by arslan_ahmed | November 4, 2022 10:52 am
By Gary Madaras, PhD, INCE, ASA, CAA, Assoc AIA
Building design standards and guidelines contain requirements for maximum permissible background noise levels in rooms due to the mechanical systems.1 A suspended acoustic ceiling is typically the only element between the noise-generating devices in the plenum and the people in the room below. Specifiers and architects seeking to ensure the mechanical equipment noise does not exceed the ceiling system’s ability to attenuate it can use a straightforward method defined in the Air-Conditioning, Heating, and Refrigeration Institute’s (AHRI) Standard 885 (2008), Procedure for Estimating Occupied Space Sound Levels in the Application of Air Terminals and Air Outlets.2
It is always best acoustically, to locate noisy mechanical equipment away from sound sensitive rooms in which occupants need to concentrate, communicate, or relax. Placing a fan-powered box over a corridor is better than positioning it directly over the students inside a classroom. There are situations when placing noisy equipment over a sound-sensitive room is unavoidable. A simple and straightforward method of ensuring the suspended acoustic ceiling has the capability of attenuating the plenum noise is laid out in AHRI Standard 885 as well as in the ASHRAE (formerly the American Society of Heating, Refrigerating and Air Conditioning Engineers) Handbook HVAC Applications (2019).3
Building professionals that are not familiar with these standards or the prediction methods within them, may instead resort to specifying ceiling panels of a certain material type, minimum weight, or minimum Ceiling Attenuation Class (CAC) rating. To understand why these approaches are inconsistent with the industry-accepted prediction methods, one must first understand how acoustic ceilings attenuate plenum noise.
Ceiling noise attenuation basics
A suspended acoustic ceiling system attenuates noise from inside the plenum in three ways.
The foundational ASHRAE research project (ASHRAE RP-755)4 leading to the prediction method in the ASHRAE Handbook and AHRI standard concludes, “Experiments have shown that, for ceiling panels supported in a T-bar grid system, leakage between the panels and the grid is the major transmission path.”
So, which ceiling panels perform the best? Every ceiling panel performs better in some respects and worse in others. Some panels do not prevent noise leaks along the grid flanges, but they limit the noise passing through the panel itself. Other panels absorb the reflected noise in the plenum and room better, and prevent noise leaks along the grid even though more sound passes through them. In conclusion, the ASHRAE Handbook HVAC Applications, when discussing the attenuation performance of various ceiling panels, states, “Experiments have shown that, for ceiling panels supported in a T-bar grid system, differences among ceiling panel types are small.”
To better understand the claims, data, and prediction method in the ASHRAE Handbook and AHRI standard, one should refer to ASHRAE RP-755. This study was conducted by the National Research Council Canada’s (NRCC) Institute for Research in Construction (IRC) (currently the Construction Research Centre). The NRCC is Canada’s largest federal research and development organization.
The RP-755 study “was initiated to investigate the transmission of sound through different ceiling types with the intent of providing more reliable design information to deal with sound transmission through ceilings close to HVAC devices.”
Within a large, acoustic chamber, the NRCC researchers tested a wide variety of ceiling panels including those made of fibreglass, mineral fibre, and gypsum board. Ceiling panel thicknesses varied from 13 to 50 mm (0.5 to 2 in.). Weights varied from 0.5 to 8.9 kg/m2 (0.1 to 1.8 psf). CAC ratings varied from 28 to 39. Sound isolation ratings for ceiling type 3 were not measured. STC ratings varied from 16 to 19, while NRC ratings varied from 0 to 1.1 (refer to Table 1). These panels represent the full range of acoustic ceiling panels available in the market at the time of the study and still today.
The NRCC researchers used various types of mechanical equipment above the ceilings as the noise sources. These devices were installed as they would be in a building, having air moving through them, and conditioning the air. For each ceiling type, and each piece of mechanical equipment, the researchers measured the resulting noise levels in the room below at occupant ear height. They compared the known sound power levels of the mechanical equipment to the measured noise level at listener height to derive the attenuation being provided by each ceiling system.
The study was empirical, it was not theoretical or based on calculations, models, or simulations. Figure 2 shows the average attenuation by frequency octave band of each ceiling across all the different types of mechanical devices used in the study.
If the existing rule-of thumb were to hold true, one would expect the heaviest weight (8.9 kg/m2
[1.8 psf]) gypsum board panels (type G13) to provide the most attenuation. In fact, it performed second to worst. The rigid material spans the high points in the supporting grid flanges, leaving gaps elsewhere through which noise leaks. Additionally, the gypsum board provides no absorption in the plenum or room. Even lower in attenuation performance is the very lightweight A2910 fibreglass panel. While it has the malleability to decrease leaks at the grid, it has low absorption (NRC 0.70) and very low weight (0.5 kg/m2 [0.1 psf]).
While stone wool ceiling panels were not offered in North America at the time of the study, their NRC rating, weight, and other attributes also falls within the range of the panels used in the study. Stone wool panels performed similarly to the thick, fibreglass panels (AHRI types 4 and 5). Both ceiling panels have the right combination of all three factors for noise attenuation. They have high NRC, moderate weight, and malleable surfaces that are soft and seal the leaks at the grid. While there are slight differences in the attenuation provided by the different ceiling panel types, those differences are not significant enough to affect building occupant well-being over time.
ASHRAE RP-755 concludes the following:
Ceiling attenuation class
CAC, measured according to ASTM International’s standard E1414, Standard Test Method for Airborne Sound Attenuation Between Rooms Sharing Common Ceiling Plenum, and calculated per ASTM E413, Classification for Rating Sound, is an acoustic metric that applies only to horizontal noise isolation of airborne sound between two adjacent rooms when the partition between the rooms stops at the height of the suspended acoustic ceiling, leaving a common plenum above the ceiling. Including CAC ratings in ceiling panel specification sections, despite being inapplicable to mechanical noise in the plenum, has unfortunately become a common mistake.
ASHRAE RP-755, as well as the ASHRAE Handbook, and AHRI Standard 885, disprove the ceiling panel CAC rating can be used to predict mechanical noise attenuation. As part of the study, NRCC researchers measured the ratings of each ceiling system (see Table 1).6 If the existing rule-of-thumb where higher specified CAC ratings result in more noise attenuation was valid, then the gypsum board panels with the highest rating of 39 should have provided the most attenuation. The mineral fibre panels with CAC ratings of 34 and 31 should have provided less attenuation than the gypsum board panels and more than the fibreglass panels, which had the lowest ratings of 28 and 31. In fact, the fibreglass panels provided the most noise attenuation even though they had the lowest CAC ratings, and the gypsum board panels with the highest rating provided next to lowest. ASHRAE concludes:
ASHRAE RP-755 also measured the Sound Transmission Class (STC) ratings of the ceilings in the study per ASTM E90, Standard Test Method for Laboratory Measurement of Airborne Sound Transmission Loss of Building Partitions and Elements.”6 While STC related more closely to the attenuation measured in the study than did CAC, the researchers concluded neither STC ratings nor frequency-specific transmission loss (TL) could be used for the prediction of mechanical noise attenuation. This is less applicable because ceiling manufacturers do not typically report the TL or STC ratings for their ceiling panels, and specifiers do not typically include these metrics in the acoustic ceiling panel section.
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.
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
Determine the maximum background noise level for each room type according to the applicable standard or Table 2.
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.
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.
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.
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 which 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.11 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 shows 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,12 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 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.
The inclusion of CAC or STC in the ceiling panel specification does not result in higher vertical sound isolation from rooms above or below or greater attenuation of noise generated by mechanical equipment located in the plenum. Specification professionals can be confident that selecting ceiling panels with high sound absorption, as defined by an NRC rating of 0.90 or higher, will meet their projects’ acoustic performance requirements in accordance with building design standards and guidelines.
1 Examples include: the International WELL Building Institute’s WELL Building Standard, Sound Concept, Feature S02 Maximum Noise Levels; the Facilities Guidelines Institute’s “Guidelines for Design and Construction of Hospitals,” (FGI Guidelines for Hospitals) Table 1.2-5 Maximum Design Criteria for Noise in Interior Spaces Caused by Building Systems; and the American National Standards Institute and the Acoustical Society of America (ANSI/ASA S12.60), Acoustical Performance Criteria, Design Requirements, and Guidelines for Schools, Part I: Permanent Schools, Table 1 Limits on A- and C-Weighted Sound Levels of Background Noise and Reverberation Times in Unoccupied Furnished Learning Space.
2 Refer to 2008 AHRI Standard 885 (with Addendum 1), Procedure for Estimating Occupied Space Sound Levels in the Application of Air Terminals and Air Outlets; free download at www.ahrinet.org/App_Content/ahri/files/STANDARDS/AHRI/AHRI_Standard_885_2008_with_Addendum_1.pdf; refer to Appendix D Sound Path Factors, Sections D1.6 Ceiling/Space Effect, Table D14 Uncorrected Ceiling/Space Effect Attenuation Values and Table D15 Ceiling/Space Effect Examples.
3 Review the 2019 ASHRAE Handbook “HVAC Applications,” Chapter 49 Noise and Vibration Control; available for purchase at https://www.techstreet.com/ashrae/standards/a49-noise-and-vibration-control-si?product_id=2073870), Section 2.8, subsection Sound Transmission Through Ceilings and Table 43 Ceiling/Plenum/Room Attenuation in dB for Generic Ceiling in T-Bar Suspension Systems.
4 ASHRAE research project RP-755 was conducted by the National Research Council Canada (NRCC) and is available online at https://nrc-publications.canada.ca/eng/view/ft/?id=df153a91-863d-48ca-a037-dfbece1f5456.
5 More information about the ceiling panels in Table 1 can be found in ASHRAE’s final report RP-755, Tables 3-1 (page 6).
6 Refer to section 13 on pages 125-127 of ASHRAE RP-755 for the measured CAC and STC data and for more detailed analysis and discussion by the original researchers.
7 Examples include: the WELL Building Standard, Sound Concept, Feature S04 Reverberation Time and Feature S05 Sound Reducing Surfaces; FGI Guidelines for Hospitals, Table 1.2-4 Minimum Design Room-Average Sound Absorption Coefficients; and ANSI/ASA S12.60, Table 1 Limits on A- and C-Weighted Sound Levels of Background Noise and Reverberation Times in Unoccupied Furnished Learning Space.
8 Caution should be taken if the building design does not comply with minimum standards and has interior partitions that do not extend full height to the floor or roof above. In this case, a specifier might consider adding CAC back into the ceiling panel specification, so the CAC of the ceiling system equals the STC rating of the partition below the ceiling.
9 The background noise requirements provided in Step 2 are from the ASHRAE Handbook, “HVAC Applications” (2019), Table 1 Design Guidelines for HVAC-Related Background Sound in Rooms. If the building must comply with a different building standard or guideline, use those values instead.
10 The values in this graph were derived using the method in the 2019 ASHRAE Handbook “HVAC Applications” and 2008 AHRI Standard 885. Octave band values for the Noise Criterion curves taken from Table 13 in AHRI Standard 885 were added to the “Environmental Adjustment Factor” in Table C1 in AHRI Standard 885. The average “Uncorrected Ceiling/Space Effect Attenuation Values” per Table D14 in AHRI Standard 885 were added to the sum to get the maximum permissible sound power levels for the mechanical equipment in the plenum.
11 For more information refer to The Construction Specifier, “Specifying ceiling panels with a high NRC;” Gary Madaras, PhD; Feb. 21, 2020; www.constructionspecifier.com/specifying-ceiling-panels-with-a-high-nrc/.
12 For more information refer to The Construction Specifier, “Effects of acoustical ceiling panel type on Vertical sound isolation;” Gary Madaras, PhD; April 30, 2021; www.constructionspecifier.com/effects-of-acoustical-ceiling-panel-type-vertical-sound-isolation/3/.
Gary Madaras, PhD, is an acoustics specialist at Rockfon. He helps designers and specifiers learn the optimized acoustics design approach and apply it correctly to their projects. He is
a member of the Acoustical Society of America (ASA), the Canadian Acoustical Association (CAA), and the Institute of Noise Control Engineering (INCE). Madaras can be reached at email@example.com.
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