Cognitive performance during irrelevant speech: Effects of speech intelligibility and office-task characteristics

In open-plan offices, it is common that colleagues talk in the background while non-auditory tasks need to be performed. The aims of the present study were, first, to investigate how much cognitive performance is impaired by irrelevant background speech with varying degree of speech intelligibility, and second, to determine whether some office-related tasks are more susceptible to distraction than others. We tested a model developed by Hongisto which predicts the decrease of cognitive performance as a function of speech intelligibility, depicted by the Speech Transmission Index (STI). Our results showed that the shape of the STI–performance curve, and the magnitude of the performance decrease, depend on task type. A semantic short-term memory task (i.e. word memory) was more sensitive to disruption by speech than a mathematical task. A word fluency task (i.e. retrieval of information from semantic long term memory) was not influenced by varying speech intelligibility. Moreover, performance on an Information search task was impaired by speech with high intelligibility only. The steepest slope in performance decrease appeared at a lower speech intelligibility value than predicted by Hongisto . It also varied between cognitive tasks. The results of this study are useful in determining an appropriate target level for acoustic design in offices: This work demonstrates that attempts to minimize speech intelligibility will yield increases in cognitive performance with a varying degree, depending on the type of focal task.