New approach: Using ASTC to control airborne sound in buildings
The noise heard by occupants in a space is the net result of all possible transmission paths from the airborne noise source, including what is transmitted directly through the separating assembly and what comes through the flanking paths.
From the noise ‘source room,’ it can be reasonably assumed that:
- there is one direct path into the receiving room directly through the separating assembly; and
- there are three flanking paths into the receiving room for each of the four edges of the separating assembly, resulting in a total of 12 flanking paths.
By considering all these paths, the sound performance of the complete system can be reasonably predicted, and meaningful airborne sound requirements can be codified. This systems rating is best described and determined by the aforementioned ASTC rating, which is a single number rating defined and measured in the laboratory using ASTM E336.
As with the STC rating, higher numbers mean better sound insulation. It is important to remember the ASTC rating is usually lower than the STC or FSTC rating because it includes the contribution from the flanking transmission. In practice, and as a means for compliance under the building code, using ASTC rather than STC will reduce ‘over-design’ and make it more efficient. It should also provide more cost-effective construction, help identify design errors before construction, and likely result in fewer complaints from building occupants.
New airborne sound control requirements in NBC 2015
The requirements for sound transmission in Section 5.9 and Section 9.11 of the 2015 edition of the NBC are markedly different from those in the 2010 edition. They are based on a systems approach to sound control using the ASTC rating.
Two compliance paths exist: one which is fundamentally a performance-based path of “measure or calculate” requiring an ASTC rating of not less than 47, and another that is largely prescriptive-based where separating assemblies must have an STC rating of not less than 50 and be combined with adjoining constructions that conform to certain new requirements placed within Part 9 Tables A.220.127.116.11A and A.18.104.22.168B intended to address flanking transmission.
A review of European sound studies and code requirements offers compelling reasons to choose an ASTC rating of 50 or greater to achieve suitable sound control. However, an ASTC rating of 47 was agreed on for this edition of the NBC because it was resolved to be somewhat ‘status quo’ with the existing requirement of an STC 50 rating. This ASTC rating will likely be increased in the successive editions of the code.
Whereas the prescriptive-path (with revised Tables A.22.214.171.124A and A.126.96.36.199B) may be enticing to use under both Parts 5 and 9 because of its apparent simplicity, there is an inherent complexity about this path, some uncertainty, and limitations in use. The tables are seen to be a little difficult to follow to ensure all the stated conditions of the solution are met for the separating and flanking assemblies. The tables only contain those separating and flanking assemblies that together achieve an ASTC rating of 47, and the ASTC rating provided by the assemblies is not specifically stated. Consequently, the tables cannot be used when designing for an ASTC rating higher than 47, and the number of assemblies identified in the tables is quite limited.
The code also cautions users that adding materials to the prescribed solution may actually reduce the ASTC rating rather than improve it. In some cases, not all the desired direct test data were available, and ASTC calculations for some assemblies were based on a conservative ‘best estimate.’ All issues considered, one might reasonably conclude the NBC encourages use of the performance path for design. Notwithstanding, masonry wall systems are well represented in these tables because of the depth of reliable information available about these assemblies at the time of creating the tables for the code’s inclusion.
The performance-based ‘measure or calculate’ path in both Parts 5 and 9 provides the most reliable and flexible design path, and speaks to all the encumbrances of the prescriptive path.
Two options are permitted:
- The ASTC rating for the separating assemblies and adjoining constructions can be determined in accordance with ASTM E413, Classification for Rating Sound Insulation, using results from direct laboratory testing and measurements in accordance with ASTM E336.
- The ASTC rating can be calculated using the requirements of International Organization for Standardization (ISO) 15712-1, Building Acoustics: Estimation of Acoustic Performance of Buildings from the Performance of Elements–Part 1: Airborne Sound Insulation Between Rooms. Where ISO 15712-1 is specified, the NBC also identifies and states a number of qualifications and clarifications needed to adapt ISO 15712-1 to the North American building acoustics environment. ISO 15712-1 has been used for many years to support performance-based sound requirements in European codes.