It’s a classic scene from movies and TV: two characters stand in an all-glass office or conference room yelling at each other, while the rest of the cast sit outside watching and asking each other what they think is being said, with no trace of sound escaping from the glass box. If you have watched the CBS drama “NCIS: LA” any time in the last several years, you have probably seen this scene play out at least once every few episodes. It appears to the viewer as if the moment the glass door swings shut, the room is acoustically cut off from the rest of the building.
So, what actually happens in the real world when we build spaces of all glass? Glass is used everywhere to enhance the feeling of openness and let in natural light. Typically, these spaces look amazing, but quickly frustrate end-users. Every conversation in gleaming glass conference rooms can be heard by others outside the room. The entire open office can hear the disciplinary meeting happening in the boss’s “private” office. Noise from one meeting can make it impossible to have another meeting in an adjacent space.
Despite what we may see on TV, glass is a poor sound isolator. Glass walls have relatively low mass and low thickness, both of which are key elements of a good sound isolator. Thick, high-mass materials disrupt the transmission of sound by changing sound energy into heat.
How’s that? You might ask! Sound is a longitudinal pressure wave, caused by molecules bumping into one another. In each of these collisions, energy is lost by the transfer of kinetic energy into heat energy. The more energy that a material changes into heat, the less energy is transferred through the material. It follows then, that the more material we can put between adjacent spaces, the less sound will be transferred. A single pane of glass provides very little material, especially when compared to a typical conference room or office wall.
The type of glass also makes a difference. When the glass is monolithic annealed rather than laminated, we lose the one remaining tool for sound isolation: change in material. Laminated glass is made of two or more panes of annealed glass joined together by a layer of plastic, or polyvinyl butyral (PVB). You can think of this joining as creating a glass sandwich (glass, plastic, glass). Some types of laminated glass are made with multiple layers (think of these as a laminated glass layer cake).
So why is the type of glass so important? When there is a change or disruption in the material the sound molecules change how they interact with each other and the adjacent material. In laminated glass systems, the change in material from glass to the laminate material and back to glass provides the opportunity to dissipate some energy. In tempered glass systems, this does not occur, as the system is a single piece of glass.
Differing materials present us with the best option for acceptable sound isolation with glass walls. By including an airspace between two panes of laminated glass, we add layers of differing materials (air being one of them) that sound must travel through, allowing for as much sound energy as possible to dissipate before reaching the listener on the other side of the assembly. In extreme applications, such as morning TV talk shows in New York City with large crowds just on the other side of the window, the airspace may be as large as a few feet.
Glass walls provide terrific architectural effects, but also present difficult acoustical challenges. It can be easy to select a system that appears effective on paper but has several Achilles’ heels in the field.
To reduce sound transmission while permitting the transmission of light, it is critical to work with an experienced acoustical engineer to develop a solution that will serve the end-user well. With careful planning and material selection at the start of design, many common end-user frustrations with modern glass office designs can be greatly reduced.
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