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. Once constructed, the acoustical properties of walls and ceilings cannot be easily changed, and when engineered and installed, neither can the sound masking system’s architecture.
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 percent 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, 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.
To maximize control over the sound, each closed room should be provided with its own loudspeaker(s) allocated to its own control zone, and each zone within open plan should not exceed three loudspeakers or 63 m2 (675 sf). They 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). Following installation, the vendor should also tune each zone 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Although outdated specifications still in circulation might allow for a wide tolerance (e.g. up to 4 dBA), a well-designed and professionally tuned system is able to keep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.
While acoustic professionals have always advocated the ABC Rule of absorbing, blocking, and covering unwanted noise, listing ‘C’ last reinforces the notion it is a final consideration and perpetuates the misplaced emphasis on isolation and absorption strategies when designing for speech privacy. Instead, the approach should be CBA: cover, block, and absorb.
By using sound masking to define and, therefore, know exactly what the background sound level will be anywhere in a facility, one can more accurately specify the remaining materials. Further, the volume can be increased at a later date if more acoustic control is needed to compensate for deficiencies in partition assemblies—a flexibility uniquely afforded by this technology.
Niklas Moeller is the vice-president of K.R. Moeller Associates Ltd., manufacturer of the LogiSon Acoustic Network and MODIO Guestroom Acoustic Control. He has more than 25 years of experience in the sound masking field. Moeller can be reached via e-mail at email@example.com.