In last month’s newsletter we discussed room modes, a standing wave phenomenon created by the geometry of a space. This month, we’ll carry on with our discussion of interior room acoustics, and focus on the control of reflected sound in a room. While recording studios and performance venues are often the first spaces that spring to mind when considering reflection control, many less prestigious spaces can benefit from thoughtful treatment, especially those that require good speech intelligibility such as lecture halls, classrooms, and conference rooms. Read on to learn more about surface finishes in the acoustician’s arsenal.
The surface treatment that most people associate with acoustics is a fiberglass panel wrapped with an open-weave fabric. Together with acoustic ceiling tile, these panels are a common sight in institutional and corporate environments, gracing the surfaces of classrooms, cafeterias, and conference rooms across the nation. The porous nature of these materials absorb sound incident upon them, dissipating the energy into heat, thereby reducing reflected sound that could otherwise lead to sound buildup, discrete echoes, and reduced speech intelligibility. Despite their ubiquity, these materials are often the bane of the interior design teams, disrupting the smooth clean lines favored by many design aesthetics.
The opposite of acoustical absorption is reflection. In spaces that require speech intelligibility or projection – such as conference rooms or auditoria – strategic use of reflective surfaces can improve the acoustics of a space dramatically. Consider a conference room for example: a reflective surface above the table can improve cross-table communication for local users, particularly those at the ends of a long table. In a concert hall, reflective ceilings and walls project sound from performers into seating areas, providing strength and intimacy for the audience.
When flat surfaces are used, reflections are specular – or mirror-like – and tend to be very focused; on the other hand, a convex panel, for example, creates diffusion. Diffusion is a useful subset of reflected sound and is often prioritized in critical spaces such as recording studios or concert halls. Specular reflections can contribute to flutter echo, slap echo, and acoustic glare, whereas diffusive elements scatter sound and counteract these undesirable effects to improve clarity while maintaining reverberation times. In older concert halls, cathedrals, and opera houses, ornate architecture and statuary was the preferred aesthetic. Whether the designers understood the acoustical ramifications of these details is up for debate, but it stands to reason that these designers used statues, niches, columns, and other decorative elements to scatter sound, dispersing energy evenly throughout these rooms. As the desire for these elements declined, purpose-built diffusive elements gained traction, and now adorn the sets of late night talk shows and podcast booths.
You don’t have to be in a concert hall or podcast booth to notice how a space sounds. Children are especially good at this; have you ever seen a child shouting to create echoes in a reverberant room or whispering in a quiet room? We encourage you to channel your inner child and notice how spaces sound and the surface treatments that make them sound that way.