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Advertising supplement provided Solutia Inc. and Arch Aluminum & Glass Co.

Frame effects

Window frames serve an important role in the sound transmission loss of windows as they themselves can compromise the sound transmission loss. A sound transmission class (STC) rating, which is predominately used in the U.S., is a single number value quantifying the ability of a material to resist the transmission of sound. The higher the STC rating, the more able the material is to resist the transmission of sound. It is not uncommon to see window frames result in an STC rating that is 1 to 5 points less than the STC rating obtained for the glass alone. For example, a window outfitted with a glass configuration achieving an STC rating of 35, might as a total window assembly (frame and glass) only achieve an STC rating of 33 if frames or installation are deficient. Window manufacturers face the challenge of designing and building frames that maximize available sound transmission loss of glass configurations used.

 

There are no fixed guidelines for designed window frames that maximize sound transmission loss, but a few observations include: The lower the sound transmission loss of glass used in a window frame, the less likely that the window frames will compromise the window STC ratings. Windows with lightweight frames that use glass configurations with high sound transmission losses should be designed so the glass surface area is as large as possible, extending as far as possible between edges of building openings (i.e., extending as deeply as possible into window frames). Window frames that rely on glass position by means of blocking should make use of continuous blocks. Continuous blocks should also be used on sides and tops of windows. Window frames should have a surface weight as great as that of the glass. Often this is not the case, particularly in windows with acoustic glass. Packing large, hollow-tube sections of frames with loose, dense glass fiberfill has been noted to improve window sound transmission loss. Frames occasionally exhibit insufficient damping or ringing that manifests itself as a reduction in transmission loss (TL) of a particular frequency, generally less than 500 Hz. Although laminated glass with a polyvinyl butyral (PVB) interlayer can contribute to frame damping, direct application of damping compounds to window frame components can more efficiently increase frame damping. Always ensure adequate sealing of all glazing components and frame joints to eliminate sound leakage.

Many of these suggestions require testing to ascertain their benefit for a particular window frame. Others may require additional tooling or labor to implement properly.

Seeking to offset TL through enhancement of only glass would be inefficient and would likely be more expensive than implementing frame improvements. For example, the enhanced glass would still yield approximately the same STC if the air infiltration of the window is not rectified. A glazing that provides damping and a properly glazed frame system ensures the lowest levels of air infiltration, which is an acoustical window design fundamental.