USING FLOOR-TO-CEILING WALLS AND SOUND MASKING
BY NIKLAS MOELLER
K.R. MOELLER & ASSOCIATES LTD.
Closed offices and meeting rooms are built with the intention of providing occupants with both visual and acoustic privacy. While the first goal can easily be achieved, the latter often proves more challenging. Some argue that this failure indicates poor design or construction. However, this position assumes that the correct approach is to rely solely on physical isolation.
In reality, a person’s ability to clearly understand a conversation is dependent on two variables: the volume of the speaker’s voice and the volume of the background sound level within the listener’s space (i.e., the signal-to-noise ratio). Traditional room construction simply reduces the signal. If the background sound level in the adjoining space is lower than the speech entering it, conversations will still be heard and potentially intelligible. With today’s building standards, this is often the case. Depending on a variety of factors—including mechanical system noise—the background sound level in closed rooms usually ranges from the low 30s to 40 A-weighted decibels (dBA).
Furthermore, modern design and construction standards do not always allow for a high level of physical containment. To preserve flexibility, designers often want walls built to below the suspended ceiling or using demountable partitions. They may incorporate substantial windows or even build from floor to ceiling in glass. Construction budgets can also limit wall options.
In any case, even if walls are built deck-to-deck, voices find their way from one room to another through a variety of pathways. Of course, an open door is a room’s biggest Achilles’ heel. No matter how well a room is designed and built, sound isolation virtually disappears in this case. While the walls of a typical private office may range from 40-55 sound transmission class (STC), the ‘cumulative’ STC rating (i.e., the combined performance of all the elements of the physical shell) drops to around 7 STC when the door is open. Sound also passes through the walls themselves, gaps along the window mullions, ceiling and floors, as well as through the plenum, ductwork and return air grills.
THE ROLE OF SOUND MASKING
In almost all situations, it is preferable to combine a reasonable amount of isolation with a raised background sound level introduced via a sound masking system.
This technology basically consists of a series of loudspeakers installed in a grid-like pattern—either invisibly above the suspended ceiling or in an open ceiling—that distribute an engineered background sound throughout the space. Most people equate the sound of a professionally tuned sound masking system with that of softly blowing air, but it is specifically designed to improve speech privacy. It also covers up noises or reduces their disruptive impact on occupants by minimizing the degree of volume change within the space.
COST SAVINGS AND FLEXIBILITY
When sound masking is included as a part of the acoustical planning for closed rooms, organizations can reduce the walls’ STC rating and, in most cases, also use floor-to-ceiling rather than deck-to-deck construction.
If a wall decreases the intrusion of voice into the room by a decibel, then the signal-to-noise ratio drops by a decibel. An identical drop occurs when the sound masking volume is raised by one decibel. Levels for sound masking in closed rooms range from approximately 40-45 dBA, depending on the size of the room and other conditions. In other words, sound masking typically adds approximately 5-12 dBA of ambient volume, which is why one sometimes hears that sound masking ‘adds 10 STC points’ to walls. Masking also provides a measure of increased speech privacy and noise control with the door open.
Budget wise, sound masking may represent one to two dollars of cost per square foot of space, but it offsets much more than that in terms of construction above the ceiling and the ease with which walls can be relocated to suit future needs.
Compared to a wall built from the floor to the suspended ceiling, the additional costs of materials and labor for deck-to-deck construction are obvious, but there are other ways that it adds to the initial budget. Each time a wall is built above the suspended ceiling, the ceiling grid must be restarted. The separated plenum space requires separate return air ducts and may necessitate additional HVAC control zones. Return ducts may need to 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. Furthermore, vigilance must be maintained to ensure that penetrations in the wall’s structure are controlled. Even minor ones can substantially reduce acoustic performance.
In terms of design, each room should be provided with its own sound masking loudspeaker. This loudspeaker should also be allocated to its own control zone; if it is not, it limits the system’s ability to meet the specified masking spectrum or ‘curve’ in each room, as well as the occupant’s ability to adjust the loudspeaker’s output according to their preference. Each control zone should offer precise volume (0.5 dBA increments) and frequency (1/3 octave) adjustments over the specified curve, which is typically from 100 to 5,000 hertz (Hz), and sometimes as high as 10,000 Hz.
The curve used in closed rooms should be identical to that used in the open plan areas; however, the overall volume level will typically be several decibels lower. This provides a good degree of consistency between the open and private spaces, but addresses occupant’s expectation that the ambient volumes in smaller rooms are lower than in large open venues.
The masking level in a closed room will not interfere with communication inside the room itself. The volume of a typical voice is 55-65 dBA at conversational levels. The distance between two people talking in a private office is not sufficient for the masking to interfere with intelligibility. That said, in some circumstances, it may be practical to include an in-room control that permits occupants in private offices to regulate the masking volume, as well as paging and background music, which are typically distributed through the same set of loudspeakers as the masking sound. While such individual control is undesirable in shared open plan areas, these rooms are personal spaces that can be adjusted according to preference.
In-room control can be provided via hardware, such as a programmable keypad or rotary volume control, a software application, or integration with third-party equipment that offers control over other aspects of the room as well. If occupants are given control over the settings in rooms that are shared, such as meeting or conference rooms, it may also be desirable to have those user adjustments reset automatically at certain times, restoring masking and paging volumes to default levels.
There are cases where one may want to implement both deck-to-deck construction and sound masking; for example, in spaces where raised voices or high volume media will be used (e.g., during video or teleconferencing activities), as well as in areas with high security needs (i.e., confidential speech privacy). Also, if the facility features an open ceiling, full height walls are recommended to ensure some degree of inter-zone isolation.
Building cost-effective and flexible closed spaces for true speech privacy can be challenging; however, combining physical barriers with sound masking can ensure effective results while helping to control the cost of initial construction and future changes.
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 at soundmaskingblog.com.