Effects of the location of sound masking loudspeakers on cognitive performance in open-plan offices: Local sound masking is as efficient as conventional sound masking

Sound masking is a well-known measure to reduce the detrimental impact of irrelevant background speech in open-plan offices. However, the impact of the location of sound masking loudspeakers on the efficiency of masking systems with regard to cognitive performance has not yet been evaluated. This aspect is particularly important in decentralised sound masking architectures that enable individualised solutions but may depend highly on spatial release from masking. This paper reports on a study in an open-plan office with laboratory-like conditions. The subjects performed a serial recall task and completed a questionnaire during twelve different sound conditions with five different loudspeaker sets. With regard to serial recall performance, a disadvantage was observed when the speech and masker signals were presented from different horizontal directions of 90° as opposed to the same direction. Moreover, a similar disadvantage occurred when the masking sound was presented over six conventional sound masking loudspeakers that were mounted in the ceiling and positioned in a grid spacing of 2.4 m. However, when the cutoff frequency of the high-pass filter that was applied to the masker signal was lowered from 200 to 100 Hz, this disadvantage did not occur. Since the sound conditions with masking from the ceiling did not produce lower error rates and were not rated as less annoying, these results imply that a local sound masking system can improve the ability to concentrate at work as efficiently as a centralised approach.