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.
Role 3: Isolation
Sound isolation is also important between vertically adjacent rooms. Energetic students in a 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 in-patient rooms in hospitals to achieve a minimum STC 50 rating. Other standards with STC 50 ratings for floor-ceiling assemblies include ANSI/ASA S12.60, Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools – Part 1 Permanent Schools (2020), and the GSA/PBS P100 for federal office buildings.
Ideally, there would be publicly available 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 among 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 that the extra weight, compared, for example, to a panel weighing 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, there was a conclusive study to determine if the weight or CAC rating of the ceiling panel influenced the assembly STC rating. The findings were published in “Effects of Acoustical Ceiling Panel Type 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.
| Core panel material | Panel weight | |
| Ceiling panel type 1 | Wet-felted mineral fiber | 5.37 kg/m2 (1.10 psf) |
| Ceiling panel type 2 | Stone wool | 3.37 kg/m2 (0.69 psf) |
| Ceiling panel type 3 | Glass fiber | 1.86 kg/m2 (0.38 psf) |
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 that 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 fiber), the assembly had the same STC rating of 54 and the frequency-specific TL performance overlapped. The third ceiling panel (fiberglass) 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 that the fiberglass panels performed differently than the stone wool or mineral fiber panels.
This insightful article highlights the importance of specifying acoustic ceilings and their four key roles in enhancing indoor environments. It offers valuable guidance for architects and designers seeking to optimize acoustics in various spaces, from offices to educational facilities. A must-read for anyone involved in construction projects aiming for superior sound management.