When the acoustic ceiling must work with the floor slab to take on the role of floor-to-floor sound isolation, it will increase the STC rating approximately six points beyond that of the floor slab alone. The type of ceiling panel material and its weight are not significant factors. The CAC rating is irrelevant to this application and should not be specified. Instead, a floor slab with an STC rating of no more than six points lower than that required by the standard should be specified.
Lastly, when the acoustic ceiling needs to take on the role of mechanical noise attenuation, most standard ceiling panels perform the same in the frequencies of concern. CAC and STC rating are irrelevant and ASHRAE warns against using these metrics as the prediction method. Instead, utilize the prediction method in either AHRI Standard 885 or the ASHRAE Handbook. Begin with the goal background noise levels. Add the attenuation provided by an acoustic ceiling. Specify the maximum permissible sound power levels of the mechanical units in the plenum.
Notes
1 Refer to Table B.4 Design Recommendations for Open Offices – Unpartitioned on page 105 of the 2017 version.
2 Refer to section 11.5.4 Reverberation Time or Ceiling Noise Reduction Coefficient (NRC) in ANSI/GBI 01-2019 Green Globes Assessment Protocol for Commercial Buildings.
3 Refer to the Sound Concept, feature S05 Sound Reducing Services of version 2 of the WELL Building Standard. The rating is for tier two performance and credits.
4 For more information refer to The Construction Specifier, “Specifying ceiling panels with a high NRC;” Gary Madaras, PhD; Feb. 21, 2020.
5 For more information refer to Acoustical Interior Construction, “A Guide on the Four Categories for Acoustics Criteria in Building Standards and Guidelines;” Gary Madaras, PhD; July-September 2016, pgs. 27-29. CISCA has granted permission for the article to be available gratis online.
6 For more information refer to Construction Canada, “Plenum barriers, speech privacy and workplace 2.0 fit-up standards;” Gary Madaras, PhD; September 21, 2017.
7 For more information refer to Construction Canada, “Effects of acoustic ceilings on vertical sound isolation;” Gary Madaras, PhD; November 5, 2021.
8 Refer to AHRI Standard 885 (2008), “Procedure for Estimating Occupied Space Sound Levels in the Application of Air Terminals and Air Outlets.” 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.
9 Refer to the ASHRAE Handbook HVAC Applications (2019), Chapter 49 Noise and Vibration Control, 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.
10 The background noise requirements provided in Figure 7 are from the ASHRAE Handbook, HVAC Application (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.
11 Refer to ASHRAE research project RP-755, “Sound transmission through ceilings from air terminal devices in the plenum: final report,” conducted by the National Research Council Canada, A.C.C. Warnock, January 1997.
12 For more information refer to Construction Canada, “Specifying acoustic ceilings and HVAC equipment to meet acoustic requirements;” Gary Madaras, PhD; November 4, 2022.
13 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.
Author
Gary Madaras, Ph.D., 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 gary.madaras@rockfon.com.
