The acoustics of façades

by Aaron Bétit and Andrew Carballeira

Photo © BigStockPhoto.com
Photo © BigStockPhoto.com

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.

Figure 1: Calculated ratings of typical acoustical assemblies. Image courtesy Acentech
Figure 1: Calculated ratings of typical acoustical assemblies.
Image courtesy Acentech

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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