31 Jan Banda Propagates Sound Knowledge About Building Envelope Acoustics
Last November, BKL was again invited to present at the BC Building Envelope Council’s Annual Conference. To lead the 45-minute session on building envelope acoustics, BKL sent Banda Logawa, an Acoustical Consultant and passionate educator. Banda is no stranger to delivering long lectures on engineering topics. Outside of his busy routine at BKL, Banda is a faculty member at the School of Science, Technology, Engineering, and Mathematics (STEM) at Capilano University.
Banda’s presentation emphasized the importance of designing proper building envelopes for residential buildings. Here is a quick summary:
Noise Criteria for Building Envelopes
During his presentation, Banda answered a lot of questions about noise criteria for building envelopes. Banda explained that, unlike sound transmission class (STC) criteria for interior wall partitions, criteria for building envelopes are typically aimed at regulating the effects of exterior noise sources on interior noise levels. While each municipality usually has its own interior noise level criteria, most are derived from those set by the Canada Mortgage and Housing Corporation (CMHC). According to CMHC, interior noise levels for residential buildings shouldn’t exceed the A-weighted 24-hour equivalent (Leq) sound level of 35 dBA, 40 dBA, and 45 dBA for bedrooms; living, dining rooms, and recreational rooms; and kitchen, bathrooms, and hallways respectively.
Characteristics of Noise Sources
Banda also showed the source spectrum for many of the noise sources we encounter in BC. Some examples include SkyTrain, buses, heavy trucks, cars, etc. He emphasized the importance of conducting on-site noise measurements. Furthermore, Banda demonstrated that, due to the different frequency spectra of common noise sources, different mitigation solutions may be required for two development locations with the same broadband noise levels.
Sound Propagation Paths
Before a sound wave reaches a building envelope, various factors attenuate or amplify the wave along the noise propagation path. These factors include divergence, ground absorption, and acoustical shielding. For long-distance noise propagation, additional details should be considered: atmospheric absorption, refractive effects from wind, and thermal gradient effects. Banda explained how 3-D acoustical modelling accounts for these factors. BKL uses software, such as Cadna/A, to model noise propagation for various projects ranging from developments of small residential houses to large hospitals and highway improvement projects.
Sound Waves and Building Envelopes
When an exterior noise hits a building envelope, multiple phenomena occur. Some of the noise reflects back outside, some converts to heat and is absorbed by the building envelope itself, and the rest transmits to the building’s interior.
STC vs OITC
Banda also tackled a misconception about the use of two single number ratings for building envelope elements: sound transmission class (STC) and outdoor-indoor transmission class (OITC). He explained that the two ratings are not interchangeable and said that OITC is a more reliable rating for exterior noise ingress since it also accounts for the low-frequency noise commonly emitted by cars, trucks, aircrafts, and more.
Increasing the Performance of Windows and Walls
During the presentation, Banda also discussed several mitigation options that can improve the performance of building envelope elements. These options include
- increasing the mass of an exterior partition;
- breaking the vibration transmission paths whenever possible;
- increasing cavity size in walls and windows;
- using sound-absorptive materials inside multiple-leaf walls;
- improving air tightness and sealing any leakage points;
- increasing damping on windows; and
- using smaller windows whenever possible.
Misconceptions About Building Envelope Acoustics
In addition to the STC–OITC misconception, Banda also brought up a few other common misconceptions. He addressed how thermal insulation performance is often thought to be identical to sound isolation performance. While avoiding thermal bridging in a building envelope will often reduce vibration (and also noise) transfer between the envelope’s outer and inner layers, one of the most common thermal insulation products, closed-cell foam, doesn’t provide any acoustical benefits. To avoid this mistake, he stressed the importance of conducting acoustical analysis independent of thermal performance analysis.