Page 23 - Fall 2007

PROGRESS IN SOUND MASKING
Robert C. Chanaud
Secure Sound Prescott, Arizona 86305
Introduction
Electronically generated sound
masking has been around for over
thirty years as a privacy tool and
great progress has been made in its use.
However, there are those who are still
wedded to the structural solution for
privacy. Those familiar with acoustics
know that sound attenuation created by
building structure is only one of three
factors that play a role in creating that
privacy. Yet many architects, and even
some acoustical consultants, cling to
structural solutions only. The glaring
weakness of sound attenuation alone is
that it is static, i.e., once installed it is usually difficult to alter and certainly almost impossible to change on a short term basis. The other two factors, the source and the background sound, are dynamic. Sound masking can be dynamic, i.e., the level can be set, either manually or automatically, at any time in any place. Another weakness of the sound attenuation approach in the design of offices is that the sound passing from one office to another will take multiple paths. Being a parallel path situation, if any one of the flanking paths is weak, the solution is weak. Specifications often go to elaborate, and expensive, extremes to eliminate weak flanking paths. All the flanking paths come together at the listener’s ear, and this is where sound masking is effective. Another factor that once inhibited use of sound masking can be exemplified by the question “How can you make it quiet by adding noise?” Now, many end users understand that the quest for privacy super- sedes the quest for quiet.
Unfortunately, many current specifications, designs, and installations have yet to take advantage of the capabilities of newer masking systems. Further integration of sound mask- ing into society will depend on these capabilities being used.
Secure applications
There is a need to protect facilities against deliberate lis- teners who may be using detection devices. The federal gov- ernment has a document1 that permits use of sound masking to protect conversations in their own facilities, such as in a Sensitive Compartmented Information Facility (SCIF), as well as those of government contractors. This is called secure masking. Generally, the room perimeter is protected, and this includes walls, windows, doors, ducts, piping, ceiling plenums and raised floor cavities. With the rise of economic espionage, commercial organizations are beginning to use secure masking to protect boardrooms, planning rooms, and research facilities. However, the sound masking equipment most often used in commercial facilities is inadequate.
“It appears that the technological evolution of sound masking equipment and methods has not found its way fully into the marketplace, but the new products are showing signs of changing that.”
It is well known that laser micro- phones exist and can detect from afar the minute vibrations of surfaces that are caused by local speech. The most obvious place for their use is on the window of a facility, but there are oth- ers. The structural solution is to have no windows, but this is not always acceptable to users. Unfortunately, windows face locations that are, most often, not under the control of the facility and the listener may have access to techniques that can recover signals buried in noise. For sound masking to provide protection, a vibration trans-
ducer must be attached to the window. Earlier, loudspeakers were placed in the ceiling above the window, but the required levels interfered strongly with room conversations. The sig- nal applied to the transducer should be non-stationary (ran- dom) to inhibit signal recovery. Further protection is achieved by having the masking signal layered, i.e., other sig- nals, such as music, voice babble, or simulated speech, should be added to and buried under the masking signal.
The structural solution for walls is a high sound transmis- sion class (STC) rating. Although that rating was not designed for speech, a sufficiently high STC wall may be able to protect against voice recognition beyond the wall, but cannot protect against other penetration methods. For example, listeners may attach vibration sensors to the inner surfaces of a stud wall to detect the gypsum board vibration caused by room speech. A normal microphone, or the relatively unknown fiber optic microphone, may be placed inside the wall cavity. The wall itself offers no protection against these devices and the back- ground level in wall cavities is quite low. Vibration maskers applied to the wall surfaces raises both the vibration masking level as well as the acoustical masking level, so if properly set they will handle all means of detection.
The structural solution for ducts is the addition of duct mufflers. They are expensive, increase system pressure drop, are difficult to install, and several may be required for each room. Again, because they are a static solution they may be able to protect conversations. The application of a vibration masker to the duct wall is a much simpler solution. It will raise internal sound levels sufficiently to protect against any sounds being transmitted through the duct.
Vibration transducers and non-stationary generators are commercially available.
Medical applications
For over forty years, persons involved in medicine (researchers, doctors, nurses, and patients), have written
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