Solving Acoustical Problems in Multifamily Construction

By Eric Miller, Architectural Testing, Inc.

As population growth, sustainable design, urban planning imperatives and work-at-home trends combine to produce higher residential densities and occupancy hours in cities, residents’ concern for privacy increases. A key element of perceived privacy is separation from unwanted noise, especially excessive traffic. The relationship between acoustics and green design is becoming stronger, with the USGBC’s LEED™ for Homes program offering “Acoustic Comfort” credit under the Innovative Design (ID) category for effective acoustic isolation of acoustically sensitive rooms.

There are two types of sound transmission that must be addressed in multifamily dwellings: airborne sound (such as voices and music) and structure-borne sound, which is transmitted in the form of vibration due to impacts (such as footfalls on the floor above, moving furniture and operating washing machines). These two modes are addressed by most building codes using two rating systems: Sound Transmission Class (STC) and Impact Insulation Class (IIC). Both yield a single rating number, higher numbers indicating better sound isolation performance.

Sound Transmission Class (STC) indicates how well a building partition—wall, ceiling, floor, door—blocks airborne sound.  For example, while normal speech is clearly understood through a partition rated at STC 35, at STC 50 loud speech may be faintly audible but not understood, and at 60+ it may not be heard at all.

Impact Insulation Class (IIC) rates a floor/ceiling assembly’s ability to block structure-borne impact sound.  As with STCs, the higher the IIC value of a floor/ceiling, the better its ability to control impact sound transmission.  An acceptable IIC rating is typically 50 or above.

Most multifamily projects must meet the acoustic requirements of the International Building Code (IBC), Section 1207. For airborne sound ratings of wall and floor/ceilings assemblies, it requires a minimum laboratory-derived rating of 50-STC (45-FSTC if field tested). For impact noise at floor/ceiling assemblies, the minimum laboratory rating is 50-IIC (45-FIIC if field tested).

For LEED ID credit, party walls in multifamily buildings must have a minimum STC rating of 55 and floor/ceiling assemblies must have minimum STC and IIC ratings of 55. If based on field tests of finished buildings, FSTC and FIIC results must be at least 50. If the building is in a HUD-defined noisy location, exterior walls and roof-ceiling assemblies must have STC ratings of ≥45 and exterior windows and doors must have STC ratings of ≥ 35.

But, there are inherent weaknesses in the STC rating system. STC ratings are heavily weighted toward speech frequencies (125 to 4,000 Hz), and are less accurate indicators at low frequencies, such as those from the typical urban environment.  For this reason, many acoustical engineers prefer using the Outdoor-Indoor Transmission Class (OITC) rating based on a noise spectrum weighted more to lower frequencies (80 to 4,000 Hertz), which is typical of aircraft, traffic and rail noise.

Employing sound control techniques

To realize the required acoustic performance for the project and/or applicable building codes—in addition to specifying sound-resistant components—designers employ one or a combination of three construction approaches. In approximate order of increasing effectiveness, these are:

•    Increasing the mass of partitions. Increasing the mass of walls and other structures, such as by adding extra layers of drywall to both sides of an interior partition, has some acoustical effect but may not be practical because of the increased weight, incremental loss of floor space and cost.

•    Cavity Absorption, such as using fiberglass insulation to fill the space in walls and floors/ceilings—interior as well as exterior—will block some mid- and high-frequency airborne noise.

•    Mechanical Decoupling. Breaking sound’s vibration paths by disconnecting two sides of a wall will significantly decrease vibration and increase the transmission loss. Staggered or double rows of wood studs, attachment of drywall using acoustic clips or resilient channels, or using acoustic drywall sandwich panels are typical measures.

Note that STC numbers differ depending on whether the results were obtained by lab or field tests, a phenomenon largely attributable to “flanking paths”—structure-borne sound transmission that bypasses the separating interior wall and travels to other building elements such as the floor, ductwork, ceiling or walls that are mechanical connected to the walls, as well as small holes, roof voids, utility penetrations, even windows and doors that decrease a wall’s acoustic insulation from the exterior. A recent study has shown how two rooms separated by a wall lab-tested at STC 67 could experience a performance reduction of 10 or more STC points in the field test because flanking transmission that undermines the wall’s sound insulation.

Recognizing that current codes, relying as they do on STC and IIC, do not adequately address flanking paths, the ICC has issued G2-2010, Guideline for Acoustics, which states: “The current level and approach of sound isolation requirements in the building code need to be upgraded.  They are currently insufficient to meet occupant needs,” because they do not account for flanking paths.

G2-2010 goes on to state that, “building acoustical performance that is predicted through an analysis of the whole building system, including flanking, during design and then verified by [mock-up and/or] field testing is preferred over solely prescriptive designs based on laboratory testing,” and that “Acoustical field testing should be considered one component of building [architectural] commissioning.”

A complete architectural commissioning program, beginning with a thorough review of the assemblies shown on the drawings and continuing through construction and occupancy permitting, can detect and cure potentially costly deficiencies. Be it green imperatives or simply ensuring resident satisfaction, acoustical performance is an issue that is not going away. In addition, architects and developers can be held legally accountable, with poor acoustics reportedly a major source of litigation for builders, second only to mold intrusion.

Demanding consumers, potential lawsuits, stringent building codes and the harmful effects of noise on human health and welfare are forcing the industry to adopt cost-effective acoustical control measures in their projects. Architects have an array of sound control methods and technologies at their disposal, including judicious Architectural Commissioning to ensure that all members of the design-build team are hearing the same message.

Eric Miller is Director-Acoustics at Architectural Testing, Inc. in York, PA.