ASTC and sound flanking paths
For many years, NBC set minimum levels for the control of noise transmitted between dwellings by defining sound insulation in terms of the STC rating. This rating measures the ability of a separating wall or floor assembly to hinder noise passing between two rooms or residential units. The higher the STC rating, the more the airborne sound transmission is restricted. Likewise, lower STC ratings mean more sound is transmitted through the wall assembly, increasing the ambient noise level of the space and resulting in a noisier interior environment with less privacy.
However, over time, code authorities discovered the STC rating was not as accurate a description of sound insulation as it needed to be—the metric overlooked noise that passes through sound flanking paths such as those from the common wall to the other walls, the ceilings, and the floor within dwellings. The more comprehensive ASTC rating was developed to address this issue. It describes the sound insulation of the complete building system and includes not only direct noise transmission through the common wall or floor, but also through the flanking paths. It is anticipated the ASTC rating will eventually overtake STC as the code’s acoustical standard.
Currently, STC and ASTC ratings are derived from sound attenuation values evaluated in the 1/3 octave bands between 125 and 4000 hertz (Hz). Acoustical engineers fit these values to the appropriate transmission loss (TL) curve to determine a final STC rating. The 2015 NBC still requires low-rise and high-rise residential buildings to achieve a minimum STC rating of 50 in wall assemblies separating two residential units, and a rating of 55 for elevator shaftwalls that border a living space. It also now demands a minimum ASTC rating of ≥47 for wall assemblies.
Fortunately, there are simplified strategies for meeting and even exceeding these numbers.
Smarter sound control strategies
Many sound control methods used in wall assemblies over the years have become known for being either too costly or difficult to install. Even minor installation errors can compromise the acoustic abilities of the entire wall assembly.
Resilient channels provide a good example. These sound-dampening metal strips are screwed in perpendicular to studs to acoustically isolate the wallboard, after which the wallboard must be very carefully screwed into the resilient channels. Any simple mistake, even something as small as positioning the resilient channels too close together or upside down, or using screws that are too long, can result in the creation of a new sound transmission ‘short circuit’ path, and thus degrade the assembly’s acoustical performance.
Another common sound control technique involves building double-stud walls or installing multiple layers of gypsum board. However, this technique is often expensive. Additionally, multiple layers of wall assembly configurations often increase the footprint of the wall, which results in a less efficient use of limited, high-priced floor space.
In response to these issues, building product manufacturers have been able to introduce smarter, more practical options for sound control in wall assemblies. One example is laminated-core, noise-reducing gypsum board.
Laminated-core, noise-reducing gypsum board
Developed to be a smarter alternative to more common sound control methods on interior walls and ceilings, this specialized gypsum board is a single-panel product that contains a viscoelastic polymer core applied between two specifically formulated thin layers of gypsum board. The final product ends up being 12.7 to 15.9 mm (1/2 to 5/8 in.) thick—the same as traditional gypsum board. This material is ideal for new construction or renovations over both wood and steel framing.
One key advantage of laminated noise-reducing gypsum boards is their ability to dampen sound transmission. The polymer core acts as a shock-absorber that dampens board vibrations and dissipates the sound energy into thermal energy. The boards perform well acoustically over an extended range of frequencies, resulting in higher STC and ASTC ratings for wall assemblies.
This type of gypsum board is an excellent acoustic solution to specify to meet NBC’s ASTC requirements, without the need for materials such as resilient channels and isolation clips. It can, however, be used to achieve extra sound transmission control in wall assemblies where resilient channels or isolation clips are desired. In such cases, the material helps reduce the negative impact of any short circuits.
Replacing traditional multi-layered gypsum systems with a single laminated noise-reducing gypsum board on one side allows contractors to build effective noise-reducing wall assemblies with a National Research Council (NRC) rating of ASTC 48. Use of this material cuts construction time and material and labour costs, and helps the project gain valuable floor space. The minimal amount of material required also contributes to the structure’s sustainability and compliance with green building practices.
In tests, assemblies utilizing a single layer of laminated-core, noise-reducing gypsum board on both sides have achieved a preliminary NRC rating of ASTC 51. This means they exceed the acoustic performance of double layers of traditional gypsum board. This gypsum board application is therefore quite effective where acoustic performance is essential, such as in condominiums, hospitals, retirement homes, and hotels.