In any study of architectural acoustics, an assessment of acoustic performance is an important metric to capture in order to support any recommendation for additional soundproofing measures, or to check if existing partitions meet government standards. Several performance rating systems acceptable to architects and acoustic engineers are available, such as the STC, FSTC and ASTC indexes.
Both the FSTC and ASTC indexes are adaptations of the Sound Transmission Class (STC) rating which relies upon measurements made in the laboratory. Under these optimum conditions, the sound pathway from a local source to a local receiver is measured – but both are configured so the sound can only travel via one possible pathway: through the partition.
In Situ Measurement
With in situ assessments, measurements are taken in the location where the partition is already installed, for example in a house or an apartment. These are not the ideal conditions found in an STC laboratory, because there are many other elements present which will influence the final rating. For example, premises may be occupied (furniture will affect the transmission of noise), floor or ceiling structures may differ from those in the laboratory, or there may simply be ambient noise present. These are all uncontrolled factors.
The FSTC Approach: A Focus on Partitions
When using the Field Sound Transmission Class (FSTC) recommended approach, laboratory conditions are recreated by lining the adjacent walls with absorbent materials. Thus, all sound transmission is focused upon the partition which is under assessment. As you would imagine, this method is complicated and time consuming, which is why it is rarely used. And in addition, this measurement is not really representative of noise transmission in normal, everyday circumstances because the environment has been significantly altered from its original configuration.
The ASTC Approach: Direct Tracking from Source to Receiver
The Apparent Sound Transmission Class (ASTC) approach is the most commonly used in situ assessment. And what makes this method so strong is that it makes no attempt to recreate optimum laboratory conditions. All site measurements are captured exactly as they occur in context. The resulting data is thus truly representative of sound transmission between the two locations, and not just through the medium of a partition.
The Measurement Process
Once the problem areas have been identified, an omnidirectional noise source which can produce sound across the widest possible range of frequencies is installed in the source room. A microphone is then placed in the receiving room to measure the difference between the noise level in the source room and the level in the receiving room.
Multiple mathematical corrections based on additional measurements of background noise, reverberation time (“echo” or sound reflection) and room dimensions are also taken into account in the final calculation. These further measurements and calculations are in accordance with those defined in the ASTM technical standards.
Interpreting the Results Using STC or ASTC Rating Index
The outcome of this process is expressed as a number on the STC rating index. And naturally, if the ASTC method is used, the outcome is expressed according to the ASTC rating index. In each instance, the higher the number the better the soundproofing.
From a legal perspective, the National Building Code of Canada (NBCC) prescribes a minimum sound transmission rating of STC 50 for any dividing walls in multi-unit residential buildings.
In addition, the National Research Council of Canada (NRC) and the Canada Mortgage and Housing Corporation (CMHC) recommend higher index ratings to provide enhanced acoustic comfort for a majority of residents. Both organizations recommend a transmission rating of ASTC 55, or higher, for dividing walls in multi-family accommodation.
The sound insulation index STC and its in situ equivalent, the ASTC index, are therefore soundproofing measurements which are helpful and informative because they include mathematical correction factors which allow a good comparison between partitions, even though the geometrical parameters of all tested partitions may not be identical.
The simple, one-digit outcome makes comparison quick and easy to understand, but can sometimes oversimplify a soundproofing problem because it offers no information on how to improve the soundproofing of a partition.
- Conseil national de Recherche du Canada (2005). National Building Code of Canada – Volume 2, Section A.188.8.131.52.(1)
- Conseil national de Recherche du Canada (2001). Solution Constructive n°50 – Des critères en matière d’acoustique dans les bâtiments.
- Société canadienne d’Hypothèques et de Logement (2003). Le point en recherche – Qualification du degré de confort acoustique dans les édifices multi-logements – Phase II.
- ASTM E413-16, Classification for Rating Sound Insulation, ASTM International, West Conshohocken, PA, 2016, www.astm.org