by Niklas Moeller
With mounting recognition of the need to support focused work and promote wellness, many organizations are looking to provide building occupants with improved speech privacy, noise control, and acoustic comfort.
Background sound is key to achieving these goals. Indeed, all acoustic designs consider this factor when determining sound transmission class (STC), articulation index, or signal-to-noise ratio. However, building professionals often neglect to use the only accurate means of controlling the minimum background level—a sound masking system—as a design tool.
By turning the traditional three-tiered approach of absorb, block, and cover—collectively known as the ‘ABC Rule’—on its head and using sound masking as the starting point for interior planning, building professionals can set the base level of background sound throughout a facility. They can then more accurately specify the blocking and absorptive elements used in their design, thereby allowing it to be delivered in a more cost-effective manner, and with greater assurance of achieving the intended results.
The variability of HVAC
Both ASTM E1130, Test Method for Objective Measurement of Speech Privacy in Open-plan Spaces Using Articulation Index, and ASTM E2638, Standard Test Method for Objective Measurement of the Speech Privacy Provided by a Closed Room, consider background sound when calculating speech privacy. (ASTM E2638 defines speech privacy class as “an objective rating of the speech privacy provided by a closed room, calculated as a sum of factors related to sound isolation provided by the room, and the background noise at the receiving point.” Similarly, when calculating speech privacy, ASTM E1130 considers the combined effects of transmission loss due to the partition assembly and reduced signal-to-noise ratio [SNR] due to the background sound level. Many acousticians use E1130 for both open and closed spaces.)
However, ASTM E2638 also reminds readers speech privacy class is only valid at the time it is measured because the background level is presumed to be provided by HVAC. Even if well-designed, this equipment’s output is only governed in that it is not to exceed maximums defined by the American Society of Heating, Refrigerating, and Air-conditioning Engineers (ASHRAE) in the 2013 ASHRAE Handbook—Fundamentals. It cannot control the minimum background sound level.
HVAC output often varies by 15 dBA or more, according to zone, time of day, and season as well as the type of equipment used. Whenever and wherever the background level falls below the 30 dBA on which STC ratings—and, hence, wall choices—are based, occupants can no longer rely on the partition assembly for speech privacy. Further, HVAC does not generate a spectrum conducive to speech privacy.
Speech privacy levels fluctuate from wall assembly to wall assembly, depending on their performance in the frequencies used to calculate STC, as well as the inconsistent noise level and spectrum generated by HVAC—not to mention sound leakages through various flanking paths. If privacy is achieved, it is largely due to good luck or overbuilding. If not, a sound masking vendor is contacted. In this scenario, the technology is consigned to band-aid status.