OITC is the appropriate parameter for determining curtain wall acoustical performance, as it more accurately represents the attenuation of noise from exterior sources. It is based on a noise spectrum weighted more to lower frequencies (around 80 Hz) typical of the noise produced by cars, motorcycles, trucks, elevated trains, and air traffic. These sources account for most of the background noise that disturbs people. OITC is determined per ASTM E1332, Standard Classification for Rating Outdoor-indoor Sound Attenuation.
The OITC rating is usually 5 to 10 dB lower than STC, primarily due to the calculation differences between ASTM methods E1332 and E413. Figure 2 illustrates this STC/OITC difference for common unitized fenestration products.
Sound control design considerations for curtain wall
Standard sound tests are conducted in the frequency range of 80-4000 Hz. However, if the building location is near an airport, on a heavily traveled highway, or closer to some other major noise source, special requirements for performance at different frequencies may be necessary. The requirements would be determined by making project-appropriate sound measurements at the proposed building site.
As noted earlier, the weak links for sound control in most walls are glazed areas and openings. A curtain wall is additionally burdened by the fact the larger expanses of glass are more flexible and will vibrate more than a smaller/thicker piece when exposed to a noise source, which results in lower values of STL, or the degree of attenuation.
Among the factors affecting sound transmission through curtain wall are frame and glass mass, glass-to-frame ratio, and seal tightness. Remedies for reducing sound transmission are essentially the same as those for promoting energy efficiency (e.g. use of insulating glass [IG], laminated glass, and configurations that minimize air infiltration). Specific product choices depend on the unique characteristics of a building’s proximity to noise, story level, window load factors, and frequency characteristics of the street noise.
How much of the intrusive sound is blocked (i.e. how large the STC or OITC index is) depends on various factors.
Air infiltration and seal tightness
Air leakage is one of the biggest factors in the acoustical performance of fenestration products. The North American Fenestration Standard (NAFS) sets a maximum air leakage rate of 1.5 L/s/m2 (0.3 cf/m/sf) for most window and door products. However, to achieve optimum STL results, consider products with a maximum air leakage rate of one-third this level (0.5 L/s/m2 [0.1 cf/m/sf]) or less.
Be aware design techniques employed to seal an operable window against air leakage and sound transmission have a direct effect on the force required to operate the unit, as the tighter the seal, the greater the friction. Industry standards and codes set limits on operating force that must be taken into account.
Fenestration system mass relative to the sound
The greater the mass of the fenestration element, the more the dampening effect. A heavier frame will exhibit greater sound attenuation than a thinner frame or glass. For example, 9.5-mm (3/8-in.) thick glass will transmit about 9 dB less sound than 3.2-mm (1/8-in.) glass. Doubling the mass per unit area can increase STL by 6 dB at some of the mid frequencies, but note the STC and OITC ratings both only increase by 3 dB. Therefore, although it may seem counter-intuitive, glass thickness is actually of limited value when it comes to improved acoustics. In fact, increased stiffness of glass limits the improvement.
Insulating glass (IG) configuration
The STC/OITC rating of double glazing increases as the width of the air space increases, typically by 3 dB for each doubling of the air space width. Note, however, there are tradeoffs at work here: thermal performance will suffer once the gap exceeds 15.9 mm (5/8 in.), due to the action of convection currents.
Suspended films will improve STL, especially at lower frequencies (~120 to 1200 Hz). Use of suspended films within IG units can improve sound attenuation, especially in the mid-frequency range of 300 to 3000 Hz, which is where most speech occurs. Argon gas infill in IG units also improves STL characteristics at higher frequencies, but air-filled units perform better at the lower frequencies associated with traffic noise. Therefore, OITC is actually better without argon. Again, the tradeoff with energy-saving goals must be balanced.