Absorption
Building design standards and guidelines typically also contain sound absorption requirements, such as minimum ceiling NRC or maximum reverberation time, so occupants are acoustically comfortable and speech is either intelligible or private.7 Many rooms and spaces have suspended acoustic ceilings overhead, mainly for their absorption performance and to ensure compliance with these room acoustics requirements.
In section 11.5.4. of the Green Building Initiative’s (GBI’s) Green Globes Assessment Protocol for Commercial Buildings (ANSI/GBI 01-2019), open offices in workplaces and patient care areas in health care facilities are required to have a minimum ceiling NRC rating of 0.90. There is also a reverberation time compliance option. Standards require high NRC ceilings based on the strength of the evidence showing its beneficial impact on physiology, behaviour, performance, and comfort. When specifiers include CAC in their acoustic ceiling panel specification sections, it often coincides with a decrease in the NRC rating. They sacrifice the beneficial absorption and the desired impact on building occupants for no improvement in the attenuation of mechanical noise in the plenum.
Three-step guide to compliance
It could require several days or more for someone to review ASHRAE RP-755, AHRI standard 885, and the ASHRAE Handbook before fully understanding the topic of ceiling attenuation of mechanical noise in the plenum. For convenience, this article provides a simple, three-step, design guide to help ensure the background noise requirements in building design standards are not exceeded.
Step 1
Determine if an acoustic ceiling is part of the building’s design esthetic and select the ceiling panel based on a high NRC for optimal room acoustics in combination with any other characteristic important for the project such as esthetics, contribution to indoor air quality, environmental/energy impact, or cost. If building design allows, consider removing CAC and STC from the acoustical ceiling panel specification.8
Step 2
Determine the maximum background noise level for each room type according to the applicable standard or Table 2.
Step 3
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 3 to determine the maximum sound power levels for the HVAC equipment in the plenum above.10 Select the appropriate device model, configuration, and operating conditions so these maximum sound power levels are not exceeded.
Special cases
If the maximum HVAC equipment sound power levels provided in Figure 3 cannot be met, consider other equipment brands or models or noise control options such as attenuators, insulated casings, or noise control jackets provided by the equipment manufacturer. Increasing the size of the device (oversizing) can reduce noise levels because the fan operates at slower speed or the air velocity inside the device is slower. Alternatively, the ceiling system manufacturer can offer options above the ceiling to reduce the mechanical noise further.
