A Brief History of Sound Masking
Believe it or not, the first sound masking technology was probably the original Musak system, launched in 1922. During the same period, there was a movement toward open-office floor plans, and Major General James O. Squier’s patented technology of syndicating music over power lines into office lobbies and waiting rooms had the unintended – but quantifiable – result of significantly improving worker productivity in those office environments. However, music isn’t uniform (how boring would it be if it were!), and when you add in singing, it can actually be a distraction. A true sound masking system was needed.
In 1955, several faculty members of the Massachusetts Institute of Technology started the R&D firm of Bolt, Baranek, and Newman (BBN) and pioneered the field of speech privacy. By the early 1960’s, the acoustics division of that firm (known as Acentech) produced the first true sound masking systems. These early systems were mounted above the suspended acoustic tiles that hide air ducts and wiring, in a space called the “plenum.” While these early systems were technically effective, they required a volume level so high that the sound masking system itself became a distraction (imagine trying work on an airport runway). These were true “white noise” machines – with a sound spectrum matching everything from a burp to a scream. It didn’t take long, though, to begin developing sound masking specifically tailored to match the spectrum of human speech, resulting in less distracting volume levels by the early 1970’s.
Recently, researchers have come out with the first truly new leap in sound masking technology in nearly 40 years: a “direct field” system using a pre-tuned sound spectrum. This new paradigm in sound masking makes it possible to achieve the same levels of speech privacy at a lower volume and with much better uniformity, resulting in more effective and at the same time more invisible sound masking than ever before.
Plenum-based systems
Since the early 1960’s, whenever someone talked about sound masking, they were referring to a set of loudspeakers positioned in a grid pattern and mounted in the plenum. Even though the basic idea is more than 40 years old, these “indirect field” systems – in particular, high-end, professionally-tuned systems – provide good quality sound.
click to animate above-ceiling demonstation
However, putting audio equipment in the plenum has some inherent difficulties. The goal of any sound masking system is to effectively mask the audio frequencies of human speech at a uniform level across the entire treated area, using a comfortable tone and keeping it at a low volume (at or below 48 dBA, which the Canadian Research Council found is something of an “annoyance threshold”). This about as easy as trying to invent a 500-Horsepower engine that still gets 82 miles per gallon.
“Indirect field” (plenum-based) sound masking systems work on the theory that the plenum will act like an echo chamber: the sound will bounce around and as a result, the sound will even out, and filter down through the acoustic ceiling evenly and quietly. In reality, though, this ideal scenario is unlikely. Because the plenum isn’t empty, sound will reflect off or get blocked by the things the plenum hides, such as heating and air-conditioning ducts, light fixtures, steel beams, and the concrete structure itself. This keeps the sound from ever evening out. In addition, sound will come through the ceiling unevenly, pouring through return air vents and the seams around light fixtures or it will be absorbed by the insulation in other areas of the plenum. The net result is that all the stuff in the plenum will cause fairly erratic sound levels varying as much as 3 to 5 decibels from one part of the office to another, even if the sound masking system itself is very good. What this means for your workers is that some will wind up getting much less sound masking than they need.
Traditionally, these limitations have been offset by simply turning up the volume to ensure a minimum level of speech privacy throughout the treated area, but this results in uncomfortably high decibel levels (often as much as double the perceived volume of the 48 dBA “annoyance threshold”) for workers in other areas.
Direct Field Systems
click to animate VoiceArrest demonstration
Direct Field systems were developed to overcome the limitations the plenum imposes on the quality of any sound masking system. Engineers pulled the system out of the plenum and mounted small, ultra wide-angle dispersion emitters (speakers) in the ceiling tiles themselves. As a result, the VoiceArrest system is able to run at a noticeably lower volume than a system mounted in the plenum while still achieving accepted speech privacy levels. Also unlike plenum-based systems, the VoiceArrest system can provide a uniform, comfortable level of sound masking across the entire treated area. Much like with LP’s, full-size station wagons, and 20-lb “laptops,” the technology has advanced, leaving the old way of doing things behind.
Benefits of direct field
It should be obvious that adequate speech privacy can be obtained if the background sound level is high enough. If a user is seated under a particularly noisy return air grille, he may well not be aware of intruding speech from a colleague and, therefore, clearly has adequate speech privacy, even if he is annoyed by the excessive noise. Similarly, a background sound system can be turned up very loud until everyone in the office has sufficient speech privacy, but this also results in annoyance. However, it is possible to design a system in which both requirements are met – adequate speech privacy and freedom from annoyance.
The first requirement is proper tonal quality or balance of frequencies in the background sound. A second requirement is that the background sound level should not exceed approximately 48 dBA, as demonstrated by recent research studies carried out by the Canadian Research Council. Many background sound systems are adjusted for levels well above 48 dBA. This is probably because a majority of the population tolerates higher levels without serious complaint, even though they find it annoying. Finally, the sound must be spatially uniform, in both tonality and sound level, at the listener’s ear elevation so that his normal moving about in the office does not result in drastic changes in the sound.
The importance of this last requirement, spatial uniformity, is not well understood even by some vendors of background sound systems. It is unusual to find systems that achieve uniformity of better than 4 or 5 dB in the important speech frequencies. Variances of this magnitude result in dramatic changes in speech privacy levels throughout the office if the system is properly adjusted so that the highest levels do not exceed 48 dBA.
As a result, most systems are adjusted for average levels of as high as 58 dBA at many locations, annoying a significant percentage of users. Alternatively, the system is adjusted to a lower average level, compromising speech privacy for a substantial percentage of the users. These factors, unfortunately, have contributed to the opinion among some users that background sound systems are either too loud or are not very effective.
The VoiceArrest™ Speech Privacy System with QT™ Quiet Technology delivers adequate speech privacy and freedom from annoyance at 45 dBA.
