Metropolitan Sytems Information Statement 1989

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DIFFERENCE BETWEEN ACOUSTICAL AND THERMAL INSULATION The transmission of acoustical and thermal energy do not follow the same physical principles. Acoustical energy flow is more easily retarded by heavy and rigid materials, while thermal energy flow is more easily retarded by materials with low thermal conductivity. An example of a good acoustical barrier but a poor .thermal barrier is a solid steel door. While the door is massive and hence minimizes sound penetration, it has a high thermal conductivity and hence does not minimize heat transmission. An example of a good thermal barrier but a poor acoustical barrier is a lightweight but thick thermal insulating panel. This barrier minimizes heat transfer but is generally a poor sound insulation. Both sound and energy can be transmitted through an open space in the wall, such as a poor seal around a window. Such a crack can increase heat flow through the wall, and can reduce the STC value of the window by up to five or more STC units. For this reason, sealing and caulking of through -the -wall penetrations is critical for good noise control. Thermal energy is diffused through a barrier at a rate determined by the thermal conductivity of the material. This thermal conductivity is strongly affected by the physical and chemical structure of the material. Acoustical energy is transmitted through a barrier in the form of wave or vibrational energy. This transmission depends upon the physical and mechanical properties of the material, as well as the structural system itself, which can vibrate in response to sound impinging on the surface. Thermal transmission through a wall can be analyzed separately for each component of the wall; acoustical energy cannot. The method used in this guide is based upon the flow of acoustical energy through a wall/window system and takes into account this difference between thermal and acoustical transmission. While an adequate thermal treatment may acoustically insulate a residence in Noise Zone IV (where a 25 dBA NLR is required), it is more likely not to do so in the higher noise zones, where special attention should be paid to the nature of acoustical energy transmission. CONCEPT OF STC (SOUND TRANSMISSION CLASS) The purpose of single -number acoustical ratings, such as the STC, is to provide a quick and simple method of building element selection to meet the desired acoustical requirements. The STC was originally established to provide some measure of speech privacy between rooms and is hence based primarily upon frequencies important in human speech. However, by analyzing the transmission loss for each frequency and comparing this with an aircraft noise spectrum, it is possible to establish the STC value needed to meet a given A -weighted decibel reduction. Sound transmission class is defined under the American Society for Testing Materials Standard E-412-73. STC is derived from the use of a standard curve that is fitted to the observed laboratory data. STC values are available primarily for interior partitions and acoustically rated windows. These are provided by the manufacturers of wall components and partitions or are found in standard reference manuals. For many common building elements, laboratory ratings are not yet available. It may be 41