Placing sound masking on the front line of acoustic design

A design that does not feature sound masking, but rather relies on HVAC to provide an assumed minimum background sound level, leads to over-specification of the room’s physical structure while nonetheless leaving its actual acoustic performance up to chance.
Images courtesy KR Moeller Associates Ltd. and Screen Solutions UK

Cost savings
If speech privacy = STCc + 30 dBA ≥ 75, then for every 1-dBA increase in the background sound level, it is possible to reduce STCc by one point and achieve the equivalent level of speech privacy. Were the background sound to be increased from 30 to 35 dBA, for instance, construction costs for partition types would start to drop significantly because the STCc can be reduced by five points.

Again, 30 dBA—and, indeed, even 35 dBA—is well below typical masking levels in closed rooms. Usually, they are set to between 40 and 43 dBA in these spaces. Depending on various factors, including occupant comfort, they may be set higher. Therefore, while 30 dBA can be used as a design benchmark, the lowest STCc rating possible to achieve an SPP of 75 is actually determined by the highest comfortable level of continuous minimum background sound.

While the established maximum levels for HVAC can form the basis for the controlled minimum background sound provided by the sound masking system, there are significant opportunities for further value engineering because the predictable overall volume and spectrum allows one to reduce the specifications for the room’s physical shell.

With a suitable design of sound masking, walls, and ceilings, it is also possible to achieve privacy with walls built to the suspended ceiling rather than to the structure, affording additional cost savings and flexibility. (For more information about this topic, see this author’s “Mind the Gap: Using Sound Masking in Closed Spaces” in the October 2012 issue of Construction Canada.)

Future flexibility
When sound masking is incorporated into the facility design, one also has the opportunity to increase the background sound level if the partition construction fails to live up to its rated level—for example, as a consequence of common deficiencies such as flanking paths—and remedial action would be cumbersome or costly. While the minimum planned level should be at least 30 dBA, as noted, the level traditionally recommended in most closed rooms is 40 to 43 dBA, leaving a range of adjustment at the facility manager’s disposal.

Of course, this rationale can also be applied to existing spaces not performing as expected. However, by waiting to install masking post-occupancy, an organization forgoes the opportunities to reduce construction costs and the specifications for other acoustic treatments.

A design that utilizes sound masking to establish a known minimum background sound level—even as low as 30 dBA—as well as a spectrum or ‘curve’ designed for speech privacy, eliminates the variability of HVAC sound, allowing acoustic goals to be more reliably achieved with the stated composite sound transmission class (STCc ).

System design and tuning
It is important to note this type of integrated acoustic design is only viable when the minimum background level is precisely generated and consistently delivered by the sound masking system.

ASTM E1111, Standard Test Method for Measuring the Interzone Attenuation of Open Office Components, acknowledges variations as small as 2 dBA can significantly influence speech privacy, while other studies indicate even a single dBA affects comprehension by up to 10 per cent and, in almost every situation, impacts articulation index by 0.0333. (See this author’s “Exploring the Impacts of Consistency in Sound Masking” in Canadian Acoustics, 42[3], 2014.)

Variations in spectral quality can have similarly negative effects. Therefore, it is incumbent on those responsible for acoustic planning to ensure the sound masking system is designed and implemented with due consideration for these stringent requirements. A poorly designed or improperly tuned system can allow as much as 4- to 6-dBA variation, meaning the system’s effectiveness is halved in unpredictable areas within the facility.

For example, having a large zone (i.e. more than three loudspeakers or 58 m2 [625 sf]) connected to one set of controls limits one’s ability to adjust the system’s output to meet the specified curve in each closed room and ensure the same masking spectrum is applied throughout the facility. If large zones span multiple private offices and/or meeting rooms, it also prevents one from adjusting the level to suit occupant preferences or needs.

To maximize control over the sound, each closed room should be provided with its own loudspeaker(s) allocated to its own control zone. Each zone should offer precise output adjustments for both volume (i.e. 0.5-dBA increments) and equalization (i.e. third-octave over the specified masking spectrum, which is typically from 100 to 5000 Hz or higher). Lastly, while occupants can be given control over the masking volume within closed rooms—for example, using a programmable keypad—the system should prevent them from setting it lower than the minimum level established for speech privacy within the facility.

Following installation, the vendor should tune all treated areas at ear height (i.e. where occupants experience the masking effects) and provide a detailed report of the results. Although outdated specifications still in circulation might allow for a wide tolerance (e.g. up to 4 dBA), a well-designed and professionally tuned system is able to keep variations in volume to ±0.5 dBA and those in frequency to ±2 dB per third octave, providing dependable coverage throughout an installation.

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