Acoustic Privacy

Incorporating sound control into the built environment
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Sponsored by LogiSon Acoustic Network
By Niklas Moeller
This test is no longer available for credit

Demonstrating the Impact

In Diagram 1, the area of intelligibility around a speaker is not circular. In Diagram 2, when sound masking is applied, the area of intelligibility shrinks.

Images courtesy of KR Moeller Associates Ltd.

In Diagram 1, the area of intelligibility around a speaker is not circular. Its shape is determined by numerous factors including, the orientation of the person speaking as well as the physical barriers and absorptive/reflective materials used within the space. In Diagram 2, when sound masking is applied, the area of intelligibility shrinks.

As shown in Diagram 1, 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.

Two workstations to test how much of an impact sound masking has on speech intelligibility.

Image courtesy of KR Moeller Associates Ltd.

In Diagram 3, AI tests were conducted between these two workstations to determine how much of an impact sound masking has on speech intelligibility, even within an otherwise acoustically well-designed space.

In any space, voices and noises diminish in volume over distance. However, background sound levels are often so low in indoor environments that speech carries intelligibly over 30 to 50 feet (9 to 15 meters) or more in open space. By increasing the background sound level, sound masking reduces the signal-to-noise ratio (SNR). As shown in Diagram 2, voices disappear below the new level after a much shorter distance.

Graph of the results of the AI tests.

Image courtesy of KR Moeller Associates Ltd.

The results of the AI tests show that, despite using absorption and blocking strategies, speech comprehension remains nearly 85 percent until sound masking is applied. Comprehension drops by an average of 10 percent for each decibel of increase in the masking volume.

The exact length is, of course, a function of the entire acoustic design of the space. However, as illustrated by the AI measurements conducted between the two workstations shown in Diagram 3, sound masking plays an integral role. This open-plan area’s acoustical design was suitably planned. The partitions are 65 inches (1651 mm) tall and perform well in terms of both absorption and isolation. The ceiling tiles are highly absorptive (0.95 NRC). The lighting system is indirect so as to not reflect too much voice/noise back down into neighboring work areas. A sound-masking system is installed above the suspended ceiling.

Graph 3 shows the results of the AI tests conducted between the two workstations. Despite the high-performance acoustical design elements, speech comprehension is nearly 85 percent 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 percent.

The AI tests also illustrate the importance of keeping tolerance as low as possible and consistently meeting the specified sound-masking curve throughout the facility. In this example, two occupants sit approximately 4.7 m (15.5 ft) apart within the open plan. As indicated by Graph 3, with the masking set to 48 dBA with a narrow tolerance of ±0.5 dBA, the listener can understand just 14 to 25 percent. When a broader tolerance of ±2 dBA is applied, the listener can understand up to 59 percent—barely an improvement over the unmasked conditions.

A Word About Workstations

While sound masking is increasingly at the forefront of open-plan design, another important strategy—blocking—is disappearing from these areas. This trend has had a dramatic impact on their acoustic performance because though other treatments can reduce overall volume levels and deal with noises generated from farther away, they have no effect over short distances. When barriers are dispensed with, local noise sources remain highly intelligible and disruptive. The ability to see and be seen further reduces privacy due to our natural capacity for lip reading.

Again, though some might argue that privacy is not expected nor needed within the open plan, understandable speech disrupts occupants’ concentration. For this reason, workstation partitions should be no lower than seated head height (1524 to 1651 mm; 60 to 65 in.). Even the direction in which a person faces affects his or her voice’s volume within the neighboring workspace; therefore, occupants should be seated facing away from each other on either side of partitions.

Speech Security

Though this course focuses on acoustic privacy rather than acoustic security (such as may be required by military facilities, corporate boardrooms, laboratories, and so on), it may be interesting for readers to know that—without the proper treatment—windows, doors, ducts, pipes, floors, ceilings, and walls present opportunities for electronic forms of eavesdropping. Speech causes vibrations on these structures, which can be picked up by probes or microphones and translated into intelligible speech. These types of listening devices are difficult to detect because they can be used at a considerable distance from the target facility.

Sound masking can apply to structures to help protect privacy

Voices cause vibrations in windows, doors, pipes, and walls, which can be picked up by audio surveillance equipment and translated into intelligible speech. Sound masking can be applied to these structures in order to help protect privacy.

If an organization suspects that it might be subject to such a threat, a sound-masking system can be connected to transducers, which transfer the masking sound to the aforementioned physical structures, impeding the use of audio surveillance equipment. In this case, it is key to ensure that the system produces a truly random masking sound (i.e., rather than on a loop) so that it cannot be filtered out of recordings.

In Conclusion

Attention must be paid to the topic of acoustic privacy within our built environment. Though this conclusion is obvious to organizations that consistently deal with sensitive information, the methods they utilize to achieve it are the same as those needed to accomplish other valuable acoustic goals—the only difference is how one sees the benefit: from the perspective of the person talking or that of the group listening. People working in an acoustically comfortable environment have an easier time concentrating on their tasks and also suffer less stress and fatigue. An organization may decide that it is more motivated by the need for a high-performance workplace than acoustic privacy, but taking the steps required to lower speech intelligibility allows it to reap both rewards.

Background sound is key to achieving this goal. By turning the traditional three-tiered approach of absorb, block, and cover—collectively known as the ABC Rule—on its head and using ‘C’ as the starting point for interior planning, building professionals can more accurately specify the blocking and absorptive elements used in their designs. In other words, this predictable minimum level becomes the canvas on which the rest of the acoustic plan is painted, allowing it to be delivered in a more cost-effective manner and with greater assurance of achieving the intended results.


Niklas Moeller is vice president of KR Moeller Associates Ltd., manufacturer of the LogiSon Acoustic Network and MODIO Guestroom Acoustic Control. He has more than 25 years experience in the sound-masking field.


LogiSon Acoustic Network logo. Today’s interiors are even more dependent on sound masking for speech privacy and noise control. The LogiSon Acoustic Network is tuned using TARGET, an application that accurately adjusts each small zone to the specified spectrum, maximizing benefits and occupant comfort. Worldwide distributors provide turnkey services and support. www.logison.com

 

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Originally published in Architectural Record
Originally published in June 2018


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