Prediction method
After establishing the goal background sound levels for each room and understanding that most ceiling panels provide the same amount of sound attenuation, one can focus on the key variables, the location, and sound power levels of the mechanical unit itself.
When possible, locate HVAC equipment over unoccupied or noisy areas such as corridors, storage rooms, and lobbies. Avoid locating HVAC equipment over normally occupied rooms with background noise requirements of NC-35 / 40-45 dB(A) or lower. When locating HVAC equipment over occupied rooms cannot be avoided, use Figure 8 to determine the maximum sound power levels for the HVAC equipment in the plenum. Specify the appropriate device model, configuration, operating conditions, and maximum sound power levels so the values in Figure 8 are not exceeded.
Key takeaways: Achieve background noise levels
The ceiling’s role in achieving acceptable background noise levels by attenuating mechanical equipment noise in the plenum can be important. Most ceiling panels perform the same. The method of starting with the mechanical unit’s sound power levels and then trying to specify a particular ceiling panel material, weight, or CAC rating is inconsistent with the industry-consensus prediction method in AHRI Standard 885 and the ASHRAE Handbook. Therefore, the CAC rating should not be included in the specification. Instead, start with the goal background sound levels for the room, add the attenuation provided by most acoustic ceiling panels per Figure 8, and specify the maximum permissible, octave band, sound power levels in the specification section for mechanical devices.
Final thoughts
While an acoustic ceiling can serve multiple roles, its primary one is to absorb sound. To ensure good room acoustics and to comply with building design standards, a minimum ceiling NRC of 0.90 should be specified in open or larger spaces with multiple occupants and noise sources.
On some projects, the acoustic ceiling may need to take on acoustic roles beyond absorption. If the interior partitions are not full-height and the acoustic ceiling’s role is expanded to include horizontal sound isolation, the ceiling alone cannot achieve high enough privacy between rooms, especially when the noise leaks caused by the penetrations for lights and air distribution devices are considered. To comply with the STC 40, 45, and 50 levels of sound isolation requirements in the standards, a lightweight plenum barrier working in combination with the acoustic ceiling is the optimal design approach. This is the only application where the CAC rating of the ceiling system and plenum barrier combined should be specified, and the value should match the STC rating of the partition.
