The acoustics of façades

by sadia_badhon | October 10, 2019 12:51 pm

by Aaron Bétit and Andrew Carballeira

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The envelope of a building defines its visual identity, addresses the thermal requirements of the environment, and provides a fire barrier to protect occupants. Another important function of the building envelope is noise mitigation. As the urban area continues to blur zoning uses to provide a more immersive experience, exterior ambient noise levels are rising in cities. Nightlife and commercial activities directly adjacent to residential developments provide exciting communities to live in, but often cause increased noise levels during conflicting uses.

The interaction of the façade and structure can present acoustical challenges on the inside of the building. Mixed-use buildings offer the benefit of social activities adjacent to habitable units, but spaces such as restaurants and bars will often operate late into the night, and require the ability to generate higher noise levels. In addition to floor/ceiling assembly concerns, providing an appropriate level of acoustical separation to allow occupants to sleep above a thriving late-night restaurant requires additional consideration at the façade/structural slab connection separating the two spaces.

The 2018 International Building Code (IBC) has provisions to limit interior noise levels for ‘habitable rooms’ (Section 1207.4). California has also implemented a maximum hourly noise level in order to help address the acoustical environment in dense commercial spaces. The established limits should be considered a bare minimum requirement. For high-quality commercial and residential spaces, the limits established by these building codes may not be sufficiently strict, and thus façade designs with higher acoustical performance are often required to provide an appropriate interior environment.

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A large variety of metrics have been developed to define the acoustical environment. To discuss the acoustical requirements of a façade, it is necessary to first define the metrics.

Community noise equivalent level (CNEL) is the sound level during a 24-hour period. It is calculated by adding the sound energy during the daytime (7 a.m. to 7 p.m.) to three times the energy during the evening (7 to 10 p.m.) to 10 times the sound energy during nighttime (10 p.m. to 7 a.m.).

The day-night average sound level (DNL) is defined as the equivalent sound level during a 24-hour day. It is calculated by adding the sound energy during daytime and evening (between 7 a.m. and 10 p.m.) to 10 times the sound energy during nighttime (10 p.m. to 7 a.m.).

The measured CNEL and DNL are typically similar, and both are appropriate to define the ambient environment.

In California, if the exterior ambient environment exceeds a CNEL or DNL of 65 dB, a noise study must be performed to estimate whether the interior noise levels meet the California Building Code (CBC) requirement. IBC uses CNEL and DNL metrics to define limits within habitable rooms, and requires all buildings to have a maximum interior CNEL or DNL of 45. The limits established by both CBC and the United States Department of Housing and Urban Development (HUD) are similar to IBC. In some cities, additional documentation about post-construction measurements can be required for spaces that are near major airports prior to receiving a certificate of occupancy from the building department.

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The acoustical separation of façades can be described by using either sound transmission class (STC) or outside-inside transmission class (OITC). The measurement process for STC is defined in ASTM E90, Standard Method for Laboratory Measurement of Airborne Sound Transmission. This procedure uses two diffuse fields created by reverberation chambers to measure the amount of sound energy blocked by a tested assembly. The resultant transmission loss is plotted and ASTM E413, Classification for Rating Sound Insulation, is used to generate the STC value. It is important to understand STC was originally developed to evaluate speech attenuation. Consequently, it may not show the whole picture for exterior noise sources that are generated at frequencies lower than speech (i.e. below 125 Hz). Busses, heavy rail, and rooftop restaurants will need a full frequency evaluation to determine the appropriate façade construction.

OITC was developed in 1990 and is defined by ASTM E1332, Standard Classification for Rating Outdoor-indoor Sound Attenuation. This measurement was intended to evaluate attenuation due to typical exterior noises such as busses, construction, and sirens, rather than speech frequencies. The OITC rating emphasizes low-frequency noise in the evaluation more than the STC rating since the ambient environment has more of these sound sources compared to speech. Figure 1 (page 77) presents typical STC and OITC rating of severa`l assemblies.

The single greatest factor influencing sound isolation is mass. Entries A and B in Figure 1 highlight an underlying rule of sound isolation called the Mass Law, wherein a doubling of mass (or a doubling of frequency) produces a 6 dB increase in sound isolation.

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A dual-mass system can often help improve acoustical separation, but the details are important and not always intuitive. In configurations such as entries A and B in Figure 1, two moving masses (gypsum wall board [GWB]) are arranged with a significant airspace separating them. The air trapped in the cavity by the drywall (i.e. the mass-air-mass system) resonates at about 50 to 70 Hz for an insulated stud wall, which is below the frequency included in the STC or OITC evaluation. Due to this, the decreased transmission loss associated with the mass-air-mass resonance does not reduce the single-number rating. Generally, airspaces less than 76 mm (3 in.) should be avoided in framed assemblies. An increase in airspace (e.g. up to 203 mm [8 in.]) can help with the reduction of low-frequency sounds as well as the overall transmission loss performance.

Unlike framed partitions, typical glazing systems have significantly smaller airspaces (e.g. 13 mm [½ in.]), and this results in resonances in the range of 150 to 190 Hz, which is more noticeable to people, relevant to speech privacy, and included in the STC and OITC evaluation. As an example, 13-mm monolithic glass has a rating of STC 34 (entry D in Figure 1), while a 25-mm insulated glass unit (IGU) consisting of 6 mm (¼ in.) glass, 13 mm airspace, and 6 mm glass (entry G in Figure 1) will result in STC 33 for the same amount of glass. It is also important to note the effect of a large airspace between glazing lites by comparing entries G and H in Figure 1.

Laminated glass can improve acoustical separation by allowing for an impedance change in the material. When sounds or vibrations experience changes in density, part of the noise is reflected back in the direction it originated. Thus, by including a thin layer of polyvinyl butyral (PVB) or ethylene-vinyl acetate between two thin layers of glass, the change in density between the glass and the lamination layer causes the reflection of some of the sound energy. A piece of 6-mm plate glass (entry C in Figure 1) provides acoustical separation of STC 31, while a piece of 6-mm laminated glass (entry E in Figure 1) offers acoustical separation of STC 35.

The interaction of the façade with the inside of the building and the provision of an appropriate level of acoustical separation requires specific attention to detail. Partitions requiring acoustical separation higher than STC 35 should be lined up with a mullion, as it is difficult—nearly impossible—to provide an appropriate seal at the glazing itself. In the past, the only true method of giving the right level of acoustical separation was to provide blocks on the mullion to match the thickness of the stud, and to extend the drywall past the stud row into the mullion (Figure 2).

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This method would result in a thicker-looking mullion from the outside of the building as the drywall extension would often be visible. Specifically made and acoustically tested systems provide caps to close the gap between the end of the drywall partition and the façade. These systems provide a cleaner-looking, less-visible connection from the outside of the building, as well as a slip connection to allow the façade to expand as it heats up. These caps also provide tested acoustical separation from STC 35 to 60. In the author’s experience, the acoustical separation provided by the partition will be significantly limited if this connection is unaddressed. If the drywall partition stops at the mullion and the gap is filled with backer rod or a neoprene gasket, a partition with a single layer of drywall on each side (including the batt insulation) should be considered, as the mullion/partition connection will limit the acoustical separation from anything higher than this partition.

The gap between the structural floor and the façade is another weak link for the acoustics within the building. Acoustical separation at the perimeter can be compromised due to the gap between these two locations. Drywall window return enclosing the gap can help address the attenuation required between these two vertical adjacencies. However, mixed-use adjacencies may need more significant construction to provide adequate acoustical separation.

In the past, zoning used to address restaurant and bar activities by providing distance between commercial and residential uses. The mixed-use concept, allowing occupants to socialize locally rather than commute to an activity, is becoming popular. While this is beneficial to the community, it requires improved acoustical separation. As mentioned, a restaurant or bar located within a mixed-use building may operate well into the nighttime, and even if live music is restricted, will cause noise levels that would be disturbing to the residences above. In leases, landlords are adding a requirement of ‘inaudibility’ and/or acoustical separation as high as STC 75 between these adjacencies. While the structural building with poured concrete slabs may already be providing this separation, without ‘box-within-a-box’ construction at the perimeter, the gap between the structure and the façade limits the acoustical separation. To achieve higher acoustical isolation, a separate façade, attached resiliently from the building structure, and closing off the acoustical ceiling or floor is required (Figure 3).

Conclusion

As a final thought, all components of the building allow the façade to properly expand and contract. Without providing slip connections and avoiding rigid connections between the façade and interior components such as the acoustical ceiling grid, pops and creaks occur as the two components heat up differently and attempt to expand.

While the visual, thermal, and fire/life safety aspects of the façade are the most discussed factors, the acoustical considerations discussed above are also an important part of the overall success of a project. Such concerns require careful detailing and construction to provide the appropriate acoustical environment to building occupants. An integrated team with the ability to address all of the relevant design considerations is crucial to maximize the mixed-use trend, while minimizing some of the concerns associated with living close to other people and different uses.

Aaron Bétit is principal consultant in architectural acoustics and mechanical systems at Acentech’s Los Angeles (California) office. His experience includes project management and acoustical recommendations including ground-up design and diagnostics for the full built environment spectrum, such as theaters, studios, education facilities, and performance spaces, as well as healthcare, commercial, public, and residential construction. Bétit can be reached at abetit@acentech.com[6].

Andrew Carballeira is senior consultant in Acentech’s Cambridge (Massachusetts) office and an Institute of Noise Control Engineering (INCE) board-certified noise control engineer. Carballeira works on various projects involving residential, commercial, and educational facilities with a concentration on architectural acoustics, noise and vibration control, and environmental noise compliance. Carballeira can be reached at acarballeira@acentech.com[7].

Source URL: https://www.constructionspecifier.com/the-acoustics-of-facades/

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