Page 38 - January 2007

The words luxury, high quality, or soundproof are sometimes used to describe projects that barely meet, or even fail to meet minimum code requirements. If builders plan to characterize their product in this manner, they are well advised to provide a level of noise control commensurate with the description. Multifamily dwellings can be grouped into the three quality categories in Table 1.
Table 1. Level of quality vs type of use
Classification
Minimum Quality
Medium Quality High Quality
Residential Use
Normal Apartments
Nursing Homes
Hospitals
Good Apartments
Normal Condominiums High Quality Condominiums
Low-cost rental property, subsistence housing, and tem- porary housing such as hotels and motels would be designed to the minimum-quality level. Note that the minimum-qual- ity design level is not the same as minimum-code level, since there must be a certain safety factor included to insure code compliance. If one were to design exactly to the code mini- mum, it would mean that the selected construction would have a 50% probability of passing a field test. This is not good design practice and since test results for a given wall or floor will vary by a few points, a 3-5 dB margin of safety is recom- mended. It is also prudent to examine the range of test results for a given configuration and to expect the lowest values in the test range rather than the highest.
The medium-quality level is appropriate for use in high- quality apartments and normal condominiums. A first-time condominium purchaser may be moving from a single fami- ly home and have an expectation of quality based on his or her previous history. Any condominium should be designed to at least the medium quality standard. If noise problems arise, the owner of a condominium does not have the free- dom of movement of an apartment dweller. Under California law the seller of a condominium must reveal any known defects to a potential buyer, including any noise problems.
Condominium homes where there is the expectation of a level of isolation similar to that found in a detached single- family residence fall into the high-quality category. In these cases due in part to the location, cost, and sales representa- tion, owners may express dissatisfaction if they can hear any activities in adjacent dwelling units. They are particularly sensitive to footfall and plumbing noise since these may occur relatively frequently.
Airborne noise standards
Many cities and states have adopted standards for the STC ratings in multifamily dwellings and these can be used to develop prudent design objectives for various levels of construction quality. The legally mandated minimum STC ratings are usually set to 50 (State of California, 1974 and the Uniform Building Code (UBC) Appendix Chapter 35, 1982), however in some cases stricter standards have been adopted.
For example the City of Redondo Beach, CA requires a min- imum STC of 55 in condominium homes.
An STC 50 may be the code required rating for a given partition. This does not necessarily mean that it yields acoustical privacy between units or that it represents a level of quality that guarantees owner satisfaction. Rather mini- mum code construction is so bad that there has to be a law against building it any worse. Other criteria such as reason- able expectation of the buyer should be used as the basis for construction decisions. Table 2 shows STC ratings for various levels of construction quality in multifamily dwellings. These ratings apply to both walls and floor—ceiling separations and are based on reasonable expectation of quality.
Table 2. Sound Transmission Class vs Level of Quality For Party Wall and Floor-Ceiling Construction
Classification Minimum Code Minimum Quality Medium Quality High Quality
STC FSTC 50 45 55 50 60 55 65 60
Structures for airborne noise isolation
High transmission loss values can be obtained by using two approaches: a high-mass single panel or a low-mass, decoupled, double panel. A simple concrete slab of sufficient thickness can provide a good floor–ceiling. A 6 inch (152 mm) thick slab has an STC rating of 55 and is sufficient by itself for a minimum quality floor. Six-inch concrete slabs with a wire-hung drywall ceiling and fiberglass insulating batt can provide enough isolation for airborne noise to be used in medium quality construction. For high quality con- struction, even with concrete slabs, a drywall ceiling sus- pended from neoprene isolators is required.
With double panel construction there is a complicated tradeoff among the partition masses, the panel spacing, and the degree of decoupling. Highly rated double-panel floor— ceilings combine a high-mass floor with a large separation between the floor and the ceiling below. The airspace space between contains fiberglass batt insulation usually 6 inches thick. The two panels should be structurally decoupled either with separate structural supports or by means of a resiliently supported ceiling or floating floor.
In wood construction, the structures are light and stiff. The problem with wood floors for airborne noise isolation is in achieving sufficient mass and decoupling. Lightweight- concrete fill, which weighs 110 to 115 lbs/cu ft (540 - 560 kg/sq m) and should be poured to a thickness of 1.5 inches (38 mm), is a common way of increasing the floor mass. Composite floor-ceiling systems fall somewhere in between wood and concrete. A composite floor can be constructed using a 3 inch (75 mm) concrete fill poured into a webbed sheet metal deck with a suspended ceiling below. With this configuration, a drywall ceiling is required even for the minimum quality design standard. Figure 1 gives examples of floor-ceiling systems suitable for medium quality con- struction.
36 Acoustics Today, January 2007