Closed offices and meeting rooms are built with the intention of providing occupants with both visual and acoustic privacy. While the first goal can be achieved easily, the latter often proves elusive because of the many ways in which sound can transfer from one space to another.

In an attempt to create sufficient speech privacy, companies may specify walls with high sound transmission class (STC) ratings. However, STC ratings are lab tested and frequently overstate real-world performance by up to 10 points. Site-tested Field STC or Noise Isolation Class ratings are better gauges, but can only be tested after the fact.

Another common tactic used to improve speech privacy in a closed space is to completely seal the room by constructing full height walls that extend from the concrete floor all the way to the deck above. While effective, this approach raises costs and reduces flexibility. Vigilance must be maintained during design, construction, maintenance and renovation to ensure that penetrations in the wall’s structure are controlled. Even minor ones can substantially reduce acoustic performance.

These challenges raise the question of whether there are alternate and preferable methods of achieving high levels of speech privacy in closed spaces, such as adding sound masking technology to closed rooms with walls built only to the suspended ceiling.

Cracks in the Armor

Each crack in a wall’s armor facilitates the transmission of sound to and from neighboring spaces. For example, wall performance is sensitive to gaps along the perimeter, such as those that can occur along window mullions or the floor. If light can pass through, so can sound, and often well enough to substantially reduce the wall’s impact. Other “imperfections” weaken the wall’s sound isolating performance; for example, HVAC elements that pass between closed spaces, and even back-to-back electrical switches and outlets. Interior windows also may contribute to sound transfer.

In the case of full height walls, the seal between the top of the wall and the deck must be maintained, which can be difficult if the surface is irregular, such as with a corrugated steel deck. The sound isolation performance of the barrier above the ceiling also may be compromised by penetrations. Openings can exist during construction or be introduced during service work or facility upgrades. Any gaps due to building structure, pipes, conduit, cables and raceways must be carefully managed to ensure the integrity of the wall. This level of care can be challenging to sustain throughout the life of the space.

Moreover, a closed space only provides acoustic isolation when the door is closed. Once open, the barrier provided by the wall is compromised. For example, an STC 40 rated wall with an open door that represents 10 percent of the wall’s area reduces its effective STC to 10. The same is true for STC 45 and 50 walls. If the door represents 20 percent of the wall area—which is the case for a standard 3-foot door in a 10- by 10-foot wall—then the effective STC is just 7.

To avoid making the door the weak link even when it is closed, it must at least match the wall’s STC rating. Any improper seals will provide a convenient route for sound to escape (or enter) the room.

Cost and Flexibility
Full height walls also present financial challenges. On top of the additional costs of materials and labor, there are other ways deck-to-deck construction can substantially add to the initial budget. Each time a wall is built above the suspended ceiling, the ceiling grid must be restarted, which is a time-consuming process. The separated plenum space requires separate return air ducts and may necessitate additional HVAC control zones. Return ducts must be treated to prevent the transfer of sound along their length from one location to another.

It is also more difficult and costly to renovate, because moving such a wall requires changes to the ceiling grid, tiles and HVAC returns.

Constructing and moving floor-to-ceiling walls is a much simpler and less costly exercise. Modular wall systems permit even more rapid relocation. However, both open up a pathway for the transmission of sound. Typically, the acoustical tile has a lower attenuation rating than the wall. Sounds pass through it, reflect from the deck above and down into the neighboring space. In this case, a tile with a ceiling attenuation class (CAC) of 35 to 40 is recommended. It lessens the flanking of sound through the ceiling and plenum, but the room is still subject to acoustic leakages.

The Speech Privacy Equation
Whether built from floor to ceiling or deck to deck, walls only address part of the speech privacy equation. A person’s ability to clearly understand a conversation is actually dependent on two variables: the volume of the speaker’s voice and the volume of the background sound level in the space. The relationship between the two is called the signal-to-noise ratio.

Traditional closed room construction attempts to provide privacy by simply reducing the signal. Even if a deck-to-deck wall is well-designed and constructed (i.e., all penetrations are addressed), it still may not provide the desired level of speech privacy. If the background sound level in the adjoining space is lower than the sound passing through the wall, noises and conversations still will be heard and potentially intelligible. With today’s building standards, this is often the case.

Sound masking technology can provide an effective background level throughout the space. This type of system consists of a series of loudspeakers that distribute an engineered sound. Though most often compared to softly blowing air, this sound is designed to mask the frequencies in human speech. It also covers up incidental noises that otherwise would impact occupants’ comfort and concentration.

Calculating the Benefits
Sound masking technology can be used in combination with walls built to the suspended ceiling or demountable partitions in order to provide a cost-effective and more flexible alternative to deck-to-deck construction.

Budget wise, sound masking may represent $1 to $2 of cost per square foot of space, but it offsets much more than that in terms of construction above the ceiling. The ability to provide private rooms with walls to the ceiling also increases the ease and cost-effectiveness of relocating them to suit future needs.

Depending on a variety of factors, including mechanical system noise, the background sound level in closed rooms without sound masking usually ranges from the low 30s to 40 dBA. Levels for sound masking in closed rooms range from approximately 40 to 45 dBA, depending on the size of the room and other conditions. In other words, sound masking typically adds approximately 5 to 12 dBA of ambient volume, which is why it’s sometimes reported that sound masking adds 10 STC points to walls.

Given that the ceiling tile in closed spaces already attenuates sound transmission to neighboring spaces, in most cases, it is unlikely that extending the wall above the ceiling will produce a greater increase in attenuation. This is particularly true if the space’s design follows the recommendation for tiles with a CAC of 35 to 40. Of course, if wall and ceiling specifications are low, then benefits may be found by upgrading those elements while also controlling the background sound levels.

Special Considerations
It’s possible to implement both deck-to-deck construction and sound masking; for example, in spaces where raised voices or high-volume media will be used for video or teleconferencing activities, as well as in areas with high security needs that require confidential speech privacy. Also, if the facility features an open ceiling, full height walls are recommended to ensure some degree of inter-zone isolation.

It is also important to note that each closed space is its own environment. Therefore, the sound masking system should allow for localized control of the masking frequencies and volume in each of these areas. If it does not and several offices are bundled together into a single adjustment zone, compromises may need to be made in terms of the settings, risking performance and occupant comfort. Current technologies can offer control for each individual space.

Building cost-effective and flexible closed spaces for true speech privacy can be challenging. Combining physical barriers with sound masking can ensure effective results while helping control the cost of initial construction and future changes. In most situations, sound masking provides not only the cost and flexibility advantages, but also as good or better speech privacy.

Niklas Moeller  is vice president of K.R. Moeller Associates Ltd., Burlington, Ontario. For more information, visit www.logison.com or www.soundmaskingblog.com.