by Niklas Moeller — Many organizations use sound masking to maintain an appropriate ambient level in their facilities, which is typically between 42 and 48 decibels (dB) in commercial interiors.
This technology basically consists of a series of loudspeakers, which are installed in a grid-like pattern above the ceiling, as well as a method of controlling their output. The sound the loudspeakers distribute has been engineered to increase speech privacy; however, it also covers up intermittent noises or reduces their disruptive impact on occupants. Many sound masking systems also provide paging and music distribution, eliminating the need for a separate product.
Over the last decade, sound masking systems have gained in popularity due to increased use of open plan, rising occupant densities and sustainable design practices—all of which have a significant impact on acoustics. The field has also changed with the introduction of networked sound masking architecture. Users are no longer limited to centralized and decentralized systems.
While any sound masking system can introduce a sound into a facility, in order to ensure it works as expected, a clear set of performance standards should be established prior to procurement. A specification that focuses on the following qualities allows competitive bids and, if its terms are upheld, ensures a high level of performance from the system ultimately chosen.
1. Small adjustment zones
The most important factor is to place an upper limit on zone size. Small zones of one to three loudspeakers (covering 225 to 675 ft2) provide a high degree of flexibility. The technician can make localized adjustments so that the specified sound masking spectrum or curve is met throughout the space. If the system uses larger zones, numerous loudspeakers are set to the same volume and frequency, and compromises must be made between performance and comfort. The greater the number of loudspeakers within each adjustment zone, the larger the area of compromise and the greater the number of people affected by it.
2. Decentralized masking sound generation
To avoid ‘phasing’ (i.e. uncontrollable variations in masking levels), each small zone should have a dedicated masking generator. In addition, each generator should provide a truly random sound (i.e. no repeat cycle). The sound must cover the entire masking spectrum — typically specified as 100 to 5,000 hertz (Hz), but sometimes as high as 10,000 Hz — providing both comfort and effectiveness, and covering a wide range of noises.
3. Fine volume and frequency control
Fine volume control (i.e. in 0.5 decibel steps) should be provided for each small zone, enabling the technician to make adjustments for local conditions. The system should also provide fine frequency control within each small zone. The range should cover the entire masking spectrum and control should be via third-octave adjustment, which is the industry standard and basis for masking targets set by acousticians.
4. Suitable loudspeaker driver
Loudspeaker drivers should be 4 to 8 inches in diameter and rated from 10 to 25 watts in order to ensure they can generate the low frequencies that are vital to occupant comfort.
5. Measured results
The process should not end as soon as the system is selected. The true gauge of whether it is performing as expected is gained from measurements done after installation and initial tuning. Masking volume is typically set to between 40 and 48 dBA in commercial interiors, and the results should be consistent within an overall range of 0.5 dBA or less. The curve should be defined in third-octave bands and range from 100 to 5,000 Hz (or as high as 10,000 Hz). 2 dB variation in each frequency band is a reasonable expectation.
Another important aspect of the specification concerns the system’s certifications. Though not critical to performance per se, they are essential to meeting regulatory requirements.
Sound masking systems must meet Underwriters Laboratories (UL) or similar standards for electrical safety. Any components installed in air-handling plenum or via cut-throughs in a suspended ceiling must also be tested to meet UL 2043, Standard for Safety Fire Test for Heat and Visible Smoke Release for Discrete Products and Their Accessories Installed in Air-Handling Spaces. Cables must be plenum rated. If using low-voltage power supplies, these should conform to the UL1310 standard for Class 2 power sources in order to avoid conduit requirements. Digital masking systems need to meet electromagnetic interference (EMI) standards.
Depending on their significance to the project at hand, the sound masking specification also may need to include secondary characteristics such as:
Timers automatically adjust the masking volume to vary in anticipation of noise levels throughout the day, balancing effectiveness and comfort. For example, the user may want the masking volume to lower at a certain time of day when there are fewer occupants in the facility.
Beyond masking zones, most systems can be zoned for a variety of functions, including paging and timer functions, as well as local occupant control (e.g. in a meeting room). In this case, the type of zoning is relevant. For example, hardwired zones require advance planning because a contractor has to re-cable parts of the system when changes need to be made in the future. Digital zones can usually be re-assigned without altering the system’s physical design. Less planning is required from the outset because any changes can be made in minutes.
Most users make significant changes to their space over time — to department location, demountable partition placement or furniture system configuration — and it is important to consider how the corresponding changes will be made to the sound masking system. 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 and/or software). In-room occupant control may be desired for private offices, conference rooms and other closed spaces.
Physical features can include housing below-ceiling equipment in locked enclosures and also ensuring enclosed rather than exposed cabling connections. Electronic measures can include monitoring, password-controlled access, encrypted communication and more. If security is a concern, additional masking generators and longer generation cycles are better because short cycles can easily be filtered out of recorded conversations.
Many sound masking systems can provide simultaneous overhead paging and background music functions. If the user requires these features, cover them in the specification.
When installed in an open ceiling, the system’s appearance should be considered, including the look of the loudspeakers (e.g. an industrial aesthetic or similar to a lighting pendant), the cable and cable connections, as well as the loudspeaker suspension methods (e.g. chain or a braided steel cable).
Own the spec
Of course, even with a well-written specification, the user could end up with a non-conforming system unless the specifier, user or another person involved in the design and procurement process is appointed as a guardian whose responsibility it is to ensure bids meet the criteria outlined.
It is also important that they 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 ongoing support.
For a sample of a generic performance-based specification that is also CSI-compliant, see: http://www.soundmaskingspecs.com/home.html
Niklas Moeller is the vice-president of K.R. Moeller Associates Ltd., manufacturer of the LogiSon Acoustic Network sound masking system (www.logison.com). He has over 25 years experience in the sound masking field and also writes an acoustics blog at www.soundmaskingblog.com.