Understanding acoustics leads to the inclusion of sound masking in office designs
Studies show that acoustics are an essential consideration in meeting what is arguably the primary goal of the office: to provide a setting conducive to optimal work performance. Participants indicate that speech privacy and comfort are key to this type of environment, and that noise problems influence both their concentration and error rates by decreasing their ability to think clearly and communicate effectively.
In fact, a survey of 400 business managers conducted by the Building Owners and Managers Association (BOMA) and the University of Maryland identifies noise control as the greatest opportunity for productivity improvements with an estimated average increase of 26 percent. The findings of another survey, commissioned by the American Society of Interior Designers (ASID), are similar. More than 70 percent of respondents said they would be more productive if their office was quieter. Such statistics suggest that the benefits of an effective acoustic environment on workplace satisfaction and profitability would be substantial.
Discussions of noise control methods in offices usually revolve around reducing noise at the source and the “ABC Rule,” meaning Absorb, Block and Cover Up. For years, facility designers applied only the first method in an attempt to reduce noise from outside sources, mechanical and office equipment. When these noises, as well as those produced during everyday activities, were not sufficiently diminished, designers then attempted to use the second and third methods to minimize their impact on office inhabitants. Today, the tendency to rely upon only three methods of noise control continues in many facilities. It is the exclusion of the fourth method, sound masking, that prevents these facilities from achieving the desired acoustical conditions.
Though sound masking systems have been used for over twenty-five years and in thousands of facilities worldwide, they remain novel to many people; therefore, it is useful to provide a brief description of this technology. Basically, a sound masking system consists of a series of speakers that distribute an electronically generated background sound within a facility. Because the human ear cannot perceive simultaneous sounds of similar volume and frequency, unwanted noises are more difficult, or impossible, to hear or comprehend.
Many people refer to such systems as “white noise systems;” however, this is a misnomer. The term “white noise” describes a very specific type of sound used in early masking systems developed in the 1970s. These systems were unsuccessful due to their inflexibility and the irritating hissing quality of the sound they produced, but the name became widely adopted. Newer sound masking products do not use a white noise signal; rather, they offer an engineered sound that is much more comfortable, unobtrusive and effective.
As with all acoustical treatments, the benefits of a masking system are maximized when used in conjunction with other noise control methods. Since sounds decay over distance, masking requires some distance to become effective, and this distance is minimized when sufficient physical barriers and absorptive materials are used in the design of the office. Furthermore, if absorption is increased, the masking system’s volume can be reduced without affecting its performance level. Use of a sound masking system can reduce costs by eliminating the need for additional insulation, extra layers of drywall, plenum barriers, high-spec walls, or permanent walls around private offices. In this way, masking also maintains the flexibility of the office space for future renovations and changes. In open plan space, masking can help maintain a level of acoustical control as density increases and workstation partitions become lower.
Sound masking greatly contributes to four key acoustical requirements: acceptable average sound level, low signal to noise ratio, low dynamic range, and maximum consistency in the acoustics across the space. This fact becomes clear when we consider those requirements and analyze one company’s attempts to fulfill them.
This company recently completed their new headquarters in Europe and was unhappy with the results with respect to noise. The office was finished with reflective surfaces on ceilings, walls and floors and no sound masking system had been installed. While sound masking and absorptive materials had been introduced during the planning stages of the project, they were eventually excluded based on poor advice from one of the professionals involved.
After the company moved in, they discovered that noise levels in their office were quite high due to the reflection of sounds from the hard surfaces back into the space. Sounds traveled a great distance as they bounced off the ceilings, floors and walls. The space was very difficult to work in. It was hard to concentrate. There was a modest amount of privacy, but only due to the high levels and quantity of noise. The company was forced to reevaluate and renovate their new facility.
How would you resolve these issues?
The first requirement is to create an average sound level that is high enough to mask noises and yet low enough to be comfortable. Neither the average sound level nor the noise floor (i.e. the lowest sound pressure levels present in a space over a period of time) should be too high or too low because overly loud environments are irritating and tiring, while overly silent environments provide little noise control or speech privacy. In a library, for example, the slightest level of activity instantly eliminates the apparent quiet provided by near silence. Since offices exist for active purposes, silent environments are not an option. Yet the typical response to acoustic issues such as those in our case study is to try to achieve silence by using the first three of the four methods of noise control: reducing and blocking noises, and adding absorptive materials to the space.
In our example, the company was advised that they could lower the noise levels in their office by completely renovating the area with absorptive materials. This action was a positive first step, but it was not sufficient to provide an acceptable acoustic environment. Absorptive ceiling tiles and furniture partitions reduce the average sound levels in a space by decreasing the volume of noises reflected back into the office. However, they also reduce the noise floor and can actually produce a space that is too quiet. While they had made the space somewhat more comfortable, these materials also eliminated the modest degree of speech privacy that had existed in the space. Conversations had become more intelligible over a greater distance.
Occupants’ level of speech privacy and their ability to concentrate is influenced by the signal to noise ratio, or the volume of conversation or sound relative to the background level or noise floor. The greater the sound relative to the background, the more noticeable and understandable it is. While sounds decay in volume over distance, low background levels also mean that they can be clearly heard from afar. A masking system dramatically reduces this distance by raising the background sound level. Any voices or sounds that have decayed to a level below the masking will be covered up and go unheard. Over shorter distances, masking may not prevent you from hearing that someone is speaking, but it will inhibit your ability to understand what is being said. This effect is a key benefit because comprehensible speech is the most distracting and least private.
Referring again to our case study, the company’s use of absorptive materials also failed to meet the third acoustical requirement, which is low dynamic range. This term is defined as the variation in the volume of sound over time or the difference between the peak sound levels and the noise floor in the space. Human senses are designed to detect such changes and it is difficult to ignore them; therefore, the higher the dynamic range, the harder it is to “block out” noise.
The absorptive materials used in the European headquarters had no effect on the high dynamic range within the space. However, masking would dramatically reduce the variation in sound levels because the lowest level of sound to occur in the space would be that generated by the masking system. All sounds beneath the masking level would become unintelligible or imperceptible. With peak levels minimized through noise reduction and containment, as well as absorption, the result would be a more comfortable and consistent acoustic environment.
The fourth requirement for effective office acoustics addresses the variations in the acoustic environment of all facilities. The type of ceiling, open air return grills, HVAC ducts or equipment, and office machinery all affect the acoustic characteristics of a space. Variations in lighting, temperature and humidity are controlled and kept within certain comfort parameters, and, similarly, the fourth requirement for effective office acoustics is to minimize variations in both sound volume and frequency. Such consistency allows individuals to move within the office and work in multiple locations with minimal impact. A sound masking system is unique among methods of noise control in its ability to minimize acoustic variations because it introduces a specific background sound at controlled levels across the space. However, in order to achieve uniformity, the masking system itself must be flexible enough to adjust frequency and volume output for small areas. If not, the masking sound will contribute to the variations in the acoustics and the final result will be unacceptable.
In the end, the sound masking distributor was called back to evaluate the effect masking would have on the European headquarters. The result of the acoustical study indicated a dramatically reduced dynamic range with only a marginal increase in the average sound level. The impact on both employee privacy and comfort would be significant. A very similar result could have been achieved with far less expenditure and time lost if the facility had been planned with absorption and masking from the start.
In all but the most difficult situations, effective office acoustics can easily be achieved – first, by understanding the basic requirements for good acoustic performance and, second, by incorporating the methods and materials necessary to meet those requirements into the office design. Armed with a better understanding of the determinants of an effective acoustic environment, we see that office designs must continue to benefit from traditional noise control methods, such as reducing, blocking and absorbing noise, and that sound masking must also be integrated into the total acoustic solution.
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