Abstract

Providing an optimal acoustical environment is an inherent challenge in open-plan offices. In order to avoid overhearing private conversations by the office workers and reduce the distraction by the background noises, many open-plan offices adopt an electronic sound masking system that generates an artificial sound to make background noises less sensible. The sound masking system should uniformly generate the targeted masking sound over the entire office area to provide the desired influence on the acoustical environments. To evaluate the uniformity of the masking sound field in open-plan offices, this study followed the ASTM E 1573-18 method which is a standard for assessing the spatial uniformity of the sound masking systems in open plan offices. More specifically, sound maps were created with a measurement grid of 0.6 m x 0.6m, to determine the spatial variations of the masking sound levels across the office space. This study also examined the parameters that can influence the spatial uniformity in the open-plan offices by using a computer-aided simulation. Moreover, to understand the importance of a uniform sound field across the office area, the intelligibility of the speech was assessed by calculating the Articulation Index (AI). The measurement results show that the variation of the sound pressure level (SPL) across the space is within ±1.5 dBA when considering the overall A-weighted SPL and ±4.5 dB when considering the unweighted octave band SPL for the frequencies from 250 Hz to 4 kHz. From the results of the simulation it can be found that the number of sound masking loudspeakers, partition height, and the scattering and absorption coefficient of the ceiling can affect the uniformity of the masking sound. For the defined parameters, the lowest variation was achieved when the number of the sound masking loudspeakers was increased to 7, followed by when the partitions were removed and when the whole ceiling was covered by the diffuser. Among these parameters, increasing the number of sound masking loudspeakers has the most effects on improving the uniformity of the sound field across the office space. Moreover, in all the defined conditions the SPL variation for more than 90% of the space cannot be less than ±1 dBA when considering the overall A-weighted SPL, and ±1.5 dB when considering the unweighted octave band SPL. After evaluating the variation of the speech intelligibility across the office, using the AI, the results show that when there is a higher variation in the sound field, the AI range change more. In order to have more than 60% of the office area within ±0.03 AI range, the variation of the sound field across the space should be less than ±1.5 dBA, and to have more than 80% of the office area within ±0.03 deviation for the AI value, beyond ±1 dBA deviation in the SMS should be avoided. However, as the open plan offices are much complicated than this model, achieving a tight range for the SPL uniformity of the sound masking system could not be realistic in most of the open plan offices. In this regard, more research should be done to find the necessity of defining tight tolerances and the effect of SPL variation of the masking sound on the speech inteligibility in open-plan offices.