Page 25 - Fall 2007

Task masking
The advent of vibration maskers for security masking has opened the door for localized masking in commercial offices, typically open office workstations. Placement of the masker under a work surface, behind a tack board, on a panel, or behind a picture, can provide privacy for an occupant with local control. Although the author had a panel mounted masking system in his workstation for several years, there seems not to be any current interest in this application by fur- niture manufacturers. However, there are personal speaker maskers that can be used for this application. One furniture manufacturer sells panel-mounted speaker maskers, but they are intended for area masking.
The masking spectrum
A required masking spectrum is defined in almost all sound masking specifications. In the poorer ones, it is given in octave bands as opposed to the one-third octave band rec- ommended by the American Society for Testing and
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Materials (ASTM). There is ample evidence that one spec-
trum is not optimum for all applications. The author has used Privacy Index7 in lieu of Articulation Index because users feel comfortable with the analogy between the degrees of privacy associated with it and school grades. Figure 2 shows the range of Privacy Index resulting from a variety of commonly used masking spectra in open offices. The overall level for each spectrum was 47 dB(A). It is clear that the spectrum contour for an open office is critical. To determine the best one for a given application, a computer modeling program was developed that creates a masking spectrum complemen- tary to the sound attenuation spectrum of the given design. This kind of modeling minimizes the amount of masking required to achieve a given degree of speech privacy. It is rec- ommended that the spectrum should be tailored to the spe- cific project, not the one saved from previous projects as is found in too many specifications. Many modern masking generators have multiple sources and equalizers, so they are capable of handling a wide range of spectrum contours.
The quest for spatial uniformity
Achieving spatial uniformity of the A-weighted sound masking level is a noble quest. One is reminded of the paper by Benoit Mandelbrot entitled “How long is the coastline of
Fig. 2. The Privacy Index achieved by various masking spectra in an open office can vary significantly. This strongly suggests that the masking spectrum contour is important in open offices.
England?” His point was that the length is determined by the scale of the measurement. As one makes the ruler smaller, the minor irregularities of the coastline are accounted for, increasing the measured length. The same concept applies to measurement of spatial uniformity. For example, if measure- ments are made everywhere in an open office, the results would show not only significant changes in level but also sig- nificant changes in the spectrum contour, demonstrating practical limits to the uniformity requirement.
One goal of spatial uniformity is to reduce the awareness of masking by not permitting detectable changes in level as one walks around a room. Experience has suggested that the small level changes within a workstation or closed office are seldom noticed. This suggests that measurements of spatial uniformity be restricted to the aisles of open offices, at occu- pant locations in closed offices, or in corridors.
The problem with all this is that the actual quest is for spatial uniformity of speech privacy. Because closed offices have very similar sound attenuation characteristics, both quests generally merge there. In open offices, however, the various panel heights and the possible proximity to vertical reflecting surfaces means that spatial uniformity of masking may not provide spatial uniformity of privacy. Zoning in open offices is the best means for solving that problem.
System zoning
Most masking system designs have insufficient zoning. A zone control changes the overall masking level created by a
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