With laminated glass, the plastic interlayer acts as a shock absorber to dampen sound waves, although the degree to which this occurs is temperature-dependent, with colder weather (which stiffens the material) reducing transmission loss. Very basically, the more rigid the glass or interlayer, the easier it is for sound transmission to pass, as opposed to the “limp mass” resulting from warmer weather that is better at absorbing sound.
Generally, engineered framing profiles with open spaces help reduce noise transmission in much the same way as insulating glass. The type of material also affects STL through the frame. Aluminum framing members will readily transmit sound if a vibration break is not utilized. For thermal isolation, aluminum extrusions with rigid polyurethane thermal breaks reduce the conduction of thermal energy. For acoustical isolation, however, less rigid materials should be used to reduce structurally transmitted vibrations.
Glass has its own characteristics for sound attenuation, as does the frame. A weighted average of both is necessary to determine the overall sound transmission. A frame material that is sound deadening should be paired with an effective sound attenuating glass. Otherwise, the weaker material will lessen the overall sound dampening of the whole unit.
In general, glass attenuates higher-frequency sound better than low-frequencies. Still, resonance effects must be avoided. Each material used has a specific frequency at which it resonates (i.e. where the amplitude builds because all frequencies are perfectly aligned and reinforce one another, resulting in a buildup of sound level).
When the frequency range of the impinging sound happens to coincide with the resonant frequency of the fenestration materials and dimensions, its ability to attenuate sound (i.e. the effective STC or OITC) will decrease or dip for those frequencies. This resonant frequency range is known as the “coincidence dip,” and it must be considered if the outdoor background noise to be controlled is heavy in those frequencies.
In IG, using different thicknesses of glass for the layers of double glazing gives greater noise reduction than using the same thicknesses for both lites due to resonant decoupling of similar components. A ratio of thickness of the glass layers of about two is most effective (e.g. 8 and 4 mm [0.3 and 0.15 in.]).
Tightness of the seal
For IG, sonic decoupling of components is also accomplished by isolating the two lites from each other to provide better STL. To accomplish this, foam spacer systems outperform the more rigid metal u-channel spacers at frequency ranges from 1000 to 5000 Hz. The use of soft, resilient seals (such as neoprene gaskets) can decrease low frequency sound transmission by several dB as well.
Curtain wall design introduces some very unique considerations of its own for attenuating sound. For example, the spandrel area of a curtain wall assembly can provide a flanking problem (i.e. sound transmission via a path other than through the fenestration portion of the wall). The STL of the spandrel area needs to be comparable to that of the curtain wall assembly, or the interior floor/ceiling assemblies need to be modified with additional mass. Figure 3 shows an example of an interior floor/ceiling assembly and its associated flanking paths.