Noise Control

Noise control is yet another consideration for private offices. If sound masking is only used in open areas, the lower ambient level within closed rooms exposes occupants to the disruptions caused by conversations and activities occurring outside their space. These interruptions force them to either endure the noise or close the door, which can be interpreted as anti-social.

Because sound masking works ‘at the ear of the listener,’ it is effective against noises or conversations regardless of how they find their way into the room and may, therefore, eliminate or reduce the need to address other acoustical pathways between spaces (e.g., sealing gaps between the walls and window mullions). This quality also makes sound masking a potentially effective tool against noises originating from outside the building. Whereas the shell of the building may not completely block the noise of traffic or passing aircraft, a masking system often easily covers these sounds or lessens their effect on occupants.

Acoustic Consistency

Achieving acoustic consistency throughout the facility is another reason that sound masking should not be limited to open plans. The system’s role is to control the acoustic conditions in the same way that one controls temperature and lighting. One does not want cold or dark areas and, similarly, one should strive to achieve a consistent acoustic environment—not have a low ambient volume in one area and an effective one in others.

If sound masking is excluded from closed rooms, there are dramatic differences between the ambient levels in these spaces and those of the open plan. The overall background sound level can differ greatly, often by as much as 10–12 dBA. The spectrum also varies. These changes call occupants’ attention to the masking system as they enter and exit treated areas.

In other words, intentionally omitting masking from particular areas of the facility runs contrary to a key design goal: ensuring the masking sound is as unobtrusive as possible. Occupants of masked spaces are supposed to forget that it is present. If sound masking is installed in all occupied areas of a facility, a more uniform sound level is maintained that, by virtue of consistency, is not noticed by occupants.

Designing Masking for Closed Rooms

As noted earlier, not all system architectures can provide effective and comfortable masking sound in the fragmented and individual environments presented by private offices and other closed rooms. So, what is current best practice in these areas?

First, each room should be provided with its own loudspeaker. In open plans, loudspeakers are typically located according to a standard grid at 15-foot spacing. Including closed rooms in this pattern leads to reduced localized control because one loudspeaker may span more than one room.

Second, the loudspeaker should be allocated to its own control zone. Ideally, this means that it is fed by a dedicated masking sound generator and that it also has dedicated volume control and third-octave equalization. Having a number of loudspeakers connected to a shared set of controls inherently limits the system’s ability to meet the specified masking curve in each office. It also limits the ability to adjust the loudspeaker’s output (e.g., masking and paging volume) according to the occupant’s preference.

Third, each zone must have precise output adjustments for volume and equalization. Third-octave equalization over the specified range of the masking spectrum is necessary, which is typically from 100 to 5,000 Hz, or as high as 10,000 Hz. Precise volume control is also needed. Modern masking technologies provide fine steps (e.g., 0.5 dBA increments) for individual zones.

In terms of system commissioning in private offices, the masking spectrum should be identical to that used in open plans; 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. Overall masking volumes in private offices usually range from 40 to 45 dBA, whereas they max out at 48 dBA in open-plan workspaces.

This best practice system design ensures that masking levels are more consistent between offices with respect to volume and spectrum, leading to uniform performance and greater occupant comfort.

That said, regardless of the sound-masking system’s design, where its loudspeakers are located, or whether they face upward or downward, the sound broadcasted by the loudspeakers changes across the interior as it interacts with various elements, such as the layout and furnishings. In order to meet the specified sound-masking curve, the system must be tuned.

Tuning should be handled after the ceilings and furnishings are in place, and with mechanical systems operating at daytime levels. Because activity and conversation prevent accurate measurement, it should also be done prior to occupation of the facility or after hours. The exact method varies by product, but basically, the acoustician or technician uses a sound level meter to measure the masking sound at ear height (i.e., the level at which occupants experience its effects), analyzes the results, and adjusts the volume and frequency settings accordingly. They repeat these steps until they meet the specified curve at each tuning location.

Though this process can be time consuming, it is essential to ensuring the expected benefits are equally enjoyed by all occupants across the facility.

Providing Occupant Control

Often, it is also practical to include an in-room control that permits occupants in private offices and meeting rooms to regulate the masking volume, as well as paging and background music. While such individual control is undesirable in shared open-plan areas, these rooms should afford a measure of personal control.

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. However, when such controls are offered, there are additional functional considerations. For instance, the user should not be given unfettered control over masking volume settings. If the user is allowed to mute or lower the volume beyond a certain limit, speech privacy will suffer. Also, frequency control should not be included because the room occupant has neither the tools nor the training required to make informed adjustments to the masking spectrum.

If occupants are given control over the settings in rooms that are shared, such as meeting or conference rooms, then it may also be desirable to have those user adjustments reset automatically at certain times, restoring masking, and paging volumes to default levels.