Other considerations
Many modern spaces are designed without ceilings, leaving the HVAC systems overhead visible. These designs must comply with the background noise requirements in the standards and guidelines. Therefore, ceilings are not required to control mechanical noise, nor to comply with the background noise requirements. Even if an acoustics ceiling is provided, it is advisable for the mechanical systems to comply with the background noise limits without the ceiling installed, so compliance is maintained if the ceiling is removed in the future.
This three-step guideline addresses only noise radiated off the HVAC device casing and transmitted through the ceiling below. Noise from the HVAC device also may be duct-borne and emitted into the room through the supply air diffusers and return air grilles. The mechanical engineer should control duct-borne noise, if necessary, with noise control devices such as discharge attenuators, duct silencers, or internal duct lining. All noise paths/sources should be combined to ensure the background noise limit is not exceeded.
Final thoughts
The importance of acoustics as part of indoor environmental quality and ultimately, the well-being of building occupants has been well-established in the minds of people involved with the design and construction of buildings and in the requirements in the building design standards and guidelines. Good acoustics leads to comfort and productivity in workplaces, recovery and privacy in health care facilities, and efficient learning in education buildings.
The standards ensure there is enough absorption to control occupant-generated noise levels in open spaces and shorten reverberance for clear speech intelligibility in enclosed rooms. The transmission of sound from one room into another is limited, so privacy and confidentiality are achieved. The background noise from building mechanical systems is limited for overall auditory comfort.
Few products or systems contribute to compliance with all three: room absorption, sound isolation, and mechanical noise control. Suspended acoustic ceilings with NRC ratings of 0.90 or higher provide high-performance sound absorption that limits how far noise travels across an open office or down the corridors of a patient ward. They decrease reverberation to ensure teachers are heard and understood by their students without experiencing ongoing voice strain. Acoustic ceilings also increase the floor-to-floor sound isolation, allowing the structural floor slab to be thinner and lower weight while complying with the minimum requirements in the standards. Lastly, acoustic ceilings attenuate noise from mechanical devices in the plenum, so the background noise level limits in the standards are met.
Many building design standards and guidelines require ceiling panels to have a high NRC rating, often 0.90 as a minimum. Most do not require ceiling panels to have a CAC or STC rating. The high NRC rating is included due to the proven benefits on building occupant health and well-being. CAC and STC are excluded because foundational research has shown that those metrics do not relate to the vertical sound isolation performance of a floor ceiling assembly or to the attenuation of noise generated by mechanical equipment located in the plenum above the ceiling. Alignment of ceiling panel specifications with the building design standards and guidelines is encouraged.
Previous work by the author shows a suspended acoustic ceiling increases the STC rating of the floor-ceiling assembly by 6 to 7 STC points, improving the performance of the slab alone in the low- to mid-40s to more than the minimum of 50 required in the standards. The type of ceiling panel in the grid does not affect the overall floor-ceiling assembly STC rating because it is largely controlled by weight of the slab 195.3 to 292.9 kg/m2 (40 to 60 psf). Small differences in the weight of the ceiling panels of less than 2.4 kg/m2 (0.5 psf) and the CAC ratings of the ceiling panels do not matter.
ASHRAE RP-755 shows all ceiling panels available in the market attenuate mechanical noise generated in the plenum the same. The findings were used to develop the prediction method in the ASHRAE handbook HVAC Applications and AHRI Standard 885. The STC and CAC ratings of the ceiling panels do not matter. The approach building industry professionals should take is to begin with the desired background noise levels in the room, add to them the sound isolation capacity of any acoustic ceiling and select a mechanical device that does not exceed the sum. Mechanical device manufacturers are aware of the attenuation limits defined in AHRI 885 and typically consider it during the design of their devices.
