Acoustics requirements in building standards utilize sound transmission class (STC) most frequently as the sound isolation performance metric. STC requirements generally range from 40 to 50, with STC 45 being the most common.5 Technically, for a partition to have an STC rating, it is required to be full height from structural floor slab to structural floor slab or roof. When the partition stops, instead, at ceiling level, some standards permit the option of combining an acoustic ceiling with a lightweight plenum barrier positioned vertically over the partition. In this instance, the combined Ceiling Attenuation Class (CAC) rating of the ceiling and plenum barrier tested together must equal the STC rating of the partition below the ceiling. Building design standards do not permit open/common plenums above ceilings because the noise can too easily pass between rooms.
When the partitions are not constructed at full height, noise can leak easily through the penetrations in the ceiling system for recessed light fixtures, air supply diffusers, return air grilles, and other ceiling-mounted devices. These noise leaks can decrease the ceiling system performance up to 10 CAC points. More importantly, it is not a uniform 10 decibel (dB) decrease across all frequencies. High frequency sound with small wavelengths, which makes speech more recognizable and intelligible, passes more easily through these leaks in the ceiling than low frequency sound. High frequency degradation to the ceiling performance is 15 to 20+ dB, making speech from the adjacent room four times louder. The article “Using Plenum Barriers to Comply with Sound Isolation Requirements” 6 offers a thorough discussion about how penetrations degrade ceiling system sound isolation performance.
Plenum barriers
Lightweight plenum barriers can be constructed of a variety of different materials. Figure 4 shows stone wool plenum barriers, which are beneficial due to their low cost, relatively quick installation, pliability, and high sound isolation performance.
Figure 5 shows the sound isolation performance differences in terms of frequency-specific transmission loss (TL) and wideband STC rating for an acoustic ceiling alone versus with stone wool plenum barriers. A standard acoustic ceiling alone—regardless of the material, weight, and NRC/CAC ratings—cannot achieve the required 40 to 50 performance level, especially when the unavoidable penetrations are considered.
It is not until a plenum barrier is added that the room-to-room isolation reaches that level. A stone wool ceiling with a single-layer, stone wool plenum barrier reaches the CAC 40 level of performance. When a second layer is added with an interstitial airspace, the performance increases to over CAC 50. See additional information about plenum barriers of different material types and their performance levels in “Using plenum barriers to comply with sound isolation requirements.”6
Summary
The relevance of the acoustic ceiling to sound isolation between rooms on the same floor is dependent on the construction of the partitions. If the partitions are full height, then the acoustic ceiling plays no role. The portion of the partition above the ceiling is providing the same sound isolation as the part below the ceiling. The sound isolation will be the same, whether or not the acoustic ceiling is present. Therefore, the CAC rating of the ceiling panel is irrelevant and does not need to be included in the ceiling panel specification. Since ceiling panels with moderate to high CAC ratings typically have lower NRC ratings, including the CAC rating in the specification can instead result in noncompliance with required absorption performance unless additional absorption is added to the floor or walls.
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.