Photo courtesy Eskenazi Hospital
Noise reduction coefficient
Material absorption performance also matters when it comes to noise control, specifically the noise reduction coefficient (NRC) and its ability to absorb sound. NRC values are determined by testing per ASTM C423-17, Standard Test Method for Sound Absorption and Sound Absorption Coefficients by the Reverberation Room Method, and calculating the average of the sound absorption at the 250, 500, 1000, and 2000 Hz one-third octave bands. The lower the NRC, the less absorptive the material. For example, an NRC of 0.75 means roughly 75 percent of sound energy impacting the material is absorbed/dissipated and 25 percent is returned to the space. When it comes to whether a material’s thickness or density has a greater influence on its sound absorption, Figure 1 shows a material’s thickness generally has a greater impact on NRC value compared to a material’s density.
Single number ratings: for screening only
Specifiers should not assume a partition with a one-point higher STC rating is functionally any better than a partition with a lower score. Testing repeatability can be plus or minus two STC points. Like the NRC rating for sound absorption, STC should not be used for design or calculation purposes. It is intended only as a quick screening tool to compare different construction assemblies. The designer should use the actual laboratory sound transmission loss values at the frequencies of interest when determining the reduction of sound between two areas. By subtracting the sound transmission loss values from the dB levels of the noise in one room for each one-third octave band and accounting for flanking in the field, the designer can estimate what the resultant noise level should be in an adjacent room at each one-third octave band.
Flanking noise
Like water or air, sound will always flow across the path of least resistance. A small opening, as small as 0.7 mm (1/32 in.), between a floor and wall can allow sound energy to flow from one space to another and reduce the wall’s STC. This flow of sound is called flanking noise. It occurs when sound finds its way around walls and floors via junctions and other openings. To prevent flanking noise, sound should be addressed in the design phase as part of a holistic design approach. Post-construction measures like furnishings and fixtures can help reduce sound levels in a room, but do not address flanking. For commercial buildings requiring STC/IIC of at least 50/50 (meaning, typical speech and walking in an adjacent room cannot be heard and walking in the room above will not be heard in the space below) managing flanking noise is a major contributor to meeting this standard.
Strategies for mitigating flanking noise
Managing flanking noise in commercial enclosures can be accomplished through four primary strategies: absorb, block, break, and isolate sound. These strategies can be combined within a structure to optimize the building’s sound profile and involve a mix of sound-forward design thinking, as well as the use of right materials to achieve the necessary mitigation.
To better understand how new, lightweight drywall types, studs and insulation options work with different assemblies and materials, the author’s firm evaluated close to 300 walls, examining how these new variables and wall configurations affect STC performance. A separate study also examined 18 combinations of pipe insulation to study how improved material combinations influence insertion loss performance. Materials tested included a variety of jacketing, the combination of cellular glass insulation with mineral wool and layering of mass loaded vinyl materials. These studies provided critical data to understand how designs can be optimized to absorb, block, break, and isolate flanking noise and other sound issues.
Sound strategy #1: Absorb
Absorption removes the acoustic energy of a sound wave. It can be accomplished by incorporating various types of materials, both structurally and as part of furnishings and fixtures. Sound absorption of materials is measured using ASTM C423-17 and ASTM E795-16, Standard Practices for Mounting Test Specimens During Sound Absorption Tests, and is expressed as NRC (read Psychoacoustics- Facts and Models by H. Fastl and E. Zwicker (2007).
The NRC rating is not perfect, so it is important to understand NRC ratings are not actually percentages but rather a ratio of the absorption to the area of the material tested. The NRC rating depends on the thickness, surface area, and perimeter of the material along with material characteristics. Small differences in NRC rating have little impact on overall sound absorption in a space. For example, a material with an NRC of 0.75 is not significantly better than one with an NRC of 0.70 when used in a space. In smaller rooms, with a limited amount of surface area for the absorption material, both would perform similarly at most frequencies. At high frequencies, most of the sound is absorbed and at lower frequencies the resulting difference in sound absorbed is not likely to be noticed. With more commercial buildings transitioning to harder surfaces, including sound absorption in the early design is a more holistic approach to managing flanking noise in commercial buildings.
