowing it to decrease transmitted sound by up to 10 decibels (db) in the critical frequency range of human hearing (1000-4000 Hertz) vs. non-laminated glass. This 10 db reduction is perceived by the average human ear to be roughly half as loud.
The use of laminated glass is just the beginning step when it comes to the science of sound control. One can use laminated glass as part of an insulated glass unit (IGU) specifically designed for further improvements. STC ratings can be further improved by using differing glass thicknesses, air infiltration, air gap, type of glass (annealed vs. laminated), type of spacer (non-metal and foam spacers are quieter), Argon gas filling, edge dampening and the use of dissimilar materials or material thicknesses to break up harmonic waves. It’s the science of change ups. Different glass or frame thicknesses and different sized airgaps in triples or quads. All this serves to disrupt the formation of harmonic waves.
I have spent a big part of my career promoting components used to fabricate windows with improved thermal properties. I have also presented quite a few seminars to architects over the years dealing with the topic of sound control. Now that the National Fenestration Rating Council (NFRC) and Energy Star have driven thermal performance to its asymptotic limit, is it now time to focus on other properties of doors and windows that could also be improved to add more meaningful improvements in consumer comfort? If so, the area of sound control could represent the wave of the future.
Technologies for designing windows for improved sound control is likely to become increasingly important as population density increases. I can envision the day when the NFRC label may also include an optional 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. Does this sound too sophisticated? Maybe so, but it just may help us sell more windows!
Yes, I was really impressed with the Soundproof Windows booth when the two doors with STC 65 were closed effectively silencing the noise from the show floor. It reminded me of the lyrics from that classic Simon and Garfunkel song, “and the vision that was planted in my brain, still remains, within the sound of silence.”