New NBC Requirement For Apparent Sound Transmission Class

Apparent Sound Transmission Class in NBC

New release of the Canadian National Building Code (NBC) has become effective early in 2016. This release of the code contains a significant change, and an improvement, in requirement for minimum soundproofing between dwelling units.


The new code replaced Sound Transmission Class (STC) requirement with Apparent Sound Transmission Class (ASTC) for wall assemblies between dwelling units.

NBC Changes to Apparent Sound Transmission Class

The old requirement of STC 50 is replaced with new requirement of ASTC 47. Apparent Sound Transmission Class (ASTC) is defined in ASTM specification E336. This value can be verified by field measurements, since it includes noise transmitted through flanking paths that are always present in a filed situation (but not in a lab test of STC value). Even though the new number is lower than the old STC value, it is an improvement over the old requirements, since the ASTC value more accurately represents the situation to which occupants of spaces are exposed to. ASTC value can be accurately verified in the field, thus making builders more accountable for the actual performance of new buildings.

Consequences of changing to apparent sound insulation class

There are several significant consequences of this seemingly minor change. Some of these are:

  • Construction contracts in the future will include verification of the ASTC value as a part of new buildings acceptance process. This will ensure that customers will get what is required by their contract, and builders will be responsible for fixing deficiencies.
  • The new NBC code defines three methods for certifying compliance with this requirement. Only one of the methods requires field testing, but it is the most reliable method and will be likely included in many construction contracts. This means that builders will have to budget for the cost of field tests and for contingency funds in case these tests fail.
  • This requirement will create a business opportunity for acoustic test companies, soundproofing companies, soundproofing consultants, building acoustics consultants and acousticians.

Even thou this revision to the NBC is a significant improvement, it falls short on another important acoustic property of buildings. That is Impact Insulation Class (IIC) for floors/ceiling partitions (as defined in ASTM spec E989). The code does not yet specify this requirement. This change is being planned for next release of the code in 2020.

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About the Author Ivan Koval

The author is the publisher of the Soundproofing.Expert website. He is a soundproofing and building acoustics consultant working in Toronto and GTA, Ontario, Canada. Telephone (416) 471-2130

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