However, if the partition is not full-height and privacy is expected between normally occupied rooms, then a plenum barrier should be combined with the acoustic ceiling to comply with the sound isolation requirement in the standards and with general user expectations. This is the only instance where a CAC rating should be specified. It should be the combined ceiling/plenum barrier rating and should equal the STC rating of the partition construction below the ceiling.
Third role: Isolation from floors above
Sound isolation is also important between vertically adjacent rooms. Students being energetic in their classroom on an upper floor of a school building should not disturb peers concentrating in the library below. In these cases, the floor construction is the primary building element controlling the amount of noise transmitting between rooms. In buildings with ceilings, it is the combination of the floor and ceiling assembly that establishes the overall noise isolation performance between rooms.
Guidelines by the Facilities Guidelines Institute (FGI) for the design and construction of healthcare facilities require floor-ceiling assemblies between inpatient rooms in hospitals achieve a minimum STC 50 rating. Another standard with the STC 50 requirement for floor-ceiling assemblies is the American National Standards Institute/Acoustical Society of America (ANSI/ASA) S12.60, Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools – Part 1 Permanent Schools (2020).
Ideally, there would be accessible, STC reports for various floor-ceiling assemblies made of concrete slabs on metal decks with acoustic ceilings suspended below. Until 2021, those tests were difficult to find. Instead, when floor-to-floor sound isolation is important on a project, a common practice amongst some architects, specifiers, and acousticians is to specify an acoustic ceiling panel that has a minimum weight of 4.88 kg/m2 (1 psf) or a CAC rating of 35. The thought is the extra weight, compared to, for example, a panel that only weighs 2.44 kg/m2 (0.50 psf), will result in a higher assembly STC rating, or a ceiling panel with a CAC rating of 35 will result in a higher STC rating than a panel with a CAC rating of 25.
In 2021, a conclusive study was conducted by the author to determine if the weight or CAC rating of the ceiling panel influenced the assembly STC rating. The findings were published in an article titled “Effects of Acoustical Ceilings on Vertical Sound Isolation.”7 The experiment compared the frequency-specific TL and wideband STC rating for a baseline concrete floor slab on metal structural deck with vinyl composite tile finish floor, versus the same baseline floor with a variety of acoustic ceiling systems suspended below.
Three different types of acoustic ceiling panels were tested. All ceiling panels were the same size and thickness with similar painted white finishes and square, lay-in edges. The main differences between the ceiling panel types were the core material types, panel weights, and acoustic performances.
The CAC ratings of the ceiling panels ranged from 20 to 35, representing the most common performance range used in the industry. The NRC ratings of the ceiling panels ranged from 0.75 to 0.95. The experiment was first conducted without recessed light fixtures and air distribution devices in the ceiling systems and then again with them. The presence or absence of the penetrations for lights and air distribution had an insignificant effect on the STC ratings of the different floor-ceiling assemblies.
Figure 6 shows the baseline concrete floor without a ceiling achieved STC 47, three points below the STC 50 rating that some standards set as minimum. When added below the floor slab, each of the ceilings increased the assembly STC rating to above the STC 50 minimum. In addition, all the ceilings increased the STC rating a similar amount. With two of the ceilings (stone wool and mineral fibre), the assembly had the same STC rating of 54 and the frequency-specific TL performance overlapped. The third ceiling panel (fibreglass) resulted in a slightly lower STC rating of 52, but still some of the frequency-specific TL values overlapped with the other ceiling types. Given the variation inherent in the test method (1.5 to 4 dB), one cannot conclude fibreglass panels performed differently than the stone wool or mineral fibre panels.
