The way a room sounds—whether it is reverberant or “dry”—is defined by the room’s size, shape, and distribution of sound-absorbing, reflecting, and scattering surfaces. The larger a room is, the more reverberant it tends to be. Conversely, the more sound-absorbing materials there are in a room, the less reverberant it will be. Materials absorbing sound tend to be rather different than materials blocking sound transmission. The former are usually lightweight, porous materials like fibrous or open-cell insulation, while the latter tend to be heavy. The key is absorbing and blocking sound are not the same. The proverbial “concrete bunker” is excellent at blocking sounds—one can have a very private conversation inside a bunker but the material is highly reflective and not absorptive. Therefore, it is very reverberant inside a concrete bunker.
A material’s absorptivity and reflectivity can be specified in terms of its sound absorption coefficients. These coefficients range from 0 (totally reflective) to 1 (totally absorptive). They vary with frequency; a mid-frequency average absorption coefficient (useful for speech) is called the noise reduction coefficient (NRC). When specifying sound-absorbing materials (e.g. fabric-wrapped panels, acoustic ceiling tiles [ACTs], or perforated wood or metal systems) in a building, it is important the absorption requirements be clearly defined. For a space where speech is the primary sound source (e.g. an office), the NRC may be sufficient; in other rooms, minimum (or maximum) absorption coefficients across the audible frequency range may need to be specified. Since materials absorb a different fraction of incident sound depending on how they are mounted, it is important the absorptivity be specified for the mounting condition utilized on the project. For example, ACT absorbs far more when suspended in a grid below the deck (in which case, absorption using the E-400 mounting should be specified, where the tile is held 400 mm [16 in.] from the substrate) than it does when surface-applied to the deck (where absorption with a type A mounting should be specified).
In real buildings, all of these concepts—noise control, sound absorption, and sound isolation—come together.
The Jamaica Plain branch of the Boston Public Library, designed by architectural firm Utile, is a good example. In a new building addition, there was interest in integrating a smooth white finish that could also provide sound absorption, which was accomplished with a stretch fabric. Acoustical consultants recommended a ceiling with an NRC value of 0.80 or greater, to control the reverberant build-up of noise in the space. Designers at Utile had a smooth, monolithic, white appearance in mind. Together, they identified a system with a fiberglass sound-absorbing core and a smooth, white, sound-transparent fabric stretched over it that met both the project’s acoustical and aesthetic needs.
The Children’s Reading Room required special acoustical attention. Since its location is directly below a staff-community meeting room, additional sound isolation at the floor/ceiling assembly was required to ensure rumpus time and storytelling sessions would not interfere with important discussions happening above. This was accomplished with a gypsum board ceiling suspended on resilient clips, along with sound-absorbing batt insulation in the ceiling plenum.
More efficient ventilation systems and mechanical noise control were needed within the building as well, which, in turn, reduced the library’s community noise impact within a bustling Boston neighborhood. Acoustical consultants modeled noise levels of exterior equipment to determine the potential impact relative to the City of Boston’s noise regulations, and also measured sound levels in the area prior to the renovation project to establish a benchmark for future assessment. The consultants then worked with the design team to meet those regulations with a combination of quiet equipment selection and strategic equipment location.
What does an NC 35 room sound like? Will an STC 50 wall be good enough? How reverberant is too reverberant for a music room? How dry is too dry for a lecture hall?
These are altogether reasonable questions, and point to a key aspect of building acoustics: subjectivity. Background noise, sound isolation, and room acoustics characteristics can be quantified and described, but the quantities are imperfect and the descriptions can fall short of truly evoking the sonic experience. When making decisions about the acoustical ramifications of design decisions, sometimes there is no substitute for actually hearing the difference.
Mock-up constructions can be an outstanding way to make these kinds of comparisons, but they are often impractical or cost-prohibitive, and some design decisions (how high should the ceiling be?) do no usually lend themselves to a mock-up. Enter auralization—the acoustical equivalent of visualization and the process of rendering the sound of a space (for one’s ears and not eyes) based on a model of the area. Designers and users can get a sense of the space as well as experience how design decisions change acoustical and visual aspects of the room before it is built.
To be most effective, an auralization should be 3D—it should give the listener a sense of depth, where the sound is coming from, and the size of a space.
The Olin School of Business, designed by Moore Ruble Yudell Architects, on the campus of Washington University in St. Louis, Missouri, features a sweeping central atrium with a sunken forum. The acoustical goals were to support an engrossing lecture in the forum, while the bustle of the glass-enclosed atrium (complete with café) soars above and around it. A 3D listening simulation was employed for the architects and Olin School administrators and facility staff to get a sense of how much sound-absorbing material would be needed in the atrium to control the bustle, to evaluate how the sound system would or would not overcome the bustle (and how the reverberance of the atrium would affect the quality of the sound system), and to predict whether faculty—whose office windows overlook the atrium—might be bothered by all the bustle.
The building owners and design team had a chance to listen to a simulation of a lecture in the forum (presented by a member of the Olin School faculty, no less), allowing them to compare various design scenarios: with and without sound-absorbing perforated wood wall panels in the atrium. Further, the intended lecture could be presented amplified or unamplified, during a simulated normal morning or busy lunch hour, and from several vantage points throughout the room. In addition to comparisons among room acoustics designs, listeners could also hear differences between different sound-isolating constructions: the activity in the atrium and forum could be observed from within a faculty office overlooking the space, with various glazing profiles in place (e.g. single-pane window and dual-pane IGU).
The simulation helped the team make design decisions regarding what materials to employ in the atrium, how much sound-absorbing material to utilize, and what kind of windows to install in faculty offices. In the end, the atrium features a plaster-like troweled sound-absorbing finish on the lower-level ceiling near the forum, micro-perforated wood panels on the walls, and other materials carefully integrated with the interior design. For the interior glazing, the simulation helped the design team and owners determine fairly standard single-pane windows would be sufficient for the faculty offices, saving the project the cost of double-pane glazing (although sufficient sill depth was left in the design to accept a future interior storm window, should more attenuation be deemed necessary at a later date).
To develop such simulations, a 3D computer model of the space is generated, and the surfaces of the model are “painted” not with color but with the acoustical properties of the materials: sound-absorption, scattering, or transmission. A sound source is then placed inside the model, and a computer program simulates how sound propagates and reverberates in the room and, ultimately, arrives at a listener location. The output of this model can be compared directly with measurements made in the completed room later—a process helping acousticians refine their models and modeling skills over time. The same output, called an impulse response, can be combined with specially recorded audio to create a simulation. The result is then played back to the listener with a specialized audio playback system designed to preserve the level, timing, and directionality of sound reflections in the simulated space, and is linked with a visual display to orient the listener. All three design concepts described earlier—background sound, sound isolation, and room acoustics—can be simulated, thereby creating a powerful tool for designers and users to make comparisons and develop a sense of how design choices affect the aural experience.
Acoustics matter and sonic design should not be an accident. When the planning, design, and specification process has the benefit of a robust understanding of acoustical design principles—particularly when it can be supplemented by an acoustical simulation—the design, engineering, and construction team can be intentional about creating the acoustical quality that will benefit the education, wellness, and productivity of the building occupants.
Ben Markham, LEED AP, is director of Acentech’s architectural acoustics group. He consults on projects involving architectural acoustics, mechanical systems noise and vibration control, and environmental acoustics in commercial, residential, and civic facilities. Markham has an avid interest in acoustical models. He is a member of the Acoustical Society of America (ASA) and was awarded the Robert B. Newman Medal for Architectural Acoustics in 2002. Markham can be reached via e-mail at firstname.lastname@example.org.