Cracks in the armor
By the same token, when a closed room fails to provide these attributes for its occupants, the failure is typically blamed on deficiencies in its design, construction, and/or maintenance.
While they might be a contributing factor, this failure cannot solely be attributed to cracks in the walls’ armor, because speech privacy is not determined by isolation alone. A person’s ability to clearly understand a conversation is dependent on two variables: the level of the speaker’s voice and the background sound level in the listener’s location. The relationship between the two is called the signal-to-noise ratio.
Traditional closed room construction attempts to provide privacy by simply reducing the signal. If a solution has not been implemented to control the minimum background sound level in adjoining areas and it is lower than the sounds passing through the wall or via various flanking paths, conversations and noises will still be heard and potentially be intelligible. (Of course, a room’s Achilles’ heel is the door. When open, the barrier provided by the wall is compromised. For example, an STC 40 wall with an open door representing 10 percent of the wall’s area reduces its effective STC to 10. The same is true for STC 45 and 50 walls. If the door represents 20 percent of the wall area—which is the case for a standard 0.91-m [3-ft] door in a 3 x 3-m [10 x 10-ft] wall—then the effective STC drops to around seven.)
Regardless, unless a sound masking system is implemented—as well as professionally tuned, and verified for performance post-installation—the minimum background sound level is not a known quantity. HVAC and other mechanical systems are sometimes thought to provide masking, but one cannot reasonably expect this type of equipment to deliver a consistent level over time/space or to even generate a spectrum conducive to speech privacy.
Accordingly, ASTM E1374, Standard Guide for Open Office Acoustics and Applicable ASTM Standards, was recently revised. The discussion of HVAC noise in the newly released ASTM E1374-18, Standard Guide for Office Acoustics and Applicable ASTM Standards, pertains only to limiting maximum noise levels rather than using this equipment for masking. Further, a sound masking system is identified as the only viable source of a continuous minimum background sound level. As the title change suggests, this standard’s scope has also been broadened; it now applies to private offices and conference rooms, not only to open plan.
Though masking technology distributes a sound often compared to softly blowing air, unlike HVAC noise, it is continuous and precisely controllable. Using it—even to apply a level as low as the 30 dBA on which STC ratings and, hence, wall choices are based—allows the expected degree of speech privacy to be more reliably achieved. After being professionally tuned post-installation, the masking system’s measured output meets a particular spectrum or “curve” specifically engineered to balance acoustic control and comfort.
Building professionals can use this predictable background sound level as the foundation for the remainder of their acoustical plan, allowing more accurate specification of the blocking and absorptive elements—and providing a means of reducing the specifications for the room’s physical shell, while still achieving the desired level of speech privacy. (For more information about using sound masking as the starting point for interior planning, see this author’s “A New Approach to Acoustics: Using sound masking as a design platform” in the January 2018 issue of The Construction Specifier.)