Electronic Sound Masking

May 1, 2008

Sound masking systems can decrease distractions and increase speech privacy and productivity in open office settings.

By Niklas Moeller

More than 40 million North Americans work in open plan offices featuring partial height panels. Granted, cubicles make better use of space and improve communication flow, but they are an acoustic challenge.

Traditional walls have given way to modular furniture systems, more employees use the same space, and everyone is seated closer together. At the same time, new construction methods, absorptive treatments, and quieter equipment have lowered the background sound level.

That may sound good—and to some degree, it is—but without adequate background sound, it is easier to hear the distracting activities happening around you.

Are closed offices the solution? You expect privacy, but in fact, sound often leaks from one office to the next through the ceiling or air transfer components. Then a closed door means nothing.

If you work in a modern facility, you can likely relate. Usually you are spending time on work that requires concentration. Disruptive conversations and noises make it harder for you to complete tasks. Errors happen more often, adding to stress. It takes more effort to focus, which tires you out, affecting your mood and, ultimately, your productivity.

This is where sound masking comes in.

What is Sound Masking?
A sound masking system consists of a series of speakers, typically installed above the suspended ceiling, that distribute an engineered background sound throughout a facility. This sound is most often compared to that of softly blowing air.

The primary benefits of this technology include increased speech privacy, reduced distractions due to noise, and a generally more comfortable working environment. It is easily installed in new or existing facilities of any size, providing an effective way of addressing acoustic problems at nominal cost and without remodeling.

How Does it Work?
If you have ever run water at your kitchen sink while trying to talk to someone in the next room, you will understand how masking works. You can tell your conversational partner is speaking, but it is difficult to comprehend what is being said because the running water has raised the noise floor in your area.

The noise floor is the level of constant sound present in a space. If it is too high, you will find it irritating. Too low, and you can easily overhear conversations and noises.

Masking creates a noise floor high enough to cover up noises and low enough for comfort, usually in the range of 42 to 48 dBA. In so doing, this technology contributes to three key acoustic requirements: low dynamic range, low signal-to-noise ratio, and maximum acoustic consistency.

Key Acoustic Requirements
The role masking plays in achieving acoustic comfort can be explained using one company's experience. When they moved into their new headquarters, the noise levels were high due to the reflection of sounds off the hard ceiling, floor, and wall finishes. Sounds traveled great distances. There was some degree of speech privacy but only due to the high noise levels. Employees found it difficult to concentrate in this environment.

Clearly, something needed to be done, but the company assumed it could lower the noise levels simply by adding absorptive materials to the space. While this was a positive first step, it was not sufficient to satisfy all the requirements.

Low dynamic range
Absorption made the environment somewhat more comfortable by decreasing the volume of sounds reflected back into the space, but the perception of a noisy environment remained because the dynamic range stayed high.

"Dynamic range" is defined as the variation in the volume of sound over time or the difference between the peak sound levels and the noise floor. Our senses are designed to detect such changes, making it difficult to ignore them. The higher the dynamic range, the harder it is to "block out" the noise.

A masking system would significantly reduce this variation. The lowest level of sound to occur in the space would be that generated by the system. All sounds beneath this level would be imperceptible or unintelligible. With peaks minimized by the absorptive materials already in use, the result would be an even more comfortable acoustic environment.

Low signal-to-noise ratio
The company's sole reliance on absorptive materials also affected speech privacy levels. Conversations were suddenly more intelligible over greater distances.

Speech privacy levels are influenced by the signal-to-noise ratio, or the volume of conversation relative to the noise floor. The greater the difference, the more understandable speech is. A low noise floor also means that conversations can be clearly heard from afar.

A masking system would dramatically reduce this distance. In most offices, speech can be understood from up to 50 feet away. Masking can reduce this distance to 20 feet or even less. Over short distances, it might not prevent occupants from hearing that someone was speaking, but it would inhibit their ability to understand exactly what was being said.

Maximum acoustic consistency
It is also important to provide acoustic consistency across the space. Variations in both volume and frequency are found in all facilities due to the type of ceiling used, the location of open air return grills, HVAC, and office equipment. A masking system would minimize these variations, just as other technology is used to keep lighting and temperature at a comfortable level.

However, in order to achieve this requirement, the system must be installed throughout the facility and offer fine frequency and volume control for small zones. If it was used as a spot treatment, it would draw attention to itself as occupants moved about. Partial installations should only occur where there is a physical break between two areas, such as a wall, doorway, or corridor.

Masking can also reduce, but not necessarily eliminate, the requirements for other acoustic treatments. Adding sufficient physical barriers and absorptive materials to the design can further minimize the distance over which noises are heard and the system's volume can be decreased without affecting its performance level.

Types of Sound Masking Systems
Though masking systems have been available for about 40 years, this technology remains novel to many people. Some mistakenly refer to it as white noise. This term actually describes a specific type of sound used in early masking systems. Their inflexibility and the hissing quality of their sound prevented widespread acceptance, but the term was readily adopted. New technologies do not use white noise.

There are a variety of system types available today. Their design and performance vary dramatically:

The earliest systems were centralized. They consist of centrally located electronic components, which are used for sound generation, amplification, and equalization, and also provide volume control. Groups of speakers are connected to the output of these components. All of the speakers broadcast the same sound and any adjustments are applied to the entire group or zone. Limited volume adjustment is also possible at each speaker. Zones are hardwired and, depending on the system's design, can be up to hundreds of speakers in size.

A centralized system

The next development was the decentralized system, which is also referred to as distributed or self-contained. These systems consist of master and satellite units. The masters house the electronics for sound generation, amplification, and output control. Up to two satellites are connected to each one and reproduce its sound. Local volume and frequency adjustments are made by entering the ceiling and accessing the master's control, typically with a screwdriver. Global volume changes and timer functions are centrally controlled. Paging and timer zones are hardwired. This type of system offers much smaller masking zones of one to three speakers.

A decentralized system

The most recent development is the networked system. It consists of distributed components that are digitally and remotely controlled using a centrally located panel or computer software. A single line of cable carries all power, control, and audio signals to the components. All types of zones are digitally assigned and can be changed without recabling. Fine volume and frequency adjustments can either be made globally and/or down to zones as small as one to three speakers (225 to 675 ft2; 21 to 63 m2).

A networked system

Desktop products are also available. These are standalone devices, not a facility-wide solution. They consist of electronics and speakers that are mounted within a workstation or sit on a desk. Because each user controls the volume, these products may reduce noise disruptions but cannot be relied upon to increase speech privacy or provide consistent masking levels.

Selection Criteria
Carefully selecting a masking system based on its performance capabilities will help ensure that your needs are met both now and in the future. When evaluating any feature, consider its acoustic impact: What will its effect be across your space and over the system's lifetime?

For most facilities, a high level of flexibility is key to achieving effective masking and maximizing occupant comfort. That is because it is not enough to just introduce any sound into your space. It has to be the right sound, in the right place, at the right time. The system's masking performance is largely determined by the following factors:

Masking generation
The number of masking generators the system includes determines the number of independently adjustable zones it offers and whether it will be affected by phasing (i.e. uncontrollable variations in the masking volume caused by the canceling and reinforcement of identical sound waves as they meet between speakers).

Determine how many masking generators the system provides and whether these are centrally located or distributed. If they are centrally located, ensure that the system is designed with at least two sound generators per zone and that the speakers are wired in an interlaced manner such that adjacent speakers emit different masking signals.

Zone considerations
Zone size affects the ability to adjust the masking to meet local needs and acoustic conditions. The smaller the zone, the more flexible the system will be because settings can be customized to each small area. Ask what size of zone the system offers and whether these are the same for both volume and frequency adjustment.

Most systems are also able to zone for paging, timer functions, keypads, and other accessories. Ask how many zones the system offers for each of these functions and whether they are independent of one another. If the system only provides one zone, it may be necessary to install several such systems in order to provide adequate control of these features within your facility.

Also ask whether zones are hardwired or digitally assigned. Hardwired zones require additional cabling and a contractor to make changes. Digitally assigned zones are changed from a control panel or computer.

Adjustment capabilities
The adjustment capabilities the system offers within each zone also affect the ability to adjust the masking sound to meet local requirements. Ask what kind of frequency (none, whole contour, 1 octave, or 1/3 octave control) and volume (large or fine increments) adjustment the system offers and over what zone size. The finer these adjustments, the greater control you have over the system's output.

Masking works best when it is inconspicuous. Total variations of more than 1 to 2 dBA (variations of +/- 0.5-1.0 dBA) can call attention to the sound and may reveal its source. Inquire as to the range of variation the system specifications permit across the space.

The system should be professionally tuned.

Speaker type
The uniformity of the sound is also affected by speaker orientation, location, and spacing. Most manufacturers offer a variety of speaker models, but the most common and best performing is still the in-ceiling, upward-facing speaker. Wall-mounted, downward-facing, underfloor, and desktop models can be useful in situations where you cannot access the ceiling or suspend anything in it, such as atriums, spaces with hard ceilings, and historical properties. However, downward-facing speakers are prone to volume variations due to a "spotlight" effect.

Where the speakers are installed also affects the system's ability to provide consistent coverage throughout your facility. For example, speakers installed within workstations are limited to areas with workstations. A versatile system can be installed in areas with suspended ceilings, hard surface ceilings, or no ceilings, and in both open and closed plan spaces.

If you have an open ceiling, consider a system with attractive speakers and tidy cabling, so that they will not detract from the design.

Control options
The method of control affects the ease, cost, and amount of disruption associated with making changes. It also impacts the likelihood that the masking and other functions will be modified if you move furniture or personnel in the future. Ask how many features can be modified and from what type of access points.

Timer functions
A timer allows the masking volume to vary in accordance with activity levels during the workday. Some systems also offer an acclimatization or ramp-up feature, which can be used to gradually introduce the masking sound in retrofit installations. Ask whether the system offers these functions and, if so, the configuration parameters it permits.

Scalability affects the system's ability to be installed in facilities of various sizes. Consider how much space the associated equipment will consume. Systems with centralized components may not be cost-effective for small installations and may take too much space in large ones. Systems with distributed components that require physical adjustment at the speaker may not be practical for large installations. Systems that offer central control of distributed components are suitable for installations of any size.

Paging and music
Most masking systems provide simultaneous paging and music distribution through the same set of speakers. However, depending on the type of system, they require different third-party equipment to be added. Ask what types of equipment you will need. If you select a system that cannot provide paging, it cannot be easily added later should your needs change.

Ask the vendor to provide documentation of the system's certifications. All components must be safety tested (UL or equivalent) and some also need to be tested for electromagnetic interference (FCC). In the United States, all masking systems installed in, or extending into, the plenum must be UL-2043 tested for flame and smoke resistance. Low-voltage systems must use Class 2-rated power supplies.

Because success depends on more than just the right product, it is important to select a system supported by professionals who can properly implement it and provide you with ongoing support. Determine what services are offered with the system you are considering.

Green Considerations
The growing popularity of green design brings additional acoustic challenges because the strategies that help with daylighting, temperature regulation, and energy conservation also tend to lower acoustic performance.

By addressing this problem, sound masking improves Indoor Environmental Quality (IEQ). Most systems have low energy needs, and some manufacturers have recycling programs in place and use RoHS-complaint parts, reducing or eliminating several pollutants.

If incorporated into the design early on, masking can also contribute to the green commitment - and lower costs - by decreasing material requirements, increasing the flexibility of the space, and helping maintain acoustic control as density increases.

Niklas Moeller ([email protected]) is vice president of K.R. Moeller Associates Ltd. He is involved in every aspect of the sound masking industry—from R&D to international market development—and frequently presents on the subject of workplace acoustics.

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