The impact of background sound levels
Most people are familiar with using walls, doors, workstations, and a well-planned layout to physically block voices and noises, as well as the benefits of installing ceiling tiles, wall panels, and soft flooring to absorb them. Fewer understand the role sound masking plays in achieving acoustic privacy.
As shown in Figure 2 the area of intelligibility around an individual is not a simple circle. Rather it is a complex shape determined by numerous factors including the speaker’s orientation, physical barriers, and absorption/reflection of the voice by the various interior finishings, furniture, and other items within the space.
In any space, voices and noises diminish in volume over distance. However, background sound levels are often so low in indoor environments speech carries intelligibly over 9 to 15 m (30 to 50 ft) or more in open space. By increasing the background sound level, sound masking reduces the signal-to-noise ratio. As shown in Figure 3, any voices will disappear below the new level after a much shorter distance.
The exact length is, of course, a function of the space’s entire acoustic design. However, as illustrated by the AI
measurements conducted between the two workstations shown in Figure 4, sound masking plays an integral role.
This open-plan area’s acoustical design was suitably planned. The partitions are 1650 mm (65 in.) tall and perform well in terms of both absorption and isolation. The ceiling tiles are highly absorptive (i.e. 0.95 NRC). The lighting system is indirect so as to not reflect too much voice/noise back down into neighbouring work areas. A sound masking system is installed above the suspended ceiling.
Figure 5 shows the results of the AI tests conducted between the two workstations.
Despite the high-performance acoustical design elements, speech comprehension is nearly 85 per cent when the sound masking system is off, because the existing background sound level is only 40.6 dBA. When the system is turned on, comprehension quickly declines. In fact, for each decibel of increase in masking volume, comprehension drops by an average of 10 per cent.
When adding sound masking, it is important to ensure the system is both designed and tuned so as to provide consistent coverage throughout the space. Outdated specifications might allow for a wide tolerance (e.g. up to 4 dBA), but as indicated by Figure 6, such variations in masking levels permit a swing of 40 per cent or more in performance. Modern, well-tuned sound masking systems are able to keep variations to just 1 dBA or less, providing dependable coverage throughout the installation.
The masking sound must be tuned to meet a sound masking spectrum or curve, which is specified by an acoustician or provided by an independent party such as the National Research Council of Canada (NRC). The specified tolerance indicates by how much the sound is allowed to deviate from that curve. The introduction of decentralized-networked technologies over the last decade has made it possible to keep variations to just ±0.5 dBA, providing a much higher level of consistency in the masking sound across a facility.