The relation between the intelligibility of irrelevant speech and cognitive performance-A revised model based on laboratory studies.

Irrelevant background speech causes dissatisfaction and impairs cognitive performance in open-plan offices. The model of Hongisto (2005, Indoor Air, 15, 458-468) predicts the relation between cognitive performance and the intelligibility of speech described with an objectively measured quantity, the Speech Transmission Index (STI). The model has impacted research in psychology and room acoustics as well as the acoustic design guidelines of offices. However, the model was based on scarce empirical data. The aim of this study was to revise the model based on a systematic literature review, focusing on laboratory experiments manipulating the STI of speech by wide-band steady-state noise. Fourteen studies reporting altogether 34 tests of the STI-performance relation were included. According to Model 1 that includes all tests, performance begins to decrease approximately above STI = 0.21 while the maximum decrease is reached at STI = 0.44. Verbal short-term memory tasks were most strongly and very consistently affected by the STI of speech. The model for these tasks showed a deterioration in performance between STI 0.12 and 0.51. Some evidence of an STI-performance relation was found in verbal working memory tasks and limited evidence in complex verbal tasks. Further research is warranted, particularly concerning task-specific effects.