Ontario Construction News staff writer
The main topic of discussion at the Construction Specifications Canada (CSC) Ottawa chapter event on Oct. 15 was the differences between lab acoustic ratings and actual construction environments.
Andrew Schmidt, who specializes in building science and acoustics for USG/CGC, walked local architects, engineers, suppliers, and specifiers through the details of sound absorption in rooms, transmission metrics, and new building materials.
A large part of his talk focused on the natural inconsistencies found in established acoustic testing. Schmidt pointed to a round-robin test involving 14 labs conducted by the ASTM E33 acoustics committee. The same wall assembly-made up of 5/8-inch Type X gypsum board on either side, lightweight studs spaced 24 inches apart, and 3.5-inch fiberglass insulation-showed Sound Transmission Class (STC) results ranging from 47 to 51 across various accredited labs.
Andrew Schmidt
“There’s quite a bit of laboratory variance in the ASTM E90 test method,” Schmidt explained, noting that tests done repeatedly in one lab can differ by about one decibel at certain frequencies. In addition, performance is usually reduced by three to five points under field conditions. This gap highlights why it’s crucial to include safety margins during design stages when aiming to comply with specific codes.
The Canadian Building Code generally requires field-tested or calculated assemblies to achieve an Apparent Sound Transmission Class (ASTC) of 47 while lab-tested assemblies need an STC of 50. Moreover, constructions separating living units from elevator or waste shafts must reach an STC of 55.
Besides sound transmission issues, Schmidt also discussed room acoustics and reverberation time influenced by total surface absorption and room size. He mentioned that standards like LEED v4.1 set a maximum reverberation time of 0.8 seconds for open-plan offices. Meeting these goals in spaces with taller ceilings often calls for higher-NRC (Noise Reduction Coefficient) ceiling tiles or extra absorptive treatments to balance out the increased volume.
He warned attendees about the tricky calculations involved with spaced sound absorbers like hanging baffles. The Array NRC rating for a baffle relies heavily on its size and spacing; closer spacing typically boosts the Array NRC while height has a smaller frequency-dependent impact. Schmidt cautioned that baffle ratings might sometimes use different calculation methods which could skew results upwards.
The rise of modern construction techniques such as mass timber and Cross-Laminated Timber (CLT) introduces new acoustic challenges. Low-frequency sounds are particularly tough to manage since CLT structures often lack the mass and decoupling found in traditional concrete setups, making airborne and impact sound control more complex.
A research initiative led by HGC-a consulting firm specializing in acoustics-in partnership with USG and other competitors aims to explore structural paths and floor treatments related to sound flanking in structural timber.
To ensure good acoustical quality in any building project, Schmidt stressed following best practices closely on-site. Since sound will always find easier routes around obstacles, he noted that unsealed gaps, duct crosstalk, open-back light fixtures, and back-to-back electrical penetrations can easily compromise sound isolation.
He suggested shifting penetrations by at least one stud bay apart, using moldable putty pads on back boxes for sealing purposes, and applying a continuous bead of non-hardening elastomeric acoustical sealant along partition edges to stop sound leaks.
If there are ceiling plenums where walls don’t extend all the way up to the deck above them, Schmidt advised ensuring that the Ceiling Attenuation Class (CAC) matches within five points of the wall’s STC rating. Additionally, he warned against short-circuiting resilient channels or placing them between panel layers since this would nullify their intended sound-isolating effects.
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