TrusSteel - Cold Formed Steel Trusses

1 2 3 4 5 6 7 8 3.13 SOUND CONTROL The Mass Law The amount of sound, or vibration, which is transmitted through floors, walls and ceilings is governed by the Mass Law, a theoretical rule that relates the mass per unit area to the control of airborne sound. The Mass Law equation estimates that each time the frequency of measurement or the mass per unit area of a single layer is doubled, the sound transmission loss (STL) is increased by about 6 decibels (dB). A 6 dB reduction in sound provides roughly a 25% reduction of the original sound level, contingent upon other factors such as temperature and the frequency (Hz) of the sound. In construction terms, a 4 inch thick concrete floor has a sound transmission loss (STL) of 42 dB at 250 Hz. Doubling the floor thickness to 8 inches only increases the STL to 48 dB. This doubling in thickness (and mass) provides only the 25% reduction in transmission loss described above. This is not an acceptable solution in today's construction market. Sound Control The subject of sound transmission is situation, or construction project, specific. The source of the sound or noise may be airborne, or structure- borne, or a combination of both. Typically the elimination of airborne noise requires a reduction in the energy level of the sound waves, which are created by fluctuations in atmospheric pressure reaching the eardrum. Structure-borne noise is created by unwanted vibrations. The designer should select, from the outset, the system and products that will deliver the appropriate results. It is normally far more economical to integrate the solution into the initial design than to attempt to create an "add-on" solution during the construction phase. There are a number of companies specializing in the engineering of noise control systems. Because increasing mass is no longer the solution of choice, these companies design systems and products that create an interruption in the noise path or create a containment barrier (at the source) to prevent the noise from reaching the receiver. These companies use four basic tools to combat noise transmission: absorption, barriers, damping and vibration isolation. A number of products, from decking and fabric barriers to mechanical devices, are used to address specific transmission loss needs. Resources In general, many sound control design methods, products and applications that work with other framing systems can work with CFS framing. Some of these products have been tested in CFS applications and the product manufacturers have published data on these applications. The building designer who is striving for a particular sound control solution should carefully examine the manufacturer's published data as well as data published by independent researchers. Here is a small sampling from the wide range of valuable informational sources on sound control: Steel Framing Alliance (SFA) www.steelframingalliance.com A Guide to Fire & Accoustic Data for Cold-Formed Steel Floor, Wall and Roof Assemblies , prepared for The U.S. Department of Housing and Urban Development (HUD) and the Steel Framing Alliance by the National Association of Home Builders (NAHB) Research Center, Inc (2004). North American Insulation Manufacturers Association (NAIMA) www.naima.org Unwanted Sound The transmission of unwanted sound, classified as noise, is one of the most common complaints made by the occupants of modern buildings. This problem has grown in recent years as material suppliers have developed products and construction methods to reduce the weight of building components. The goal has been to conserve material and reduce both component cost and construction time. Unfortunately, the goals of lighter weight building materials and the containment of noise often come into direct conflict. Noise Is Measured in Decibels (dB) Whispers: about 20 dB Normal conversations: about 60 dB City traffic: about 80 dB Lawn mower/leaf blower: about 103 dB Repeated exposure to sounds over 85 decibels is considered dangerous to hearing, and the louder the noise, the less time it takes to damage hearing. Methods of Sound Propagation reflection absorption transmission ENGINEERED BY ALPINE S P E C I F Y I N G / D E S I G N I N G

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