by Elaina Adams | October 1, 2011 10:01 am
By Niklas Moeller, MBA
Everyone has heard the old adage “silence is golden,” but just as with lighting and temperature, the comfort zone for the volume of sound is actually not zero. In fact, if the background sound level in a space is too low, conversations and noise can easily be heard, even from a great distance, impacting speech privacy and disrupting one’s concentration.
Many organizations use a sound masking system to maintain an appropriate ambient sound level in their facilities, which is typically between 42 and 48 decibels (dB) in commercial interiors. This technology consists of a series of loudspeakers, which are installed in a grid-like pattern in or above the ceiling, and a method of controlling their output. While the sound the loudspeakers distribute has been specifically engineered to increase speech privacy, it also covers up intermittent noises or reduces their impact by decreasing the change between baseline and peak volumes. Although the background sound level is technically higher, occupants perceive the space as quieter. Many systems also provide paging and music distribution, eliminating the need for a separate system.
Sound masking systems have been used in various applications for decades, including:
In recent years, they have gained even more popularity because of the increased use of open-plan space and demountable partitions, rising densities, and sustainable design practices––all of which have a significant impact on acoustics.
The field has also changed with the introduction of new sound masking systems. Users are no longer limited to a choice between centralized (Figure 1) and decentralized products (Figure 2), but can now select a digital or networked technology (Figure 3). However, what often gets lost in the shuffle are the key design and performance features that can have a substantial impact on the outcome within each space.
The specification gap
Sound masking is a critical design choice for which one does not want to leave a lot of room for interpretation. After all, when purchasing a system, the user is not seeking the mere pleasure of owning the equipment. Without a set of performance standards, poor procurement decisions can be made. The desired level of speech privacy, noise control, and occupant comfort may be sacrificed, as well as the user’s ability to easily and cost-effectively adjust the system in the future.
To keep the focus on design and performance, the manner in which sound masking systems are specified needs to be updated. Currently, they are often specified according to the aforementioned types, limiting the number of vendors that can bid on a given project. Bidding opportunities are further restricted when the specification incorporates propriety elements such as the dimensions of components, types of inputs/outputs, and other minor details. At the other end of this spectrum are specifications that merely state “provide a sound masking system.” When compared to the manner in which most other building systems such as HVAC or fire alarms are specified, the contrast is striking.
The best practice approach for sound masking is to write a performance-based specification focusing on the qualities that are critical to the system’s effectiveness and occupant comfort.
Key performance criteria
A sound masking system’s performance is determined by the following criteria:
These six elements are vital to every project’s success. Clear requirements can be set for each one, in addition to various masking technologies that can meet those standards. In other words, a specification focusing on these elements allows competitive bids and, providing the terms of the specification are upheld, also ensures a high performance level from the system selected.
Adjustment zone size
Acoustic conditions and user needs vary between private offices, meeting rooms, corridors, and reception areas, as well as across open-plan spaces (Figure 4). Sound masking designs with small adjustment zones (i.e. individually controllable groups of loudspeakers) enable the user to adjust the frequency and volume to meet these diverse needs.
Conversely, designs using large adjustment zones––from eight to hundreds of loudspeakers––require the user to make compromises that increase the system’s effectiveness in some areas while diminishing occupant comfort in others, or vice versa (Figure 5, page 2).
The impact of these compromises is far from minimal. A few decibels of variation in masking volume can dramatically impact the system’s effectiveness, even without taking into consideration the consistency of frequency levels. In many situations, users can expect a 10 per cent reduction in performance for each decibel variation below the target masking volume. A poorly designed system can allow as much as a 6-dB variation (i.e. ±3 dB), meaning the system’s effectiveness will be halved in some areas of the user’s space.
Zone size also affects the ease with which the user can make changes to the system in the future. Churn rates and renovations require systems that can be quickly, easily, and cost-effectively readjusted. Large zones limit the user’s ability to reconfigure the sound masking system without first physically changing its design, moving loudspeakers, or re-wiring parts of the system.
In other words, the most important factor within a sound masking specification is to place an upper limit on adjustment zone size. In this case, less truly is more––one to four loudspeakers in each zone provide a high degree of flexibility.
Masking sound generation
Each small adjustment zone should have a dedicated masking sound generator to avoid a phenomenon called phasing (i.e. uncontrollable variations in masking levels), which occurs when numerous speakers adjacent to each other emit the same masking signal.
To maximize unobtrusiveness, each generator should also provide a sound that occupants perceive as being random (i.e. with no noticeable repeat cycle). The sound produced by the generator must cover the entire masking spectrum of 100 to 8000 Hz––frequency output beyond this range is unnecessary.
Volume adjustment capabilities
The masking sound is greatly affected by the overall workplace design, including the materials used, furnishings, location on the floor, and items above the ceiling. These elements have an impact no matter how the loudspeakers are installed (e.g. upward-facing above a suspended ceiling or direct-facing cut through a ceiling). For this reason, ASTM E 1573-09, Standard Test Method for Evaluating Masking Sound in Open Offices Using A-weighted and One-third Octave Band Sound Pressure Levels, requires measurements to be taken in areas representative of all workspace types.
If the adjustment zones are large, numerous loudspeakers are set to the same output level, but after interacting with the variables in the space as noted above, the masking volume fluctuates. Variations of 2 dB or more call attention to the masking sound, reveal its source to occupants, and diminish results.
Large-zoned designs attempt to mitigate these volume variations by including audio transformers as volume controls on each loudspeaker. However, they only provide rough adjustments of 3 dB each. When the volume cannot be finely adjusted in small areas, users need to set a volume that is best ‘on average,’ compromising comfort or effectiveness at various, unpredictable points across their space.
The specification should call for fine volume controls for each small zone. Increments of 0.5 or even 1 dB enable the user to adjust the sound whenever needed in order to accommodate variable acoustic conditions. The specification should also require the final masking volume be consistent within a range of 1 to 1.5 dB in all areas desired. Again, the benefits are consistent performance and comfort.
Frequency adjustment capabilities
The sound masking system should also provide fine frequency control within each small adjustment zone.
The range of masking sound is generally specified to be between 100 to 8000 Hz. The system should provide control over these frequencies via third-octave adjustment because it is both the industry standard and the basis for masking targets that are set by acousticians.
However, providing third-octave adjustment is not enough when these controls are paired to large adjustment zones. A well-designed system provides equalization for each group of one to four loudspeakers.
As long as the masking system can meet the volume and frequency targets established by the specification, it is not essential to specify the loudspeaker’s size, wattage rating, or other parameters. Yet, it is worth noting very small loudspeaker drivers (i.e. less than 76 mm [3 in.]) are unlikely to generate sufficient levels below several hundred hertz (i.e. down to the required 100 Hz). These low frequencies are necessary to create the full masking spectrum. While they play a relatively minor role in reducing speech intelligibility, they are vital to occupant comfort. Most masking loudspeakers are 102 to 203 mm (4 to 8 in.) in diameter and rated from 10 to 25 watts.
The true gauge of whether the sound masking system ultimately selected is performing as required is gained from post-adjustment measurements.
The specification should require specific results that are measured and documented. Best practice is to require a test in each 93-m2 (1000-sf) area, and have the vendor adjust the sound masking system within that area as needs dictate. (Some systems may be able to outperform this requirement, but it is a good baseline.) Two or three types of measurements should be required:
It is important to remember there are no independent standards for masking performance, only standards relating to measurement such as ASTM E 1573-09. A specification stating the sound masking system is or should be ‘compliant’ with any ASTM standard is misleading. Instead, it is essential that it outlines all the above requirements for masking output.
Depending on their significance to the project at hand, some secondary characteristics may also need to be included in the specification, including:
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 a lower masking volume at a certain time when there are fewer occupants in the facility.
Considerations for the specification include:
Masking systems may also offer a ramp-up feature. It is best to specify this in retrofit situations because it is used to gradually introduce the masking sound, allowing occupants to easily acclimatize to the change in their acoustical conditions.
Beyond masking zones, most systems can be zoned for various 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 advanced planning because a contractor will have to re-cable parts of the system when future changes need to be made. 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.
The method of controlling the system impacts the ease, cost, precision, and amount of disruption associated with making initial and future adjustments. Some designs provide central control over a limited range of features. Others provide central control over a few features and local control over others. There are also designs offering control over all features from a central location.
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).
Depending on the user, security may be another key consideration. In this case, the specification should describe both the physical and electronic security features for the sound masking system.
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, and encrypted communication.
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, they should be covered 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 esthetic 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).
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 UL 1310, Class 2 Power Units, to avoid conduit requirements. Digital masking systems need to meet the electromagnetic interference (EMI) standards.
If sustainability is a goal within the space, users might also voluntarily require compliance with the European Restriction of Hazardous Substances (RoHS) directive, which limits the quantities of certain materials used in the system’s components.
Even if the sound masking technology the vendor proposes adheres to a generally worded design guide, the vendor may intend to implement it in a different manner. Therefore, it is important whenever possible, to require drawings as part of the bid submission process (Figure 6).
These documents can help identify differences between sound masking proposals because they show the components, quantities, and locations, making it easier to spot design shortcuts and subsequently discuss those deviations with the vendor.
Ideally, of course, the drawings should be included as part of the specification itself, allowing the user to set the adjustment zones for each area. For example, there may be areas where the client wishes to use zones smaller than the four-loudspeaker maximum, such as in private offices and meeting rooms. These drawing should be created by the user in conjunction with an acoustical consultant or trusted vendor.
Another useful document to request in the specification is a compliance form. Vendors should be asked to submit a statement indicating their adherence to each aspect of the specification. They should also be required to note any deviations, describing how their system’s design differs.
Own the spec
Acoustics are an integral part of a project’s long-term success and should be planned from the outset. While every sound masking system introduces a sound into the space, overall performance can vary dramatically. A well-constructed specification is essential to ensure the technology and the system’s design meets the user’s current and future requirements (Figure 7). If not, the sound masking system may be ineffectual, underused, or become a source of irritation itself and possibly turned off.
However, 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. Many times the value of a well-designed specification is nullified because no one is asked to ensure all proposals––and, indeed, the system ultimately selected––conform to the desired performance levels.
It is also wise to learn what services are offered in conjunction with each proposal under consideration. The sound masking system should be supported by professionals who can properly design and implement it and provide the user with ongoing support.
Niklas Moeller, MBA, is vice-president of K.R. Moeller Associates Ltd. (Burlington, Ont.), a global developer and manufacturer of sound masking systems for more than 30 years. He has been in the sound masking business since 1998. Moeller can be reached via e-mail at firstname.lastname@example.org.
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