Canadian Design and Construction Report staff writer
Navigating the gap between laboratory acoustic ratings and real-world construction conditions was the central focus of a technical presentation at the Construction Specifications Canada (CSC) Ottawa chapter event on Oct. 15.
Andrew Schmidt, building science lead and acoustics subject-matter expert for USG/CGC, guided local architects, specifiers, engineers, and suppliers through the intricacies of room sound absorption, transmission metrics, and evolving building materials.
A significant portion of the presentation highlighted the natural variability in established acoustic testing. Schmidt referenced a 14-lab round-robin test conducted by the ASTM E33 acoustics committee. Testing the exact same wall assembly — featuring 5/8-inch Type X gypsum board on both sides, 24-inch-on-center lightweight studs, and 3.5-inch fiberglass insulation — yielded Sound Transmission Class (STC) results ranging from 47 to 51 across the different accredited laboratories.
“There’s quite a bit of laboratory variance in the ASTM E90 test method,” Schmidt said, explaining that repeated tests in a single lab can also vary by roughly one decibel at individual frequencies. Furthermore, field conditions typically reduce performance by another three to five points. This gap underscores the importance of incorporating safety margins during the design phase, particularly when striving to meet specific code requirements.
Under the Canadian Building Code, field-tested or calculated assemblies generally require an Apparent Sound Transmission Class (ASTC) of 47, while laboratory-tested assemblies require an STC of 50. Construction separating dwelling units from elevator or refuse shafts must meet an STC of 55.
Beyond sound transmission, Schmidt addressed room acoustics and reverberation time, which are heavily influenced by total surface absorption and room volume. He noted that standards like LEED v4.1 mandate a maximum reverberation time of 0.8 seconds for open-plan offices. Achieving these targets in spaces with higher ceilings often requires higher-NRC (Noise Reduction Coefficient) ceiling tiles or additional absorptive treatments to offset the increased volume.
He also cautioned attendees about the complex mathematics behind spaced sound absorbers, such as hanging baffles. A baffle’s Array NRC rating depends heavily on the element’s size and spacing. Closer spacing generally increases the Array NRC, while suspension height has a smaller, frequency-dependent effect. Schmidt advised that published baffle ratings may sometimes use inconsistent calculation methods, which can artificially inflate the numbers.
Modern construction methods, notably mass timber and Cross-Laminated Timber (CLT), present new acoustical hurdles. Low-frequency sound is particularly difficult to control, and CLT structures often lack the necessary mass and decoupling found in traditional concrete assemblies, complicating both airborne and impact sound control.
To address the uncertainties surrounding sound flanking in structural timber, a research project led by acoustical consulting firm HGC, in collaboration with USG and competitors, is planned to test structural paths and floor treatments.
To maintain acoustical integrity in any structure, Schmidt emphasized strict adherence to field best practices. Because sound will find the path of least resistance, he noted that unsealed gaps, duct crosstalk, open-back light fixtures, and back-to-back electrical penetrations easily undermine sound isolation.
He advised offsetting penetrations by at least one stud bay, using moldable putty pads on back boxes, and applying a single bead of non-hardening elastomeric acoustical sealant at the partition perimeter to prevent sound leakage.
For ceiling plenums where walls do not extend full height to the deck, Schmidt recommended ensuring the ceiling’s Ceiling Attenuation Class (CAC) is matched within five points of the wall’s STC rating. Additionally, he warned against short-circuiting resilient channels or sandwiching them between panel layers, which effectively neutralizes their sound-isolating benefits.
