Page 27 - Fall 2007

Fig. 5. When both the intensity and duration of exposure of an employee to acousti- cal distractions is expressed as dB-Minutes for several masking levels, the impact on employees is changed. It is based on the criterion that L10-L90 is greater than 5 dB.
Figure 5 shows the data in terms of the number of dB- Minutes greater than the criterion. Although the percentage of time a listener would be exposed to distractions was sig- nificant at 45 dB(A), this graph indicates that the severity of that exposure was greatly reduced. This figure also shows how important it is to set the masking level properly, and that a predetermined fixed level based on tradition is inadequate. Since we are concerned with cumulative effects, consider an average workday exposure. The average exposure is reduced from 4 dB to 1.4 dB by increasing the masking level by only 2 dB. Adding another 2 dB produces only minor improve- ment. It should be clear that too little masking can result in much distraction, and that too much masking adds little ben-
levels are abstracted and the difference between them is com- pared with a difference criterion. If the level difference is greater than the criterion, the masking level is raised slowly. If the level difference is less than the criterion, the masking level is lowered. Generally, the rate of decrease is less than the rate of increase.
To develop some feel for what that criterion should be and what masking levels are required to control distractions, we looked at a typical day in a busy office. Figure 3 shows the L10 and L90 values for a twenty four hour period in an open office with a suspended ceiling and with sound masking at a fixed level of 44 dB(A).
At the present time, it is considered reasonable to set the level difference criterion at 5 dB. Figure 4 shows the total exposure time for the data in Fig. 3 as a percentage of the workday for several levels of sound masking in an open office. Levels of 45 dB(A) and above provided a significant reduction in the amount of time one was exposed to distrac- tions. Raising the level from 43 to 47 dB(A) would have pro- vided a 45% reduction in exposure. The figure suggests that a level of 49 dB(A) would be required to provide good priva- cy at all times. Most professionals consider that level exces- sive and unacceptable to occupants.
Most dissatisfaction with the acoustics of a work envi- ronment is an accumulation of distractions both in time and in the degree of severity. A metric such as dB-Minutes, (sim- ilar to that used by the Occupational Safety and Health Administration)9 would add in that factor.
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