Updating Performance Standards

Sound masking is a critical design element for which one does not want to leave a lot of room for error. That is why ASTM Subcommittee E33.02 on Speech Privacy—part of ASTM Committee E33 on Building and Environmental Acoustics—is currently working on proposed standard WK47433, Performance Specification of Electronic Sound Masking When Used in Building Spaces. They are also in the process of updating: • ASTM E1130, Test Method for Objective Measurement of Speech Privacy in Open Plan Spaces Using Articulation Index;
• ASTM E1374, Guide for Open Office Acoustics and Applicable ASTM Standards;
• ASTM E1573, Test Method for Evaluating Masking Sound in Open Offices Using A-Weighted and One-Third Octave Band Sound Pressure Levels; and
• ASTM E2638, Test Method for Objective Measurement of the Speech Privacy Provided by a Closed Room.

Preparing a Performance-Based Spec

Indeed, without a set of performance standards, the user may not achieve the expected level of speech privacy, noise control and occupant comfort.

Preparing or even evaluating a sound masking specification can be challenging, but the payoff is worth the effort. These systems typically have a long lifespan. When properly designed, they help to keep acoustics under control even as densities swell and, if included in a project’s planning stages, they also increase flexibility, typically allowing an organization to remain in their facility for a longer period. In other words, the user can end up living with the system for quite some time.

Currently, sound masking systems are often specified according to the aforementioned types—centralized, decentralized, or networked—limiting the number of vendors that can bid on a given project. Bidding opportunities are further restricted when the specification incorporates proprietary 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 a HVAC or fire alarms are specified, the contrast is striking.

The best approach is to focus on the six qualities that are critical to effectiveness, comfort, and flexibility. Whether one’s role is to write the specification or evaluate an existing document, these performance-based factors provide a solid foundation for a successful sound masking installation in any type of facility:

• Adjustment zone size
• Masking sound generation
• Volume adjustment
• Frequency adjustment
• Loudspeaker requirements
• Measured results

With these six factors forming the framework for the document, one can easily add other requirements, the importance of which varies by project (e.g. the appearance of the loudspeakers in an open ceiling), application (e.g. individual room control in a hospital patient room) or geographical region (e.g. UL 2043 compliance in the United States).

Adjustment zone size

To recap, adjustment zones are groups of loudspeakers for which the technician can establish individual volume and frequency settings. From a performance perspective, one loudspeaker in each zone is ideal, but it can be an acceptable compromise—and more budget-friendly—to have up to three, at least across open plans. In fact, most systems use a mix of these small zone sizes.

Acoustic conditions and user needs vary between private offices, meeting rooms, corridors, and reception areas, as well as across open plans. Designs that use large adjustment zones (e.g. involving more than a few loudspeakers) require one to make ever-increasing compromises. For example, if the volume needs to be raised to improve the system’s effectiveness in one area, it might be too loud in another, affecting comfort. If comfort is desired, the masking’s effectiveness may be diminished in some areas. Furthermore, as shown in the graph, the larger the zone, the greater the number of occupants it affects. The numbers ramp up very quickly, from an average of 1.3 people in a single loudspeaker zone to literally hundreds of people and tens of thousands of square feet in a hundred loudspeaker zone.

Zone size also affects the ease with which the technician can make changes in response to, for example, renovations or moving personnel. Large zones require the system’s design to be altered, which usually involves moving loudspeakers and rewiring.

In this case, less truly is more: one to three loudspeakers in each zone (i.e. 225 to 675 ft2 [30 to 62 m2]) provides a high degree of control and flexibility, enabling technicians to adjust their volume and frequency to achieve a consistently effective and comfortable masking sound.

Masking sound generation

Each small adjustment zone should feature a dedicated masking sound generator in order to avoid a phenomenon called phasing (i.e. uncontrollable variations in the masking levels).

To maximize unobtrusiveness, each generator should also provide a sound that occupants perceive as random (i.e. with no noticeable repeat cycle). If there is no noticeable loop, and the masking can also be finely tuned to suit the needs found throughout the space, occupants do not focus on the sound.

The sound produced by the generator should cover the entire masking spectrum of 100 to 5,000 Hz or as high as 10,000 Hz.

Volume Adjustment

The workplace design, furnishings, and other materials used within it will impact the masking sound regardless of how the loudspeakers are installed. For this reason, the ASTM standard for measuring and evaluating masking performance in open offices (i.e. ASTM E1573, 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 that are representative of all workspace types.

If the zones are large, many loudspeakers are set to the same volume setting, but the masking nonetheless fluctuates across the space as it interacts with the variables mentioned above. Some centralized designs try to mitigate this problem by providing audio transformers on each loudspeaker; however, they only offer coarse adjustments in 3 dBA steps.

When the masking volume cannot be finely adjusted in small areas, the technician needs to set it to a level that is best ‘on average,’ compromising comfort or effectiveness at various unpredictable points across the space. Users can typically expect a 10 percent reduction in performance for each decibel below the target masking volume. A poorly designed system can allow as much as 4 to 6 dBA variation, halving the system’s effectiveness in some areas. Furthermore, these peaks and troughs call occupants’ attention to the sound as they move through the space.

The sound masking system’s performance is further reduced if it must be tuned to a lower overall volume in order to avoid exceeding the typically recommended maximum of 48 dBA in some areas.

Therefore, the spec should call for fine volume control for each small zone of one to three loudspeakers. Increments of 0.5 dB enable adjustment wherever needed in order to accommodate variable acoustic conditions. Also require the final masking volume to be consistent within a range of 1 dBA or less in all areas desired. Again, the benefits are comfort and consistent performance across the space.

Frequency Adjustment

The system should provide fine frequency control for each small adjustment zone. The range of masking sound is generally specified to be between 100 to 5,000 Hz or as high as 10,000 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 set by acousticians.

Loudspeaker Requirements

As long as the 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. However, it is worth noting that very small drivers (i.e. less than 3 inches [76 mm]) are unlikely to generate sufficient levels below several hundred hertz (i.e. down to the required 100 Hz), which are necessary to create the full masking spectrum. While they play a relatively small role in reducing speech intelligibility, they are vital to occupant comfort. The masking loudspeaker drivers should be 4 to 8 inches (102 to 203 mm) in diameter and rated from 10 to 25 watts.

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 performed after installation and initial tuning. Therefore, the spec should require specific results that are measured and documented.

A minimum performance guideline is to require the masking sound to be measured in each 1,000 square feet (90 square meters) open area and each closed room, at a height between 4 to 4.7 feet (1.2 to 1.4 meters) from the floor (i.e. at ear height rather than directly below a loudspeaker). Some systems can adjust for smaller areas, but this is an acceptable baseline. Masking volume is typically set to between 40 and 48 dBA, and the results should be consistent within a 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). A reasonable expectation is ±2 dB variation in each frequency band.

The vendor should adjust the masking sound within that area as needs dictate and provide a final report verifying the final results; the report should also indicate areas where the masking sound is outside tolerance and why (e.g. noise from mechanical equipment or HVAC).