The increased use of open floor plans, movable walls and sustainable design practices make sound masking systems an indispensible part of today’s interior landscape. While any system will introduce a background sound into the client’s space, the manner in which it is implemented and the types of adjustments it offers dramatically impact its ability to provide speech privacy, noise control and occupant comfort, as well as be manipulated to accommodate future needs.
A well-developed specification that focuses on the system’s performance is essential to ensuring its benefits are not left to chance. Six elements are vital to the success of every installation: zone size, sound generation, volume adjustment, frequency adjustment, speaker characteristics and measured results.
If the specification focuses on these qualities rather than a particular type of sound masking system, the pool of possible vendors grows. It also increases the likelihood the system will deliver on its promise of solving any number of acoustic issues.
Zone SizeIt is important to place an upper limit on zone size (i.e., groups of speakers) in the specification. The smaller the zone size, the better.
One to four speakers in each zone provides a high degree of flexibility, allowing the user to adjust frequency and volume to meet different acoustic conditions and occupant needs in reception areas, closed offices, boardrooms, hallways and across open floor plans. It also allows changes to be made quickly following renovations or the relocation of departments or personnel.
Conversely, designs featuring large zones—from eight to dozens or even hundreds of speakers—require the client to make compromises. If one area requires a particular performance level, other areas may have to endure louder volumes or diminished effectiveness. Large zones also restrict the user’s ability to alter the system’s settings without first moving speakers or changing the cabling.
Sound Generation
Each small zone should feature a dedicated masking sound generator. Otherwise, phasing can occur (i.e., when identical sound waves meet and interfere with each other, causing noticeable variations in the masking level). Phasing can be circumvented when each speaker is installed adjacent to another that is supplied by a different generator.
The sound produced by each generator should cover the entire masking spectrum
of 100 to 8,000 Hz.
In addition, each generator should provide a sound that occupants perceive as random. Its actual generation can be pseudo-random, but the repeat cycle should be as long as possible. If there is no noticeable loop and the masking can be finely tuned to suit the needs of the entire space, occupants will not focus on the sound.
Volume AdjustmentThe materials used in the building’s design, the location on the floor plate, the items above the ceiling and the furnishings will impact the masking sound regardless of where the speakers are installed (e.g., upward-facing above a suspended ceiling or downward-facing cut through a ceiling).
If the zones are too large, these variables cause the masking volume to fluctuate across the space, despite the fact numerous speakers are set to the same exact level. Variations of more than 1 or 2 decibel A-weighting (dBA) can call attention to the sound and reveal its source, which is contrary to the system’s goals. The level of speech privacy and noise control also can be compromised.
To provide more local volume control, some large-zoned designs offer audio transformers on each speaker. However, these adjustments are 3 dBA each, which is significant.
To offset the impact of the surroundings and achieve acoustic consistency throughout the space, the specification should call for fine volume adjustments in each small zone. Increments of 0.5 or even 1 dBA enable the user to adjust the volume in order to accommodate variable acoustic conditions.
The specification also should stipulate the final masking volume is consistent within a range of 1 to 1.5 dBA in all areas desired. As a result, the sound will be less noticeable and its source less discernable to occupants.
Frequency AdjustmentThe sound masking system should provide fine frequency control within each small zone.
The system should provide control over frequencies between 100 and 8,000 Hz, the range of masking sound. A wider range is unnecessary.
Frequency control also should be implemented in a way that provides third-octave adjustment. This ensures the design provides a control for each one-third of an octave frequency “band” throughout the masking spectrum, making it easier to achieve the goals for each area.
Speaker CharacteristicsIf the sound masking system can meet the volume and frequency targets established by the specification, it is not essential to specify the speaker’s size or wattage rating. However, very small speaker drivers (e.g., less than 3 inches) are not likely to generate levels below a few hundred hertz. These low frequencies are necessary to create the full masking spectrum. While they do not significantly contribute to reducing speech intelligibility, they are vital to occupant comfort. Most masking speakers measure between 4 and 8 inches in diameter and are rated between 10 and 25 watts.
Measured Results
The specification should request measured results for the entire space. Best practices require measurements for each 1,000-square-foot area. Some systems may be able to outperform this requirement, but it is a good baseline. Measurements should include:
- Overall volume and variation tolerances. Masking volumes typically range between 42 and 48 dBA, depending on the type of space and the client’s performance requirements. The results should be consistent within a range of 1 to 1.5 dBA or less.
- Masking frequency curve.A general curve exists, created in third-octave bands, that the acoustical industry considers effective and comfortable. The specification should set maximum variations for each frequency band. For example, a +/– 2 dBA variation is a reasonable expectation.
- Temporal uniformity.This refers to the consistency of the masking volume over time. While this attribute can be assessed, it is usually not an issue and is less frequently specified and evaluated.
No independent standards for masking performance exist, only standards relating to measurement, such as ASTM E1573-09. Therefore, the specification must outline all requirements for masking output. A specification that claims the sound masking system should “comply” with ASTM standards is misleading.
Additional ConsiderationsThe following features aren’t essential to masking performance, but may be required in the client’s application.
- Timer functions. A timer allows the client to program the masking volume to vary in accordance with anticipated activity levels throughout the day. For example, the client may want the masking volume to lower when there are fewer occupants in the facility. Aspects to consider include the degree to which the timer function varies the rate of volume change; the number of independent timer zones provided by the design and if those zones are hard-wired or digital; whether daily schedules should be independent; and if unique schedules can be programmed for specific days of the year (e.g., holidays).
- Zoning methods. In addition to providing adjustment zones for masking, most systems provide zones for paging and timer functions, as well as local volume switches. Specifying a design that provides a large number of small zones for each of these functions offers the greatest flexibility. In this case, the type of zoning is as relevant as the size. For example, hardwired zones require advance planning because a contractor has to re-cable parts of the system when changes need to be made. Digital zones usually can be re-assigned without altering the system’s physical design.
- Control methods. The method of controlling the speaker zones and their output impacts the ease, cost, precision and amount of disruption associated with making changes to the sound masking system. Some designs provide central control over a limited range of features. Others provide central control over a few features and local control over others. Other designs offer control over all features from a central location. The specification can include the types of features and settings that need to be controlled and from what kind of access point (e.g., hardware or software).
- Security features. The specification should describe both the physical and electronic security features for the sound masking system. Physical features can include keeping below-ceiling equipment in locked enclosures and ensuring cabling connections are made inside the enclosures rather than being exposed. Electronic measures can include monitoring, password-controlled access and encrypted communication.
- Paging functions. Many sound masking systems can provide simultaneous overhead paging and background music functions.
- Aesthetics. If the masking system will be installed in an open ceiling, its appearance should be considered. This part of the specification could include details regarding the look of the speakers (e.g., an industrial look versus something similar to a lighting pendant), as well as that of the cable, cable connections and speaker suspension methods (e.g., braided steel cable rather than chain).
Safeguarding the SpecEven if the sound masking system the vendor proposes can provide one to four speaker zones in principle, it may choose to implement it in another manner. Requesting drawings that show specific components, their quantities and locations can make it easier to compare bids, as well as to identify deviations and discuss changes with the vendor.
Another useful document to request is a compliance form. Vendors should be asked to submit a statement indicating their adherence to each aspect of the specification. They also should be required to note any deviations and describe how their system’s design differs.
Someone involved in the design and procurement process should be deemed responsible for ensuring bids meet the criteria outlined in the specification. It is also important to determine what services are offered in conjunction with each of the proposals under consideration. The sound masking system should be supported by professionals who can properly implement it and provide the client with ongoing support.