Factors Affecting Acoustics

Environmental vs. Architectural Noise

There are two types of noise: environmental and architectural. Sources of environmental noise include automobiles, locomotives, rail cars, jet aircraft, industrial sources, telecommunications equipment, entertainment and construction vibration. The impacts of environmental noise include sleep disturbance, speech interference, occupant annoyance, reduced worker productivity, and prolonged patient recovery in health-care settings. Environmental noise can be controlled through measures such as noise barriers, sound-rated walls, and sound-rated windows. Building codes dictate different land-use categories that each have varying levels of acceptable exterior noise exposure, measured in decibels (the levels are Normally Acceptable, Conditionally Acceptable, and Unacceptable).

Source: US CLT Handbook

Simplified illustration of sound (pressure wave) propagation through air/solid structure (Kappagantu, 2010). In this figure, C signifies regions of compression, and R signifies regions of rarefaction of the air molecules. Furthermore, the “0” pressure line in the graph represents the atmospheric pressure level.

Land-use categories are categorized as:

• Residential, Hotels, and Motels
• Outdoor Sport and Recreation
• Schools, Libraries, and Museums
• Auditoriums, Concert Halls, and Amphitheaters
• Industrial, Manufacturing, Utilities, and Agriculture

Land uses where people live and work have more stringent codes and standards for acceptable noise exposure than uses such as stadiums and manufacturing plants.

Architectural noise includes environmental noise but also mechanical/equipment noise, structural vibration, and room acoustics, which can affect occupants in all building types, including health care, schools, institutional/commercial, residential, performing arts, civic, and industrial. The practice of architectural acoustics attempts to achieve good speech intelligibility, improve privacy, enhance the quality of performances in studios or venues, and suppress noise to make schools, offices, and residences more productive and pleasant places to work and live. This can be achieved by carefully specifying and detailing each building element and system to ensure the materials used work well together acoustically. Because of the complicated nature of this practice, architectural acoustic design is often performed by acoustic consultants in tandem with the rest of the design team.

Source: US CLT Handbook

Sound generated by various sources and their pressure (Pope, 2003).

Airborne vs. Impact Sound

Transfer of sound between adjacent spaces takes two forms. The first is the transfer of airborne sound, such as speech or music. In airborne noise, the medium carrying the sound energy is air. The ability for a building element such as a wall or floor/ceiling to prevent the transfer of airborne sound is measured by STC ratings. The second form is the transfer of impact sound, such as footfall. Impact sound is often more disruptive than airborne sound. For example, if a residential building is constructed of concrete, an 8-inch slab will effectively block the sound of voices from the suite above, but the sharp reports of someone walking in hard-soled shoes will be clearly audible. In contrast, it has been observed that very stiff wood-joisted floor/ceiling assemblies present greater low-frequency impact sound insulation. The ability of a floor/ceiling assembly to prevent the transfer of impact sound is measured by IIC ratings. Designers should determine the most appropriate rating to satisfy codes, regulations, and owner requirements.

Source: US CLT Handbook

Floor surface as flanking path (IRC, 2002). The red arrow indicates the direct path, and the blue arrows indicate the flanking path.

Acoustical Codes and Wood Buildings

The International Building Code (IBC) provides the minimum requirements for sound insulation of demising walls and floor/ceiling assemblies between adjacent dwelling units or between dwelling units and public areas, such as halls, corridors, stairs, or service areas. In residential buildings, the IBC provides a minimum design requirement for unit-to-unit acoustical protection between floors. It requires an STC rating or IIC rating of 50, unless the authority having jurisdiction has its own more stringent requirement. The International Residential Code (IRC) has an appendix that provides for a minimum design separation of STC 45 for townhouses, where specifically adopted by the authority having jurisdiction. This minimum requirement is the same for entry-level housing, market-rate housing, and luxury housing, whether it is dorms, apartments, or condominiums. Beyond that, it is the responsibility of the design team to develop an acoustical design that meets owner/developer/tenant expectations for the project.

In a mixed-use project, consideration must be given to the acoustics between all of the adjacent spaces—not just dwelling unit to dwelling unit. Acoustical separation between residential and other occupancies can be a significant challenge, and this condition is not addressed in the IRC or IBC, though it is starting to be addressed in green building codes such as the California Green Building Standards Code (CALGreen).

CASE STUDY: STUDENT HOUSING GETS EXTRA COLLEGE CREDIT FROM WOOD
UNIVERSITY OF WASHINGTON WEST CAMPUS STUDENT HOUSING

Photos: Benjamin Benschneider | Architect: Mahlum Architects

Mixed-use student housing integrates layers of public and private amenities. West Campus Housing, Elm Hall and Elder Hall, University of Washington, Seattle.

In 2012, the University of Washington (UW) completed a $109-million, five-building construction project, adding nearly 1,700 student housing beds. Known as West Campus Student Housing – Phase I, the 668,800-square-foot project was the first of four phases planned by UW to add much-needed student housing to its Seattle campus, which has an enrollment of more than 42,000 students. The West Campus structures were the first new residence halls to be built on UW’s main campus since the early 1970s.

UW’s need was great, but the budget was limited. The IBC allows five stories of Type III wood-frame construction when the building is equipped with an automatic sprinkler system that complies with the National Fire Protection Association’s code NFPA 13. Designers across the country are increasingly choosing this option as a lower-cost alternative to steel and concrete. However, Seattle’s building code is unique in that it also allows five stories of wood with a Type V-A structure (when the building has an NFPA-compliant sprinkler system), which is even more cost-effective and being considered by a growing number of other jurisdictions as a way to encourage urban infill development. With this in mind, Mahlum Architects worked with engineers at Coughlin Porter Lundeen to make the most of the urban campus location, designing each of the buildings with five stories of light-frame Type V-A wood construction over a two-story Type I-A concrete podium. This two-story podium helped them meet both ambitious design goals and the university’s tight budget.

Acoustics are important for any multifamily housing project, but particularly so for student housing. Mitigation measures must be weighed against the budget, which is why the design team brought in experts from Seattle-based SSA Acoustics. While the science of sound is fairly complicated, many mitigation measures are relatively simple. For example, SSA recommended a strategic combination of staggered-stud and double-stud walls to minimize sound transmission between residential units themselves, between the units and common spaces, and between the units and service areas. Because it knew single-stud walls would not provide adequate performance, SSA recommended staggered-stud walls between residential units.

“Since there is no rigid connection between the gypsum board on each side except at the plate, a staggered-stud wall performs better than a single-stud wall,” says Mohamed Ait Allaoua with SSA Acoustics. “Double-stud walls perform better than a staggered-stud design because plates are separated by an air space, so we specified double-stud walls between residential units and common spaces (lounges, staircases, elevators, etc.) and service areas.”

Details count when it comes to acoustics so all penetrations were sealed using resilient caulk. Whenever possible, contractors located junction boxes using minimum 24-inch spacing and avoided placing them back to back. When this was not possible, they placed putty pads on the backside of the junction boxes. “In the floor/ceiling assembly, we paid careful attention to the installation of resilient channels, which are often one of the main causes of failed floor/ceiling assemblies from an acoustical standpoint,” Ait Allaoua says. “In fact, there is a difference of 8 to 10 IIC and STC points between assemblies with resilient channels versus those without.” SSA specified straightforward requirements for channel installation; for example, the length of screws was specified for the first layer and for the second layer of gypsum board to never touch the framing behind the resilient channel.

Bathrooms and kitchens feature drop ceilings to accommodate ducting and plumbing, which provided additional noise reduction between units. Where the finish floor was stained concrete, a resiliently suspended gypsum wallboard ceiling was installed using neoprene clips to reduce footfall impact noise below.

Fire-protection measures often benefit acoustical efforts. Where putty pads were required at electrical boxes for fire code (in 1- or 2-hour fire-rated wall assemblies), there was no additional acoustical mitigation required. Penetrations through 1- or 2-hour rated demising walls, corridor walls, shaft walls, floor/ceiling assemblies, and others were sealed with fire-resilient caulk, which also met acoustical recommendations.

SSA also made structural recommendations to reduce floor vibration. It recommended that the live load deflection of the floor assembly between residential units achieve a max L/480; the code only requires L/360. SSA also recommended that plywood sheathing be placed only on the outer side (not the inner side) of double-stud walls since air space between the layers of mass on each side of the studs is critical for achieving the specified acoustical performance.

Sustainability also appeals to an increasing number of students, and the University of Washington prides itself as being “one of the country’s preeminent leaders in environmental practices,“ committing itself to offer students what it calls an “urban eco-lifestyle.“ Cedar Apartments received certification at the LEED Silver level, while Poplar, Alder, and Elm Halls earned a LEED Gold designation.

In addition, four of the five buildings in West Campus Student Housing – Phase I meet The 2030 Challenge (requiring 60 percent reduction over baseline fossil fuel energy consumption) with the purchase of green power. Design strategies included use of high-efficiency heating and ventilation systems, low building envelope air infiltration, elimination of building envelope thermal bridges, efficient light fixtures and lighting control systems, and others. While on-site renewable energy production was not part of the project, provisions were made on the roof structures to allow future installation of solar hot water systems, which significantly increased roof loading.

The five buildings are testaments to the fact that wood construction can not only save time and money but also create elegant, durable, urban structures that contribute positively to city and campus vitality. The West Campus Housing project represented a paradigm shift at the University of Washington, symbolizing its first embrace of large-scale light wood-frame construction.