CREJ - Multifamily Properties Quarterly - July 2015
Architects need to design multifamily housing buildings to accommodate many interacting components including functionality, - structural integrity and fire separation. However, one factor that is typically on the mind of the occupants in the units is sound attenuation. In other words, it is important to know if and how much they hear the neighbor next door. In multifamily housing, sound attenuation, governed by the International Building Code, can be a major leasing factor. An architect can design methods and details to attenuate sound if they have a strong understanding of how sound behaves. In physics, sound is a vibration that typically propagates as an audible mechanical wave of pressure and displacement through air, solid material or liquid. Sound is measured in pressure level units called decibels. The larger the vibration of the source, the louder a sound is heard by the human ear. Standard noise levels of some common sources consist of a “quiet home” at 25 to 35 dBA, “human voice at three feet” at 55 to 60 dBA and the “average TV or radio” at 80 to 100 dBA. There are also two types of sound – airborne and impact. Airborne sound is vibration, like music or conversation. Impact sounds are generated by foot traffic on the floor above or the pounding of a hammer on the wall. The sound rating classifications established by U.S. Department of Housing and Urban Development and the National Bureau of Standards are recognized by the building construction industry and found in building codes. There are two rating systems that compare the acoustic quality of various building assemblies. In each classification, the higher the number, the better the sound isolation performance of the assembly. Sound Transmission Class rates a wall assembly’s resistance to airborne sound transfer at the speech frequencies (125 to 4000 Hertz). Impact Insulation Class rates a floor/ceiling assembly’s resistance to structure borne or impact noise transfer. The IBC contains requirements for sound isolation between dwelling units in apartments and condominiums. It requires partitions and floor/ ceiling assemblies to have an STC rating of 50 (lab tested). It requires that floor/ceiling assemblies have an IIC rating of 50 (lab tested). This is recognized as a minimum industry standard. Architects consider the quality of the dwelling unit when determining STC level for design. The minimum code requirement is not necessarily best design practice. Multifamily housing can be grouped into three distinct occupancy types: 1. Minimum (standard apartment): Code required STC/IIC 50, recommended STC/IIC 55 2. Medium (good apartments and normal condominiums): recommended STC/IIC 65 3. High (luxury condominiums): recommended STC/IIC 75 In general, the more mass a wall has, the better it is at attenuating airborne sounds. Most projects cannot be constructed economically with solid masonry or concrete demising walls. Most projects are constructed out of lighter wood-framed partitions and floor/ceiling assemblies. While the lighter assemblies do a better job of attenuating higher-frequencies sound verses lower-frequencies sound, it is the mass of the materials used and the isolation of materials to one another that really make the difference. In wood-framed partition construction, methods that separate the layers of materials can reduce both airborne- and impact-sound transmission. The common demising and corridor partitions are best constructed using staggered stud spacing on sill plates that are set in sealant on the subfloor. Gypsum board should not be attached directly to the studs, but installed on resilient metal channels on at least one side of the stud framing. This isolation reduces the sound transmission through the structural frame. A second layer of gypsum board can increase the STC rating of the partition by about 9 dB. Filling at least two-thirds of the cavity with absorptive insulating material increases sound transmission by 8 to 10 dB, if properly isolated. The perimeter of the wall and any penetration must be sealed airtight with a non-hardening acoustic sealant. In wood floor/ceiling construction, methods and details that attenuate sound are similar to that of partitions. This includes heavy material layers that are not solidly attached to the framing members and separated by a thick cavity that is filled with absorbing insulation. To achieve the minimum STC rating, the ceiling material of the floor/ ceiling assembly should be a double layer of gypsum board attached to resilient channels. The cavity should contain sound absorbing insulation. Adding 1½ inches of concrete aids in increasing the STC ratings by effectively attenuating the high-frequency airborne noises, but it does little to attenuate the IIC ratings low-frequency noises for the floor/ ceiling assembly. The addition of a “floating floor” system is one way of attenuating the lower-frequency noise. This system requires an isolation layer of a resilient cushion or interlayer of foamed rubber, plastic mesh or cork between the wooded subfloor and the concrete mass. Ceramic tile and hardwood flooring will reduce the IIC rating over resilient flooring or carpet and pad. The majority of noise concerns also can be alleviated through proper space planning. Quiet areas should not be located near potentially noisy areas. If you can mirror uses and spaces back to back along a demising partition then the relative background noise of each space helps dampen the sound generated within adjacent spaces. Potentially annoying sound transmission from floor to floor can be attenuated through the vertical stacking of uses and spaces. Since one of the biggest concerns with occupants of multifamily living units is noise from adjoining units, good sound-attention design can greatly affect the value of these projects.