Considerations for closed rooms
Maintaining an adequate background sound level is also important in closed rooms. Generally speaking, an occupant’s expectation of privacy is higher in this type of space than within an open plan; however, doors and even deck-to-deck walls are often not enough to provide it.
Walls, windows, doors, ceiling tiles, and flooring reduce the volume of voice coming through the room’s physical structure, but even minor penetrations can seriously compromise its acoustic performance by allowing sounds to transmit into adjoining spaces. If the background sound level in those spaces is lower than the speech passing through the wall, it will still be possible to hear and understand a conversation. In other words, the degree of speech privacy experienced in closed rooms is still largely determined by the signal-to-noise ratio. While masking levels should be set to achieve between 45 and 48 dBA within an open plan, closed rooms should typically be lower at 40 to 45 dBA.
The doorway is a major challenge for a closed space. Even when closed, the door usually presents the weakest link, but when it is open, it does not matter how well the walls have been constructed, the level of sound isolation dramatically drops. For example, the effective rating of a 50 STC wall drops to 7 when the door to a typical 3-m (10-ft) wide office is opened. Most organizations do not want the doors to private offices to be closed at all times. Sound masking, absorptive materials, and layout (e.g. staggering doorways along a corridor) should be used in order to continue to provide some degree of acoustic privacy when they are open.
Speech security
Of course, eavesdropping can also be intentional, and handled in a much more sophisticated manner than leaning one’s ear against a glass and putting it up to the wall.
Though this article focuses on acoustic privacy rather than acoustic security—such as what may be required by military facilities, corporate boardrooms, or laboratories—it is important to know 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. Further, these types of listening devices are difficult to detect because they can be used at a considerable distance from the target facility.
If an organization suspects 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 the system produces a truly random masking sound (i.e. rather than on a loop) so it cannot be filtered out of recordings.
Conclusion
Attention must be paid to the topic of acoustic privacy within the built environment. Though this conclusion is obvious to organizations consistently dealing with sensitive information, the methods they use to achieve it are the same as those needed to accomplish other valuable acoustic goals—the only difference is how one sees the benefit: that is, from the perspective of the person talking or that of the group listening.
Building occupants 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 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 them to reap both rewards.
Niklas Moeller is the vice-president of K.R. Moeller Associates Ltd., manufacturer of the LogiSon Acoustic Network sound masking system (logison.com). He also writes an acoustics blog, which can be visited at soundmaskingblog.com.
