Plenum barriers, speech privacy, and workplace 2.0 fit-up standards

Photo courtesy Rockfon

By Gary Madaras, PhD, Assoc. AIA
When someone enters an enclosed room, whether it is a private office or conference room, there is an expectation he or she is achieving not only visual privacy, but also sound privacy. Whether this is actually the case depends on how the room’s envelope—including the slabs, walls, doors, windows, and at times, ceiling (and what happens above)—is constructed. It is possible to have little or no visual difference between a room that has high sound privacy and one that does not. This is largely why analysis of post-occupancy survey data from office buildings shows occupants have been dissatisfied with their acoustic environment. By a wide margin, the lowest scoring metric on the surveys is sound privacy. (For more, see M. Frontczak et al’s “Quantitative Relationships Between Occupant Satisfaction and Satisfaction Aspects of Indoor Environmental Quality and Building Design,” published in January 2012 by the Center for the Built Environment’s Center for Environmental Design Research at the University of California, Berkeley.)

Workplace renewal is an ongoing priority for the Canadian government. In the “Eighteenth Annual Report to the Prime Minister on the Public Service of Canada,” the Clerk of the Privy Council stated:

A modern, healthy workplace supports greater productivity, a more engaged workforce and better results for Canadians. Deputies and managers have a responsibility to create workplaces that support the well-being, wellness and productivity of our employees.

Driven by this challenge for workplace renewal, Public Services and Procurement Canada (PSPC) has taken the lead in facilitating change across the government, through the development and implementation of the government-wide Workplace 2.0 initiative. It provides PSPC employees, client departments, agencies, and the private sector with direction and guidance on the fit-up of federal office accommodations. It applies to all office accommodation projects, and tenant services projects managed by PSPC. Workplace 2.0 is creating a modern workplace that will attract, retain, and enable public servants to work smarter, greener, and healthier to better serve Canadians. (This comes from the “Message from the Assistant Deputy Minister, John McBain,” located on page ADM-i of the Government of Canada Workplace 2.0 Fit-up Standards.)

Sound privacy in Workplace 2.0
The construction required to achieve sound privacy in general-purpose office spaces is described in the table of section A3.3 of Workplace 2.0. Three levels of privacy are established: standard privacy, enhanced privacy, and secured privacy. Standard privacy is equated to a sound transmission class (STC) rating of 35 for the wall or, similarly, a ceiling attenuation class (CAC) rating of 35.  (In the United States, the American equivalent to Workplace 2.0, General Services Administration [GSA] P100, Facilities Standards for the Public Buildings Service, has moved to noise isolation class [NIC] as a metric, rather than STC.) The wall should be insulated and extend from the floor slab to the underside of the suspended acoustic ceiling. This design leaves an open plenum above the suspended ceiling, meaning there are no upper walls to block sound transfer between rooms.

The Government of Alberta’s Department of Infrastructure (previously known as the Department of Public Works, Supply, and Services) has conducted extensive research on sound isolation between offices with suspended ceilings.  (See K. Kruger’s “The Effect of Various Parameters on the Sound Isolation between Offices with Suspended Ceilings,” in Canadian Acoustics [16 (2)], 1988.) It states attempting to match the isolation performance of a ceiling to the demising wall can lead to disappointing results. It is important to know the combined effect of the wall and ceiling, along with any flanking that might be introduced through the latter, such as ventilation penetrations.

This low level of privacy (i.e. STC/CAC 35) is only permitted for unoccupied spaces, such as storage rooms and equipment rooms, and short occupancy spaces where privacy is not expected or required, such as kitchenettes. It can be deduced that ‘standard privacy’ as used in Workplace 2.0 is the same as ‘no privacy,’ and therefore unacceptable for spaces normally occupied by people while working.

Most regularly occupied spaces—such as enclosed, private offices, meeting rooms, training rooms, quiet rooms, and telecommunication rooms—should be constructed to achieve enhanced sound privacy, which is equated to an STC/CAC rating of 45. The higher-STC wall should be insulated and extend from the floor slab to the underside of the suspended acoustic ceiling. The main difference between standard privacy and enhanced privacy construction is the additional requirement of a plenum barrier extending vertically from the top of the wall to the underside of the slab above. This plenum barrier blocks the sound that transmits through the suspended acoustic ceilings, increasing the overall performance.

Another conclusion from the Alberta research is the most effective method of reducing sound transmission through the ceiling is to introduce a barrier into the plenum. The plenum barrier can be limited in length—it only has to be positioned above the wall between the two adjacent rooms. The plenum barrier does not need to extend around the entire perimeters of the rooms. A limited plenum barrier allows return air to still flow freely through the plenum.

When plenum barriers do need to surround the entire perimeter of the room, a hole of the appropriate size based on air volume and desired velocity should be cut in the plenum barrier over the door into the room. Workplace 2.0 requires solid-core wood doors, but does not call for perimeter seals or acoustic ratings. It even permits sliding doors, which are difficult to seal acoustically. It is highly likely the door will be the weakest link and transmit more noise than the return air opening in the plenum barrier above the ceiling.

Most specifiers, architects, interior designers, and contractors know how to design and construct an STC 45 wall. If they do not already have a standard detail on their wall-type sheet, they can go to any of a number of gypsum board manufacturers and quickly find details and test reports for an STC 45 wall. Conversely, there has been little to no information available to designers, in Workplace 2.0 or the industry’s literature base, about recommended materials and installation methods to use for a plenum barrier that, when combined with the ceiling, will perform at the same or higher level. This was one of the research goals of the Optimized Acoustics Research Program, a group for which this author serves as director. (The Optimized Acoustics Research Program is an ongoing, multi-year, multi-organization investigation into cost-effective means of designing and constructing interior architecture compliant with the acoustic requirements in industry standards, guidelines, and building rating systems. It began in 2014, and progress updates of the findings, such as this article, have been presented at and published in the proceedings of InterNoise 2015, NoiseCon 2016 and 2017, and Acoustics Week Canada 2015 and 2016, as well as published in Sound & Vibration and Canadian Acoustics. For more, visit www.optimizedacoustics.com.)

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