CREJ - Building Dialogue - September 2016
With any housing project – single- or multifamily – exterior noise should always be of concern. This exterior noise entering the occupants interior living space is problematic. There are many studies concerning noise and the negative impact. One conducted in 2003 with 1,700 persons shows noise affects our long-term health and increase medical care costs. The two sources of “exterior noise” are noise from the surrounding neighborhood – like sirens traffic, trains, etc. – and possibly equally annoying is the noise stemming from the activities of the neighbors. With respect to outside noise, location has been a long-time real estate mantra as the location of a property is often as important as the building itself. Location is also an important consideration with respect to exterior noise levels. The table lists typical sound levels for various outdoor environments and activities along with subjective descriptions. “A-weighted” sound levels (dBA) are used because they represent how people hear low to high frequencies. Federal laws are in place to help regulate noise level of airplanes and highway noise. The Department of Housing and Urban Development requirements for housing have existed for many years with a focus on limiting the noise levels inside the living areas. The HUD design goal is 45 dBA maximum inside the bedroom or living room. The World Health Organization recommends 40 dBA. It has been our experience that most people prefer quieter (e.g., 30 to 35 dBA). The exterior walls, windows and doors of single- or multifamily buildings are the first line of defense against outside noise entering the living spaces. Usually, the windows are the “weakest link” and need to be upgraded to increase the noise insulation they provide or the amount of glazing can be reduced. The American Society for Testing and Material issues standards to determine exterior noise isolation (i.e., Composite Sound Transmission Class and, more recently, Composite Outside to Inside Transmission Class). STCc is an acoustical calculation of the overall ability of a building element to block exterior noise to inside rooms. The individual STC ratings of the roof, exterior walls, doors and windows are analyzed and based on the square foot of each, a composite sound isolation is calculated. The calculations include the frequencies between 125 and 4,000 hertz, which are speech frequencies. It doesn’t work well if the noise includes lower frequencies (<100 Hz) caused by trucks, train engines, aircraft flyovers, etc. OITCc is a more recent ASTM test standard (E-1332) also used to rate the sound isolation performance of the exterior of a building. It was adopted because it includes frequencies down to 80 Hz. This better represents aircraft, truck and rail noise because it is weighted more to lower frequencies. OITCc should be used if the residential project is located near sources having significant low-frequency noise content. However, OITC is more difficult to use since OITC ratings for many building elements are not available. It is important to select the appropriate method to evaluate a project. If you apply OITCc analysis to a project where there is no significant low frequency noise impact on the building, the results can lead to higher costs due to overdesign. Conversely, if you use STCc for the building analysis and there are low-frequency noise sources nearby, complaints are likely. It is important to first evaluate the location and then measure exterior noise levels at the project site to determine the appropriate method to use in the analysis and to determine how to reduce noise inside the living spaces. If the project is in a quiet suburb, STCc probably applies. On the other hand, a project located near highways or rail lines should probably be analyzed using OITCc. elogsdon@dlaa.com