Selecting a sound masking system
Acoustical requirements vary from office to office. One of the first considerations, when designing a sound masking system, is to determine the most advantageous speaker type and placement. The system capabilities must also be determined. For example, the system could be a basic dedicated sound masking system or a system with many zones of sound masking, paging and music.
sound masking fills in the existing ambient background sound
Sound masking is often confused with noise cancellation technology. Noise cancellation works well with headphones but is not applicable to use throughout an office where there are many noise sources and many listening positions. The lack of privacy in most offices is actually due to too little ambient background sound, making conversations and other activities more noticeable and distracting.
Typically an office without sound masking will have an ambient sound level of under 40 decibels. Conversational speech levels tend to be near 65 decibels causing conversations to be understood, and distracting to others, from up to 45 feet away. Adding sound masking to increase ambient sound levels to around 47 decibels does not impede local conversation, but limits the radius of distraction to around 15 feet.
In most open office environments, adding sound masking acoustically triples the distance between workers. In other words, workstations would have to be three times larger to get the same degree of privacy as can be achieved by adding sound masking.
how is open office speech privacy quantified?
Privacy Index is a rating of speech privacy that ranges from 0 (complete intelligibility) to 100 (complete privacy). It is important to note that the speech privacy rating is not linear. Analogous to academic grades, a Privacy Index of 50 does not represent a "passing" grade for open office. The goal for open office is a PI of 80 or above. The goal for closed offices is a PI of 95 or above. Note that panels, acoustical ceilings and sound masking all contribute to the resulting speech privacy level.
Networked sound masking systems
The most significant industry trend is toward networked, addressable sound masking systems. These systems use network hardware allowing reconfiguration or adjustment of the system via a networked PC or control panel. It is important to verify that the design provides for every speaker to be individually addressed. It is not good practice to connect multiple speakers to a single address.
Network systems have blurred the distinction between centralized and distributed systems. A network system should provide multi-channel networked audio for masking, paging and music signals. Sound masking signals can be broadcast through the network or generated at the speaker controller level. Beware of addressable or networked systems that specify "primary" and "secondary" speakers as the secondary speakers are not individually adjustable. Also beware of the practice of placing more than one speaker on a "speaker channel" as the speakers can only be adjusted as a group, defeating the intended purpose of an addressable system.
Centralized sound masking systems
In the classic centralized system there is a central equipment rack housing the system generator, mixers and amplifiers. Speakers are wired into specific zones. Every speaker has a volume control. This approach is still common and can offer more volume adjustment, per speaker, than some flawed network approaches.
Distributed sound masking systems
In a distributed system there is no central generator or amplifier. Each speaker contains it's own generator and amplifier, which is an ideal solution for very small systems, with little or no growth planned.
Ceiling plenum speakers and Open structure speakers
For offices with suspended acoustical ceilings the best choice is to specify and install upward firing plenum mounted speakers. Speaker are typically installed on 15-16 foot centers and fill the room below with non-directional, ambient sound. One of the traits of a professionally done sound masking system is that one cannot easily discern from where the sound is coming. Plenum mounted speakers produce reflected sound above the ceiling producing an unobtrusive and uniform sound in the room below. In many cases, closed offices can be masked by spillover from the open office speakers.
A popular construction trend for green buildings and those spaces which are designed to make an architectural statement, is to omit the
suspended ceiling. This approach works well, acoustically, when there is sufficient height to the structure above. Depending on the height
of the structure, speakers may need to be placed slightly closer together in order to assure spatial uniformity. In open structure the best
speaker selection is upward firing speakers similar to the type used in the plenum above a suspended ceiling.
Under access floor speakers
Under floor sound masking systems have risen in popularity due to the increased use of access floors. The reverberant under-floor cavity creates an ideal location for sound masking speakers. There are, however, some caveats. Floor penetrations and the perimeter of the floor must be properly sealed to avoid hot spots. Under floor sound masking systems can outperform other systems for spatial uniformity, however they are not suited to all access floor environments and require a consultant or contractor skilled in the design and installation of under-floor sound masking systems.
Downward facing or Direct field speakers
In most applications, the least desirable choice for sound masking distribution is the "direct field" speaker. These speakers are typically
installed in suspended ceilings and aimed downward at the room occupants causing a "spot-light" effect. Regardless of the broad
dispersion pattern produced by modern direct field speakers or emitters, in most applications they cannot produce the spatial uniformity of
speakers which fill the plenum and the room below with indirect ambient sound filtering through every ceiling tile.
In some cases a direct field masking system may be more easily tuned, since there are no plenum variables to consider during the tuning process. The design consideration that is often not considered with direct field speakers is that one must now consider in-room obstructions such as tall panels which can cause acoustical shadowing, especially where there are tall panels and relatively low ceilings.
Direct field speakers are the correct choice when there are no other options, such as a room having drywall ceiling with little access above. They can also work well with high ceilings and low panels.
In closed office environments a direct field speaker must be put into every office as there is no "spill-over" effect from the plenum space as is the case with upward firing speakers. In all cases a sound masking system using direct field emitters will require more speakers than an ambient system. The emitters will likely also have to be relocated during office reconfigurations. These systems are also more expensive to relocate as a hole is typically cut into the ceiling tile during installation.
Sound masking was created in the 1970s to address the lack of speech privacy in open office workstations. Upward firing speakers, above the ceiling, were used over typical downward firing paging speakers in order to create spatial uniformity and to create sound masking systems that were effective and unobtrusive. Downward firing speakers, or emitters, have improved over the years, but as the laws of physics have remained constant, directional speakers, aimed at the occupants, are by their very definition less uniform. One can prove this by experiencing both types of systems. With a plenum based system occupants cannot locate the position of sound masking speakers visually or acoustically, because the sound is coming through all the ceiling tiles. With exposed downward firing speakers, occupants can easily identify speaker locations both visually and acoustically.
For many applications, the classic centralized system is still a valid choice, however network systems will continue to be the state of the art in sound masking. As all other building systems become network based and controlled by facility managers, users will expect the same level of control over speech privacy and paging systems.
Verify when selecting a networked sound masking system that every speaker is individually addressable.