January 11, 2021
by Steven Saffell
The sounds of transportation, construction, and industry—of civilization itself—are stimulating to some as part of the vitality of the urban environment. Yet, over the long haul, these sounds become intrusive when they filter into indoor environments. Numerous studies document there is much more at stake than mere annoyance, as the effects of background noise suppress productivity, learning, and healing within offices, schools, and hospitals, respectively. Additional facilities commonly affected by noise problems can include places of worship, hotels, multi-family dwellings, libraries, and courthouses.
Generally, sound transmission increases as the glass temperature decreases, indicating sound intrusion may be perceived as worse during the winter, precisely when people are spending more time indoors (possibly exacerbated by COVID-19 shutdowns).
Efforts to decrease the amount of noise reaching occupied spaces commonly focuses on fenestration—at times an entry point if not properly designed for the noise control mission. This is especially true of relatively lightweight construction such as metal curtain wall since sound transmission through any barrier is inversely proportional to the mass of the barrier. However, with careful detailing, based on an understanding of the mechanism of sound transmission, metal curtain walls can reduce intrusive noise.
Sound is characterized by its frequency (“pitch”) and amplitude (loudness). The amplitude of a sound pulse is the difference between the maximum and minimum sonic pressure that is developed between the crest and trough of the sinusoidal sound wave. This difference is represented as the sound pressure level (SPL) and is perceived as the “loudness” of the sound. The greater the amplitude of a sound wave, the higher the SPL and the louder the sound.
SPL of a sound is measured in decibels (dB). A comparison between SPL and relative loudness is shown in Figure 1.
It is important to note the relationship between SPL and perceived loudness is a logarithmic scale; thus the difference between the loudness of a private office is twice that of a bedroom (16 vs. 8 relative loudness), not 1/3 greater (40 vs. 30 SPL). Similarly, the challenge in reducing loud street noise to that of a bedroom translates to reducing the noise level by a factor of 64 (512 vs. 8 relative loudness), not just 2.33 (70 vs. 30 SPL).
When it comes to designing walls to dampen the transmission of unwanted sound, a variety of performance indices are available.
Sound transmission loss (STL), for example, measures the ability of a particular material or wall configuration to reduce impinging sound by indicating the amount in dB by which sound of a particular frequency band is reduced (attenuated) when passing through the material or wall. For example: 12.7 mm (1/2-in.) drywall has an STL of 15 dB for an impinging low-frequency sound at a frequency of 125 hertz. The method for defining STL is given by ASTM E90-09(2016), Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements.
However, the ability of a material or fenestration product to attenuate sound is better indicated by the two common methods of product classification: Sound transmission class (STC) and outdoor-indoor transmission class (OITC). For both, the higher the number, the more the intruding sound is blocked.
Like decibels, STC scores are on a logarithmic scale so each number is higher than the one before. For example, an increase in STC from 28 to 38 equates to a 90 percent reduction in noise. STC is determined per the methods detailed in ASTM E413, Classification for Rating Sound Insulation. Although STC is generally satisfactory for ranking the ability of building envelope components to reduce higher frequency noise, it does not properly rate insulation against noise sources from the typical outdoor urban environment, which generally has a significant low-frequency component.
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.
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.
A reliable methodology must be employed to obtain uniform and comparable measurements that enable evaluation of different products in a fair and consistent manner. To accomplish this, the Fenestration and Glazing Industry Alliance (FGIA) recently updated its document, American Architectural Manufacturers Association (AAMA) 1801, Voluntary Specification for the Acoustical Rating of Windows, Doors, and Glazed Wall Sections, which describes STL measurement procedures.
AAMA 1801 prescribes the use of STL test data obtained per ASTM International E1425-14, Standard Practice for Determining the Acoustical Performance of Exterior Windows and Doors. This information is then used to calculate STC and OITC ratings.
Products are evaluated in accordance with a sound spectrum that has been shown to correlate well with the spectra from three typical transportation sources (aircraft, highway traffic, and railroads). This rating system uses OITC classification numbers.
As specified in ASTM E1425, the test specimen is installed into a test opening of a filler wall that has a significantly higher sound transmission loss (at least 10 dB at every test frequency) than the specimen that was being tested.
The gap between the test opening and specimen, which allows for some adjustment if the product is slightly out of square, may not exceed 10 mm (393 mils). However, the perimeter of the test specimen may not be in contact with the test opening. A closed-cell foam isolator pad is placed on the bottom of the test opening to support the test specimen and isolate it from the test opening, and up to a 9.5-mm (3/8-in.) gap is required at the top and sides. The specimen is sealed in place with a dense mastic-type sealant, such as a duct seal compound, at the exterior and interior of the top, bottom, and sides, but not covering more than 6 to 12 mm (1/4 to 1/2 in.) of the test specimen frame so that additional damping is not applied. Weep holes or slots are not covered and a minimal number of shims should be used to adjust the test specimen in the opening.
The average temperature in both rooms of the test area during all acoustical measurements is maintained within the range of 22, ±5 C (71.6, ±9 F). For laminated glass or other temperature-sensitive materials, the average temperature in both rooms and the average surface temperature of the specimen should both be kept within the range of 22, ±2 C (71.6, ±3.6 F). These temperature conditions will minimize thermal effects on the measured STL and ensure better reproducibility among laboratories.
Finally, products are classified and rated based on their acoustical performance in accordance with ASTM E1332 (OITC). Additionally, products are rated based on their acoustical performance in accordance with ASTM E413 (STC).
A guide specification to ensure the appropriate acoustical testing is provided in AAMA Curtain Wall Manual (CWM), an FGIA document:
184.108.40.206 Sound Transmission Perform acoustical tests in accordance with ASTM E90 and ASTM E1425 on the glass type(s) specified in Section 08 80 00 Glazing, rigidly supported in aluminum framing of the same product family. “Glass-only” test results shall not be acceptable.
220.127.116.11.1 Products shall be classified based on their acoustical performance in accordance with ASTM E413 (STC) and ASTM E1332 (OITC). Products shall be rated using OITC classification data.
18.104.22.168.1.1 Outdoor-Indoor Transmission Class (OITC) shall not be less than .
Preventing exterior noise from degrading an otherwise quality interior environment requires detailed project specifications, a thorough understanding of the acoustic performance of different combinations of glazing and framing, and reliable test results or certification. Acoustically rated fenestration products are available in many different styles, framing materials, and operating systems, and proper application of AAMA and ASTM standards will help in developing successful design solutions.
Referenced documents including AAMA 1801, AAMA CWM, and AAMA TIR-A1 can be purchased in FGIA’s online store.
Steven Saffell oversees the standards, product certification, and codes and regulatory affairs aspects of the Fenestration and Glazing Industry Alliance (FGIA). His background is a tapestry of architectural firm work, modular design, as well as residential and commercial fenestration experience. He also spent three years teaching as an adjunct professor. He is experienced in managing technical teams, including employee development, operational strategy, and financial management. He can be reached at ssaffell@FGIAonline.org.
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