Describing Levels of Speech Privacy in Open-Plan Offices

The primary goal of the acoustical design of conventional open-plan offices (cubicle style) is to achieve an acceptable level of acoustical or speech privacy. That is, we would like to make it more difficult for occupants to hear and understand speech sounds from adjacent workstations. We can therefore measure speech privacy by measuring the intelligibility of speech sounds from adjacent workstations. Speech privacy or speech intelligibility ratings can be related to signal-to-noise ratio type measures that indicate how much louder the speech signal is relative to the ambient noise. The most common objective measures of speech privacy in open-plan offices are the Articulation Index (AI) and its more recent replacement the Speech Intelligibility Index (SII). Both are frequency-weighted signal-to-noise-ratio type measures. That is, they include measures of signal-to-noise ratios at a range of frequencies and sum them according to the relative importance of each frequency band. Both AI and SII have values varying between 0 and 1. A value close to 1 indicates near-perfect speech intelligibility. A value close to 0 indicates very low intelligibility, which would correspond to a high degree of speech privacy. Criterion values of AI=0.15 (equivalent to SII=0.20) have been suggested as a maximum value for achieving 'acceptable' or 'normal' speech privacy in open-plan offices. However, these criteria are not based on experimental studies and seem to have been estimated from practical experience. The literature contains several descriptions of the meaning of 'normal' or 'acceptable' speech privacy as well as different criteria for achieving them other than those given above. The present work included controlled experiments in which subjects evaluated a wide range of simulated acoustical conditions in open offices. In one experiment subjects performed speech intelligibility tests for conditions of varied AI and SII. In a second experiment, subjects rated the speech privacy and the degree of distraction of the conditions while working on representative office work tasks. At the criterion values of AI and SII, the median subject could understand 88% of the words in phonetically balanced test sentences. For these criterion values they rated speech privacy as a little less than 'acceptable' and the degree of distraction as almost 'moderate'. These AI and SII criteria are a good choice for design goals because they are practically achievable while representing a significant amount of speech privacy. They represent the point at which speech privacy starts to substantially improve with decreasing SII or AI value. …