Disruption of writing by background speech: The role of speech transmission index

Speech transmission index (STI) is an objective measure of the acoustic properties of office environments and is used to specify norms for acceptable acoustic work conditions. Yet, the tasks used to evaluate the effects of varying STIs on work performance have often been focusing on memory (as memory of visually presented words) and reading tasks and may not give a complete view of the severity even of low STI values (i.e., when speech intelligibility is low). Against this background, we used a more typical office-work task in the present study. The participants were asked to write short essays (5 min per essay) in 5 different STI conditions (0.08; 0.23; 0.34; 0.50; and 0.71). Writing fluency dropped drastically and the number of pauses longer than 5 s increased at STI values above 0.23. This study shows that realistic work-related performance drops even at low STI values and has implications for how to evaluate acoustic conditions in school and office environments. Undesired background speech in offices and similar environments potentially impairs work-related skills like writing [1–3], reading comprehension [4,5], proofreading [6–8], and prose memory [9,10]. Occupational noise, and speech in particular, are amongst the most often mentioned sources of annoyance at work [11–13], decreases satisfaction with the work environment [14], and are stressful [15,16]. Thus, performance and health are challenged when working in open-office environments wherein background sound, like talking colleagues, ringing phones, noise from copying machines, and so on, is common place. One objective predictor that is used to evaluate the acoustic environment is the Speech transmission index (STI) e.g., [17]. STI is a physical measure of speech intelligibility (i.e., the possibility to hear what is said) and is standardized by IEC 60268-16 [18]. It ranges from perfect speech intelligibility (i.e., 1.00) to no intelligi-bility at all (i.e., 0) and depends mostly on signal-to-noise ratio, reverberation and the amount of early reflections between the source and the receiver. A number of studies have investigated the relation between STI values of background sound and work-related performance e.g., [8,19–21]. A dominant view developed by Hongisto [17] is that performance drops most drastically when the background speech has an STI around 0.30 and 0.40, and that the decrement in performance fades out after an STI of 0.50. Hongisto's model is based on a generalization across several, different cognitive tasks. More recent studies have investigated the influence of STI on specific tasks and those studies …