Measuring acoustic performance
For anyone who has looked at a wall assembly or opened an acoustical ceiling catalog, they have likely come across the method for measuring and quantifying building materials. Noise Reduction Coefficient (NRC), Sound Transmission Class (STC), and Ceiling Attenuation Class (CAC) are ways to evaluate how materials will perform acoustically in each environment. NRC is a scalar representation of the amount of sound energy absorbed upon striking a surface, represented as a decimal or percentage of sound energy absorbed and rounded to the nearest 0.05. An NRC of zero indicates perfect reflection, and an NRC of one indicates perfect absorption. If half the sound energy is absorbed by a material, like in the earlier example, then it has an NRC of 0.5. This is average sound absorption over four, 250, 500, 1000, and 2000 Hz. This is a similar measurement to the sound absorption average (SAA), but the SAA is over a wider variety of Hz, starting at 200, and going to 2500 Hz. It is rounded to the nearest 0.01. Although SAA is a more exact measurement of sound absorption, the industry standard for most acoustical products remains NRC.
STC roughly measures the decibel drop from one side of an obstruction to the other. The word decibel comes from the root deci, meaning 10, and its inventor Mr. Alexander Graham Bell. A change in 10 dB indicates a doubling or halving of a soundwave’s amplitude. STC can be between interior partitions, ceilings/floors, doors/windows, or exterior to interior. If there is a 70 dB sound incident on one side of an interior partition, and 25 dB can be heard on the opposite side, the partition has a 45 STC. CAC is a measurement that has come into existence with more open office landscapes, partitions that go under grid, and not slab to slab. CAC is like STC in that it measures a decibel drop, but STC is a two-part reduction of sound energy. If there is a sound incident on one side of a partition, one must measure in CAC the sound energy that goes through the ceiling, bounces off the structure above, and comes down into an adjacent space. The drop in decibels will be the ceiling’s CAC. Like STC, the higher the CAC, the better the performance of a ceiling (i.e. a 25 CAC is considered low performing, while a 35 CAC is considered high performing). When using a high CAC ceiling, wall construction with a minimum STC of 40 should be specified. CAC and STC are more about blocking or confining sound energy, while NRC is about absorbing it.
For years, the most common way to deploy sound absorption in a space has been using acoustical ceiling tile (ACT). The ceiling is a large, cost-effective place to create an acoustically absorbent plane, plus house electrical and HVAC components. However, continuing demands to be more sustainable and renewable have led to more resilient and recyclable materials to address these areas. Perforated aluminum is becoming a desirable means to addressing the world’s need to be better stewards of the planet. By micro-perforating aluminum panels one can create a place for sound waves to enter, and then be trapped by any number of sound absorbing materials. These materials can be both recycled and recyclable. Cellulose fiber acoustical batt (CFAB) is made from recycled cotton and polyester that is treated to have a Class A fire rating, and a perfectly absorbing NRC 1.0. This material is a non-irritant and can be recycled at the end of its use. Facing this with a micro-perforated aluminum panel creates a high-performing recyclable wall or ceiling to last the life expectancy of the building.
A good case study is the Wayne County Detention Center in Goldsboro, North Carolina. The project is a holding and correctional facility with large open spaces—nothing but concrete and other hard reflective surfaces. This made verbal communication and speech intelligibility low, in a space where miscommunication could escalate into life or death situations. The challenge was to introduce acoustical absorption with extremely resilient surfaces without providing opportunity to escape or hide contraband. The solution: a heavy gauge micro-perforated security ceiling backed by acoustical fleece. The tamper-proof fasteners created added reassurance against infiltration, and the acoustical gains created better speech intelligibility and a safer environment for both workers and inmates.
Metals do not only provide resilience, beauty, and acoustical performance, but gains in paint technology make for an unlimited resilient pallet for the design and viewing community. Paint coatings with 70 percent PVDF resin formulation now mimic wood, stone, and other natural materials, offering all the natural beauty but with minimal upkeep. Further, they are suitable for indoor or outdoor use. Other finishes and coatings can be applied to metal to resist chemicals used for cleaning in critical environments that demand this process. For large expansive spaces, metal is the perfect option for wall panels and/or baffles. This gives ample opportunity to explore color, texture, geometry, gloss, and sheen, all in a single material that is abuse resistant, resilient, and does not go into the landfill once its architectural life is over.
Kevin Cox, CSI, CDT, is the director of interior business development for ATAS International. He has more than 25 years of experience in the commercial construction industry, including positions within operations, sales, and estimating for a large commercial interior contractor. He has held positions in architectural business development and ran his own commercial contracting company for half a decade. Cox has extensive knowledge of exterior and interior wall construction, as well as interior ceilings and room acoustics. He is a member of the Construction Specifications Institute (CSI), is a director for the CSI Raleigh-Durham, NC chapter, and is a certified Construction Documents Technologist (CDT).
