A model predicting the effect of speech of varying intelligibility on work performance

Speech is the most distracting sound in (open-plan) offices. Several laboratory studies have shown that speech impairs the performance of, for example, reading and short-term memory. It is not the sound level of speech that determines its distracting power but its intelligibility, which can be physically determined by measuring the Speech Transmission Index (STI). The aim of this study was to develop a mathematical model that predicts how much the performance is reduced due to speech of varying intelligibility. The model was based on the literature according to which performance decrements have been 4-45% depending on the task. The best performance occurs when speech is absent (STI = 0.0), and the strongest performance decrement occurs when speech is perfectly heard (STI = 1.0). The shape of the performance vs. STI between 0.0 and 1.0 was adopted from the general speech intelligibility theory. The performance starts to decrease when STI exceeds 0.2. Highest performance decrease is reached already when STI exceeds 0.60.