Glass is a universal building material that is attractive to architects and clients, while posing a variety of challenges to acousticians.
Due to its transparent nature, glass creates an open and pleasing atmosphere. Curtain walls, skylights and windows allow for a view both outward and inward; connecting occupants to the building’s natural or urban setting. The use of natural light can lower electricity bills, brighten the rooms of a building, boosting the mood of the occupants. Glass is also a renewable building material, with 30% of new glass comprised of recycled materials. For all these reasons and more, glass will continue to play a major role in architecture in the future.
However, glass has a number of acoustical properties that can contribute to a poor occupant experience. To illustrate this, let’s take a closer look at what happens when sound interacts with glass.
When sound encounters a window, the glass converts some energy into thermal and kinetic energy (resonate vibrations), allows some sound to pass through, and reflects the rest back.
Glass only “absorbs” sound near its resonant frequency (and subsequent harmonics). The resonant frequency of glass is dependent on many factors, including density, thickness and panel size. As is the case with many “hard” building materials, the absorbed sound accounts for only a small fraction of sound energy’s interaction; most sound is either reflected or transmitted through the glass. Sound reflection and sound transmission are two separate acoustic issues with separate solutions.
Sound Reflection – Reflected acoustic energy from an internal sound source can cause a number of issues for occupants. Large, uninterrupted spans of hard materials like glass and gypsum cause specular reflections (echoes) and contribute to excessive reverberation and noise levels. These conditions can contribute to a poor acoustic environment in which speech is difficult to understand and music clarity suffers.
Specular reflections are compounded when there are other hard surfaces in the room. Flutter echo, heard as “ringing”, happens when sound bounces back-and-forth between parallel reflective surfaces (between walls or floor-to-ceiling). Flutter echoes greatly degrade speech intelligibility and music definition. This is a big problem in studios, offices, conference rooms and theater/media rooms. If there is an abundance of reflective surfaces, background “noise” from latent energy will cover up or distort the direct sound.
Typically, these issues are corrected with sound absorbing materials. However, we cannot simply “resurface” the glass with sound absorption, like we would with concrete or gypsum, without impacting transparency. Until someone invents invisible acoustic foam or fiberglass, sound reflections off glass will continue to be a challenge that needs accounted for.
Sound absorptive materials like thick curtains or acoustic shades provide adequate sound absorption and coverage flexibility. Other creative solutions include “stand alone” furnishings like tall, leafy plants or translucent perforated plastic sheets mounted over top the window. Essentially, any irregular surface you can introduce in front of the glass will help diffuse sound and break up harmful wall-to-wall reflections.
Sound Transmission – More than 90% of all exterior noise comes in through doors and windows. This can be partially attributed to poor weather stripping. “Leaky” windows will not only cause uncomfortable drafts, but allow sound to more easily work its way into our homes and businesses. Sound is a little like water; it will “pour out” through any gaps in the building assembly. Improving sound-loss across glass often starts with replacing the weather stripping and properly sealing any joints with non-hardening acoustic caulk.
Air-tight, limp, massive materials are the best at blocking sound. Glass is rigid, and its heft is limited by transparency requirements that keep it thin. Glass transmits a lot of sound energy, particularly at low frequencies. Laminated glass and insulated glazing assemblies both reduce sound transmission through glass by reducing resonance and adding air-space.
Including an acoustic consultant early in the design process will allow architects and owners to make well-informed decisions. An acoustical consultant will best identify potential pitfalls of using glass and recommend glazing systems and construction techniques to minimize future headaches. This measured approach will result in more beautiful looking (and sounding) spaces!